Content:

EMRP – Earth Magnetism & Rock Physics

EMRP1.2 – Advances in Rock Physics and coupled THM reservoir processes

EGU2020-7237 | Displays | EMRP1.2

Upscaling laboratory measurements: Quantifying the role of hydrothermal alteration in creating geothermal and epithermal mineral resources

Michael Heap, Darren Gravley, Ben Kennedy, Albert Gilg, Elisabeth Bertolett, and Shaun Barker

Hydrothermal fluids can alter the chemical and physical properties of the materials through which they pass and can therefore modify the efficiency of fluid circulation. The role of hydrothermal alteration in the development of geothermal and epithermal mineral resources, systems that require the efficient hydrothermal circulation provided by fracture networks, is investigated here from a petrophysical standpoint using samples collected from a well exposed and variably altered palaeo-hydrothermal system hosted in the Ohakuri ignimbrite deposit in the Taupō Volcanic Zone (New Zealand). Our new laboratory data show that, although quartz and adularia precipitation reduces matrix porosity and permeability, it increases the uniaxial compressive strength, Young’s modulus, and propensity for brittle behaviour. The fractures formed in highly altered rocks containing quartz and adularia are also more planar than those formed in their less altered counterparts. All of these factors combine to enhance the likelihood that a silicified rock-mass will host permeability-enhancing fractures. Indeed, the highly altered silicified rocks of the Ohakuri ignimbrite deposit are much more fractured than less altered outcrops. By contrast, smectite alteration at the margins of the hydrothermal system does not significantly increase strength or Young’s modulus, or significantly decrease permeability, and creates a relatively unfractured rock-mass. Using our new laboratory data, we provide permeability modelling that shows that the equivalent permeability of a silicified rock-mass will be higher than that of a less altered rock-mass or a rock-mass characterised by smectite alteration, the latter of which provides a low-permeability cap required for an economically viable hydrothermal resource. Our new data show, using a petrophysical approach, how hydrothermal alteration can produce rock-masses that are both suitable for geothermal energy exploitation (high-permeability reservoir and low-permeability cap) and more likely to host high-grade epithermal mineral veins, such as gold and silver (localised fluid flow).

How to cite: Heap, M., Gravley, D., Kennedy, B., Gilg, A., Bertolett, E., and Barker, S.: Upscaling laboratory measurements: Quantifying the role of hydrothermal alteration in creating geothermal and epithermal mineral resources, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7237, https://doi.org/10.5194/egusphere-egu2020-7237, 2020.

EGU2020-6028 | Displays | EMRP1.2

Acoustic signature of fluid substitution in reservoir rocks

Christian David, Joël Sarout, Christophe Barnes, Jérémie Dautriat, and Lucas Pimienta

During the production of hydrocarbon reservoirs, EOR operations, storage of CO2 underground or geothermal fluid exchanges at depth, fluid substitution processes can lead to significant changes in rock properties which can be captured from the variations in seismic waves attributes. In the laboratory, fluid substitution processes can be investigated using ultrasonic monitoring. 

The motivation of our study was to identify the seismic attributes of fluid substitution in reservoir rocks through a direct comparison between the variation in amplitude, velocity, spectral content, energy, and the actual fluid distribution in the rocks. Different arrays of ultrasonic P-wave sensors were used to record at constant time steps the waveforms during fluid substitution experiments. Two different kinds of experiments are presented: (i) water injection experiments in oil-saturated samples under stress in a triaxial setup mimicking EOR operations, (ii) spontaneous water imbibition experiments at room conditions.

In the water injection tests on a poorly consolidated sandstone saturated with oil and loaded at high deviatoric stresses, water weakening triggers mechanical instabilities leading to the rock failure. The onset of such instabilities can be followed with ultrasonic monitoring either in the passive mode (acoustic emissions recording) or in the active mode (P wave velocity survey).

In the water imbibition experiments, a methodology based on the analytical signal and instantaneous phase was designed to decompose each waveform into discrete wavelets associated with direct or reflected waves. The energy carried by the wavelets is very sensitive to the fluid substitution process: the coda wavelets are impacted as soon as imbibition starts and can be used as a precursor for remote fluid substitution. It is also shown that the amplitude of the first P-wave arrival is impacted by the upward moving fluid front before the P-wave velocity is. Several scenarios are discussed to explain the decoupling between P wave amplitude and velocity variations during fluid substitution processes.

How to cite: David, C., Sarout, J., Barnes, C., Dautriat, J., and Pimienta, L.: Acoustic signature of fluid substitution in reservoir rocks, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6028, https://doi.org/10.5194/egusphere-egu2020-6028, 2020.

EGU2020-6615 | Displays | EMRP1.2

Laboratory testing for monitoring of reservoir properties during water injection

Davide Geremia, Christian David, Christophe Barnes, Beatriz Menéndez, Jeremie Dautriat, Lionel Esteban, Joel Sarout, Sara Vandycke, and Fanny Descamps

Since the performances of geological reservoirs are continuously changing during the production history, tools capable of characterizing these changes are becoming day by day of relevant importance. A time-lapse monitoring is an important and widely used way to explore the variations induced by the oil depletion. In an enhanced oil recovery scenario, the seismic survey method has been mostly used to monitor the remaining oil fraction with respect to the injected water, however no particular attention has been addressed to the effects generated by the water-rock interaction, which might induce deformation with no stress variation. Indeed, it is well known that water can induce important mechanical weakening in reservoir rocks.

For that purpose, we performed injection tests on carbonate rocks in a conventional triaxial cell. An essential characteristic of these tests is the very low injection pressure, in order to minimize changes in the effective stresses and focus specifically on the rock - fluid interaction. The test consists in injecting water from the bottom in a critically loaded sample, initially in a dry state, until deformation is induced by the water-air substitution and failure is reached. While testing, the rock sample is instrumented with either 6 (at UCP, GEC lab) or 16 (at CSIRO, Geomechanics and Geophysics lab) P-wave transducers allowing us to perform an active ultrasonic survey with a narrow time intervals.

The described methodology allowed us to monitor how P-wave attributes (amplitude, velocity and frequency), elastic moduli, as well as, permeability and injection rate change while water is flooding the sample increasing the water saturation, and damage is produced by the water – rock interaction. For instance, injecting water into a dry rock sample could produce several patterns of variations in the P-wave velocity, which we ascribed to 1) partial water saturation; 2) water-induced damage with no failure; 3) water-induced failure and, in some cases, 4) total water saturation. More experiments are planned to mimic real EOR operation, like injecting water in an oil-saturated rock sample, with acoustic monitoring as well.

The outcome of this study indicates that combining multiple data sets from different sources is an effective tool for monitoring the exploitation of underground resources. This can certainly enhance our understanding of reservoir properties changing over time and target the attention toward the areas of greatest interest.

How to cite: Geremia, D., David, C., Barnes, C., Menéndez, B., Dautriat, J., Esteban, L., Sarout, J., Vandycke, S., and Descamps, F.: Laboratory testing for monitoring of reservoir properties during water injection, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6615, https://doi.org/10.5194/egusphere-egu2020-6615, 2020.

EGU2020-22373 | Displays | EMRP1.2

Anisotropy in soft rocks

Julia Leuthold, Elina Gerolymatou, and Theodoros Triantafyllidis

In this work, the mechanically induced compaction process in highly porous rocks is studied with experimental investigations and constitutive modeling. The focus of the study is on the influence of the inherent anisotropy on the mechanical properties. From a practical point of view, such behavior is of particular interest when considering reservoirs in soft, porous rocks. The reduction in pore pressure, which is linked to the production, leads to the possibility of compaction in the vicinity of the borehole. One effect is the risk of the loss of stability or of increased sand production. Another is the reduction of the permeability locally. The probability of such occurrences and the magnitude of such effects is currently under debate.

Although the formation of compaction bands in porous rocks has already been investigated in several studies, both in the laboratory and in situ, the extent data about the influence of the inherent anisotropy on the mechanical properties of porous rocks is limited. Baud et al. [1] documented an influence of the orientation of the bedding plane on the mechanical behavior of Diemelstadt sandstone and Louis et al.  [2] documented an influence of the bedding plane on the formation of discrete and continuous compaction bands in Rothbach Sandstone.

On the basis of an extensive experimental program of triaxial and isotropic compression, triaxial extension tests as well as investigations with ultrasonic pulse method, the mechanical behavior of a highly porous rock (Maastricht Calcarenite) is analyzed with a special focus on the formation of compaction bands. The test program is performed with samples cored under different inclinations to the bedding plane to study the influence of the inherent anisotropy on the mechanical properties.

Based on the experimental results, the applicability of a constitutive model for the description of the mechanical properties is tested. Furthermore it is examined how the inherent anisotropy may be considered in the constitutive model and different approaches are discussed.

For the numerical simulation a nonlocal model is suggested to simulate the formation of compaction bands. Finally, conclusions are drawn and an outlook on experimental investigations of the influence of compaction banding on the hydraulically properties is given.

 

[1]P. Baud, P. Meredith und E. Townend, „Permeability evolution during triaxial compaction of an anisotropic porous sandstone,“ Journal of Geophysical Research, May 2012.

[2]L. Louis, P. Baud und T.-f. Wong, „Microstructural Inhomogeneity and Mechanical Anisotropy Associated with Bedding in Rothbach Sandstone,“ Pure and Applied Geophysics, July 2009.

 

How to cite: Leuthold, J., Gerolymatou, E., and Triantafyllidis, T.: Anisotropy in soft rocks, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22373, https://doi.org/10.5194/egusphere-egu2020-22373, 2020.

EGU2020-5417 | Displays | EMRP1.2

Time-lapse X-ray imaging of deformation modes in organic-rich Green River Shale heated under confinement

Maya Kobchenko, Anne Pluymakers, Benoit Cordonnier, and François Renard

Shales are layered sedimentary rocks, which can be almost impermeable for fluids and act as seals and cap-rock, or, if a shale layer hosts a fracture network, it can act as a fluid reservoir and/or a conduit.  Organic-rich shales contain organic matter - kerogen, which can transform from solid-state to oil and gas during shale burial and exposure to heat. When the organic matter is decomposing into lighter molecular weight hydrocarbons, the pore-pressure inside the shale rock increases and can drive propagation of hydraulic fractures and strongly modify the permeability of these tight rocks. Density, geometry, extension, and connectivity of the final fracture network depend on the combination of the heating conditions and history of external loading experienced by the shale reservoir. Here, we have performed a series of rock physics experiments where organic-.rich shale samples were heated, under in situ conditions, and the development of microfractures was imaged through time. We used the high-energy X-ray beam produced at the European Synchrotron Radiation Facility to acquire dynamic microtomography images and monitor different modes of the shale deformation in-situ in 3D. We reproduce natural conditions of the shale deformation processes using a combination of vertical load, confining and heating of the shale samples. Shales feature natural mineral and silt lamination and hydraulic fractures easily propagate parallel to these laminae if no overburden stress is applied. However, if the principal external load becomes vertical, perpendicular to the shale lamination, the fracture propagation direction can deviate from the horizontal one. Together horizontal and vertical fractures form a three-dimensional connected fracture network, which provides escaping pathways for generated hydrocarbons. Our experiments demonstrate that tight shale rocks, which are often considered as impermeable, could have hosted transient episodes of micro-fracturing and high permeability during burial history.

How to cite: Kobchenko, M., Pluymakers, A., Cordonnier, B., and Renard, F.: Time-lapse X-ray imaging of deformation modes in organic-rich Green River Shale heated under confinement , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5417, https://doi.org/10.5194/egusphere-egu2020-5417, 2020.

EGU2020-8454 | Displays | EMRP1.2

Seismo-electric Conversion in Sandstones and Shales using 2 Different Experimental Approaches, Modelling and Theory

Paul Glover, Rong Peng, Piroska Lorinczi, and Bangrang Di

The development of seismo-electric (SE) exploration techniques relies significantly upon being able to understand and quantify the strength of frequency-dependent SE conversion. However, there have been very few SE measurements or modelling carried out. In this paper we present two experimental methods for making such measurements, and examine how the strength of SE conversion depends on frequency, porosity, permeability, and why it is unusual in shales. The first is based on an electromagnetic shaker and can be used in the 1 Hz to 2 kHz frequency range. The second is a piezo-electric water-bath apparatus which can be used in the 1kHz to 500 kHz frequency range.

The first apparatus has been tested on samples of Berea sandstone. Both the in-phase and in-quadrature components of the streaming potential coefficient have been measured with an uncertainty of better than ±4%. The experimental measurements show the critical frequency at which the quadrature component is maximal, and the frequency of this component is shown to agree very well with both permeability and grain size. The experimental measurements have been modelled using several different methods.

The second apparatus was used to measure SE coupling as a function of porosity and permeability, interpreting the results using a micro-capillary model and current theory. We found a general agreement between the theoretical curves and the test data, indicating that SE conversion is enhanced by increases in porosity over a range of different frequencies. However, SE conversion has a complex relationship with rock permeability, which changes with frequency, and which is more sensitive to changes in the petrophysical properties of low-permeability samples. This observation suggests that seismic conversion may have advantages in characterizing low permeability reservoirs such as tight gas and tight oil reservoirs as well as shale gas reservoirs.

We have also carried out SE measurements on Sichuan Basin shales (permeability 1.47 – 107 nD), together with some comparative measurements on sandstones (0.2 – 60 mD). Experimental results show that SE conversion in shales is comparable to that exhibited by sandstones, and is approximately independent of frequency in the seismic frequency range (<1 kHz). Anisotropy which arises from bedding in the shales results in anisotropy in the streaming potential coefficient. Numerical modelling has been used to examine the effects of varying zeta potential, porosity, tortuosity, dimensionless number and permeability. It was found that SE conversion is highly sensitive to changes in porosity, tortuosity and zeta potential in shales. Numerical modelling suggests that the cause of the SE conversion in shales is enhanced zeta potentials caused by clay minerals, which are highly frequency dependent. This is supported by a comparison of our experimental data with numerical modelling as a function of clay mineral composition from XRD measurements. Consequently, the sensitivity of SE coupling to the clay minerals suggests that SE exploration may have potential for the characterization of clay minerals in shale gas and shale oil reservoirs.

How to cite: Glover, P., Peng, R., Lorinczi, P., and Di, B.: Seismo-electric Conversion in Sandstones and Shales using 2 Different Experimental Approaches, Modelling and Theory, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8454, https://doi.org/10.5194/egusphere-egu2020-8454, 2020.

EGU2020-7213 | Displays | EMRP1.2

Hydraulic characterization of a karstic limestone vadose zone based on multi-methods geophysical measurements and lab testing

Clara Jodry, Céline Mallet, Jacques Deparis, Salma Ammor, Jean-Michel Baltassat, and Mohamed Azaroual

The vadose zone (VZ) is a highly heterogeneous and dynamic system that have a huge impact on fluid flows and heat transfer, from the soil to the saturated zone. In order to characterize flow patterns within the vadose zone, a comprehensive knowledge of spatial hydraulic parameters distribution is necessary. In this matter, geophysical techniques have proven to be efficient, providing various physical parameters and imaging of the underground. Nonetheless, these techniques are mainly used for water-saturated media and an appropriate calibration of the standard petrophysical relationships is necessary for VZ.

This study is carried out in the framework of the implementation, in an agricultural field, of the platform “Observatory of Transfers in the Vadose Zone” (O-ZNS, Centre – Val de Loire, France). The O-ZNS aims to understand and quantify mass and heat transfers in the VZ thanks to an exceptional well (depth – 20 m and diameter – 4m) associated with boreholes dedicated to geophysical measurements and instrumented piezometers. The emphasis is put on developing high-resolution investigations and focused monitoring techniques and sensors for the vadose zone. This observatory offers a unique support to study and establish the relationships to convert physical responses into hydraulic parameters, especially water content, in the VZ of a limestone aquifer.

The geophysical field investigations, conducted prior to the digging of the well, included various scales of observation with 3D Electrical Resistivity Imaging, 2D Magnetic Resonance tomography and crossholes Ground Penetrating Radar tomography. These highlighted three main lithological groups with a few meter-thick soil, a heterogeneous karstified limestone and a massive fractured limestone, all part of the same geological formation. The results put forth the importance of the karstified level heterogeneity on transfers’ behaviour in the VZ, highlighting the presence of clay lens and a disparate water content distribution.

Going further, laboratory investigations have been carried out using field cores in order to characterize the VZ of the Beauce Limestone aquifer. Laboratory analyses enable us to establish Topp’s, Archie’s and CRIM (Complex Refractive Index Model) empirical relations and model. The objective now is to link quantitatively these geophysical field measurements, primarily electrical conductivity and dielectric permittivity, to the medium’s hydraulic parameters (e.g., hydraulic conductivity, porosity, water content). Results from this analysis should bring valuable information on the hydrogeological behaviour of the aquifer system and underline the influence of the observation scales on the estimation of the hydraulic parameter values of the vadose zone.

How to cite: Jodry, C., Mallet, C., Deparis, J., Ammor, S., Baltassat, J.-M., and Azaroual, M.: Hydraulic characterization of a karstic limestone vadose zone based on multi-methods geophysical measurements and lab testing, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7213, https://doi.org/10.5194/egusphere-egu2020-7213, 2020.

Keywords: Thermo Hydro-Mechanical (THM), Model Order Reduction (MOR), Parametric solutions, Real time simulations.
 
Radioactive waste is a by-product of nuclear power generation. It is hazardous to all forms of life and the environment. Its radioactive activity naturally decays over time, so waste has to be isolated and confined in appropriate disposal facilities for a sufficient period until it no longer poses a threat. Deep geological repositories constitute one of the most promising options for isolating this type of waste from human and environmental interactions. The analysis and prediction of the behaviour of such systems relies on coupled THM models [1]. The coupled nature of the problem is explained as follows [2]: i) a thermal part including the heat released by the wastes; ii) the mechanical behavior of the canister holding the wastes, the isolation system and the underground host rock; and, iii) the flow of natural water present in any underground porous media.

A coupled THM problem depends on space, time, and on material parameters (for instance, elastic modulus (E), heat conductivity (κ) and hydraulic conductivity (K)) and geometric parameters (for instance, the distance between canisters). We seek for families of solutions depending on these parameters. We would like to provide a real time numerical simulation of the THM problem for any value of the parameters within a range. Real time here, means a solution provided in a few seconds (instead of several hours). Such a solution can be used within an inversion problem, to obtain an best fitting value of the parameters based on some observations, or even in a control situation, where the prediction of the simulation is used to take some decision in the field.
 
Reduced Order Methods (ROM)  are a family of numerical methods able to provide such a solutions. In this work we will present several parametric problems, and show how ROM  [3] can provide real time solution to (simple) THM problems.
 

REFERENCES:

[1] Toprak, E.; Mokni, N.; Olivella, S.; Pintado, X.: Thermo-Hydro-Mechanical Modelling of Buffer. Synthesis Report. August 2013.

[2] Selvadurai, A.P.S; Suvorov, A.P.: Thermo-Poroelasticity and Geomechanics.CAMBRIDGE UNIVERSITY PRESS, 2017.

[3] Diez, P.; Zlotnik, S.; Garcia-Gonzalez, A.; Huerta, A.: Encapsulated PGD algebraic toolbox operating with high-dimensional data. Accepted In Archives of Computational Methods in Engineering, 2019.

How to cite: Moaven, A., Massart, T. J., and Zlotnik, S.: Real time solutions of Thermo-Hydro Mechanical problems with application to the design of Engineered Barriers via Reduced Order Methods, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10171, https://doi.org/10.5194/egusphere-egu2020-10171, 2020.

EGU2020-22565 | Displays | EMRP1.2

Analysis of THCM coupling in heterogeneous sediments using high-pressure flow-through testing systems

Christian Deusner, Shubhangi Gupta, Andrzej Falenty, Elke Kossel, and Matthias Haeckel

The experimental and numerical investigation of THCM process coupling is important to better understand reservoir geotechnical behavior and sub-surface processes. In particular, when THCM process coupling is dominated by focused fluid migration and localized chemical or microbiological reactions, bulk sediment and, thus, reservoir geotechnical behavior becomes poorly predictable. To improve the understanding of these complicated processes and process coupling on relevant time and spatial scales, it is necessary to combine experimental and numerical simulation approaches, and to develop complementary investigation strategies.    

We use different high-pressure flow-through experimental systems with triaxial testing units in combination with tomographical imaging tools (e.g. X-ray CT and ERT) to simulate and analyze relevant processes in ocean and earth systems. Our geotechnical studies are carried out at high hydrostatic pressures up to 40 MPa and temperatures between -30°C and 80°C. The experimental systems allow testing of large sample specimen (up to a diameter of 150 mm and a height of 400 mm). In particular, we investigate scenarios with heterogeneous phase distributions and dynamic flow conditions, which cannot be interpreted based on the assumption of homogeneous phase distributions in a sensible manner.

Here, we focus on discussing experimental and numerical strategies and problems towards understanding geotechnical behavior of heterogeneous sediments, including issues from gas migration in fine-grained sediments (e.g. silty clays), gas hydrate formation under two-phase flow conditions, and localized failure and shear banding in cemented soils. We present results from recent studies on underground usage including gas production and injection scenarios, which are relevant for the understanding of reservoir behavior, storage scenarios and, overall, marine sediment and slope stability. One of the most important aspects is to improve current strategies for combined and complementary experimental and numerical studies, considering that the overall objective is to understand processes on a reservoir scale.

How to cite: Deusner, C., Gupta, S., Falenty, A., Kossel, E., and Haeckel, M.: Analysis of THCM coupling in heterogeneous sediments using high-pressure flow-through testing systems, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22565, https://doi.org/10.5194/egusphere-egu2020-22565, 2020.

Maximizing heat exploitation in geothermal systems is crucial for the economic efficiency of many geothermal systems. As the hydraulic flow in most geothermal systems is primarily due to fracture flow, heat transfer processes along the fracture surfaces are essential. However, while flow and mass transport in a single fracture have been studied experimentally and theoretically to a great extent, heat transfer processes have been rarely investigated. Laboratory experiments show the influence of the fracture surface morphology on flow and heat transfer processes, though a physical interpretation has been missing so far. Further, in many geothermal systems but also in many natural hydrothermal systems, the solid and fluid phases are not in local thermal equilibrium. Parameterization of local thermal non-equilibrium models was originally developed for porous media and adoptions to fractures have been cumbersome. In this work, I present a numerical study on heat transfer processes across rough fracture surfaces. Using a three-dimensional steady-state flow model, heat transfer across the fracture surface is studied for both scenarios: assuming and neglecting a thermal equilibrium across phase boundaries. Also, separate fracture morphologies have been studied using natural sandstone probes as well as synthetically generated fractures. The numerical simulations results are compared to laboratory experiments using artificially generated and 3D-printed fracture surfaces of various fracture morphologies for code validation. The full three-dimensional simulations reveal the role of flow channeling effects on the heat transfer taking place along rough surfaces, which is not captured by simulations with reduced spatial dimensions. The simulations results suggest a re-examination of the effective heat transfer coefficient for fractured reservoirs under local thermal non-equilibrium conditions incorporating characteristics of fracture morphology. The simulations results can also be linked to thermal stress generation and possibly explaining the deformations of fracture surfaces observed in the laboratory. However, parameterization of surface roughness is neither distinct nor trivial. Various parameters exist, such as the joint roughness coefficient, Hurst exponent or statistical descriptions, but none has been successfully linked to flow, transport or transfer characteristics. Relating fracture morphology with results of numerical simulations and laboratory findings regarding transfer and transport processes indicate a shortfall of conventional roughness parameterizations to sufficiently describe the observed variation in heat transfer parameters.

How to cite: Heinze, T.: Numerical study of heat transfer across rough fracture surfaces, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4195, https://doi.org/10.5194/egusphere-egu2020-4195, 2020.

EGU2020-1217 | Displays | EMRP1.2

Temporal change of permeability in macro-fractured granite by accumulation of fine-grained minerals

Yoshitaka Nara, Masaji Kato, Tsutomu Sato, Masanori Kohno, and Toshinori Sato

It is essential to understand the long-term migration of radionuclides when considering rock engineering projects such as the geological disposal of radioactive waste. The network of fractures and pores in a rock mass plays a major role in fluid migration as it provides a pathway for fluid flow. The geometry of the network can change due to fracture sealing by some fine-grained materials over long-term periods. Groundwater usually contains fine-grained minerals such as clay minerals. Therefore, it is possible that the accumulation of such fine-grained minerals occurs within a rock fracture under groundwater flow. In this case, the aperture of a fracture may decrease, which brings about the decrease of the permeability. It is therefore essential to conduct permeability measurements using water including fine-grained minerals in order to understand the permeability characteristics of a rock. However, this has not been investigated well. In this study, we use a macro-fractured granite sample to investigate the temporal change of the permeability that occurs under the flow of water that includes two different amounts of clay.

It was shown that the clay accumulated in the macro-fracture and that the permeability of the macro-fractured granite sample decreased over time. It was also recognized that the decrease of the permeability was more significant under the water flow with the higher clay content. As a result of the observation using microscope, it was recognized that the clay minerals accumulated in the macro-fracture in the granite sample, which decreased the aperture of the fracture. We concluded that the accumulation of clay minerals in the fracture decreased the permeability of the rock. Furthermore, it is concluded that the filling and closure of fractures in rock is possible under the flow of groundwater including clay minerals.

 

How to cite: Nara, Y., Kato, M., Sato, T., Kohno, M., and Sato, T.: Temporal change of permeability in macro-fractured granite by accumulation of fine-grained minerals, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1217, https://doi.org/10.5194/egusphere-egu2020-1217, 2020.

The carbon storage and energy development activities in deep geologic zones that potentially affect water quality in shallow aquifers are of central importance in the energy-water nexus. The extraction of natural gas involves hydraulically fracturing deep shale formations. The storing of carbon dioxide in deep geologic formations is pursued to mitigate global warming. Both these activities have the potential to contaminate the shallow aquifers used for potable water and return the greenhouse gases to the atmosphere. In the case of carbon storage, both during and post-injection phases, it is possible for the CO2 and formation brine to leak through natural faults, pressure-induced fractures, or failed well casings. Two scientific challenges have to be addressed to safely store the carbon in the deep formation while protecting the shallow aquifers. The first, characterizing the affected geologic formations, and the second is monitoring the leakage. Monitoring involves determining leakage locations and tracking of the gas and brine plume through the geologic formation between the deep confining layer used for storage and the shallow aquifer. Challenges to the characterization derive from the limitations and sparsity of observational data in deep formations.  Effective monitoring poses both scientific and engineering challenges as the leakage locations not known, and the resulting pathways cannot be predicted easily. This paper presents two studies where intermediate-scale testing systems were used to understand the processes that occur during the leakage of stored supercritical CO2.  The focus of the first study was to better understand the process of CO2 gas exsolution after a leak from the deep confining formation. The second study addresses the issue of monitoring brine leakage from the confined formation where supercritical CO2 is stored. The improved understanding of these leakage processes will help to develop assessment and monitoring systems for storage permeance and protecting shallow sources of potable groundwater. It is not feasible to conduct experiments in the field to obtain both the fundamental process understanding and test and validate developed modeling and monitoring tools due to lack of control of boundary and initial conditions and expense in fully characterizing deep formations. Tests in intermediate scale synthetic aquifers where highly controlled experiments can be conducted to obtain accurate data provide an alternative to overcome this challenge. However, designing test systems and performing tests under ambient laboratory conditions different types of challenges. This paper will present some of the challenges and how they were overcome. The results on new process insights, how the data was used to assess the natural capacity of the aquifer attenuation of leaking gas, and validating inversion methods for site characterization and leakage detection will be presented. Even though this study focused on CO2 leakage, the results will be of value in problems of natural has leakage during hydraulic fracturing in alternate energy development.

 

How to cite: Illangasekare, T. and Askar, A.: Understanding the leakage of greenhouse gasses from seep geologic formations during geologic carbon storage and hydraulic fracturing: intermediate scale testing challenges , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19592, https://doi.org/10.5194/egusphere-egu2020-19592, 2020.

EGU2020-805 | Displays | EMRP1.2

A Pore-Scale Investigation of Fluid Displacement and Residual Trapping Under Intermediate-Wet Conditions

Rumbidzai Nhunduru, Amir Jahanbakhsh, Omid Shahrokhi, Krystian Wlodarczyk, Duncan Hand, William MacPherson, Susana Garcia, and Mercedes Maroto-Valer

Residual trapping in which ganglia of fluid are isolated and immobilised in porous media by capillary forces is innate to several subsurface engineering applications including carbon geo-sequestration. Residual trapping is highly significant in carbon dioxide (CO2) sequestration, as entrapment of supercritical CO2 in rock pore spaces, limits upward migration of the buoyant CO2 plume and enhances long-term CO2 storage security. It is estimated that residual trapping contributes up to 40% of overall trapping CO2 in the first century following injection (1). The amount of residual trapping depends largely on the wettability of the porous rock.

Brine filled saline aquifers have been identified as having the largest potential for CO2 storage with an estimated cumulative storage capacity of 104 Giga-tons of CO2 (2). Likewise, the focus of many studies has been devoted to investigating residual trapping in water-wet, brine filled sandstone reservoirs, and little attention has been given to intermediate-wet and oil-wet carbonate reservoirs. However, until CO2 storage technology reaches maturity, initial CO2 sequestration projects will most likely be conducted in depleted and oil producing carbonate reservoirs due to economic benefits associated with CO2 enhanced oil recovery and the existence of installed infrastructure which can be reassigned for CO2 injection purposes (3). 

Accordingly, in this work, the intrinsically water-wetting surfaces of laser fabricated glass micromodels (4); which are two-dimensional representations of natural porous rock structures, were chemically modified to imitate intermediate-wet reservoir conditions through a silanization procedure. Imbibition experiments were conducted in the micromodels using two proxy, CO2-brine fluid pairs; deionized (DI) water and n-decane as well as DI water and air.

Fluid displacement under intermediate wettability was analysed and compared with water-wet conditions and residual fluid saturations were quantified for different porous structures. The Volume of Fluid method was used to simulate the experiments in OpenFOAM. Results from the micromodel experiments were used to validate the simulations.

This work has demonstrated that fluid displacement during the imbibition process occurs through a series of cooperative pore-filling events under intermediate-wet conditions and the presence of dead-end pores was found to enhance residual trapping of the non-wetting fluid. Coupling experimental and simulation studies provides a unique insight to multiphase flow under intermediate wet conditions.  

 

Acknowledgements

This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (MILEPOST, Grant agreement no: 695070). This paper reflects only the authors’ view and ERC is not responsible for any use that may be made of the information it contains.

 

References

  1. Li X, Akbarabadi M, Karpyn ZT, Piri M, Bazilevskaya E, Experimental Investigation of Carbon Dioxide Trapping Due to Capillary Retention in Saline Aquifers, Geofluids, 2015;15(4):563–76.
  2. Benson; GEA; Iiasa. Chapter 13: Carbon Capture and Storage. Global Energy Asssessment. 2012.
  3. Al-Menhali AS, Menke HP, Blunt MJ, Krevor SC. Pore Scale Observations of Trapped CO2 in Mixed-Wet Carbonate Rock: Applications to Storage in Oil Fields. Environ Sci Technol 2016;50(18):10282–90.
  4. Wlodarczyk KL, Carter RM, Jahanbakhsh A, Lopes AA, Mackenzie MD, Maier RRJ, Hand DP, and Maroto-Valer MM, Rapid Laser Manufacturing of Microfluidic Devices from Glass Substrates. Micromachines. 2018; 9(8)

How to cite: Nhunduru, R., Jahanbakhsh, A., Shahrokhi, O., Wlodarczyk, K., Hand, D., MacPherson, W., Garcia, S., and Maroto-Valer, M.: A Pore-Scale Investigation of Fluid Displacement and Residual Trapping Under Intermediate-Wet Conditions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-805, https://doi.org/10.5194/egusphere-egu2020-805, 2020.

EGU2020-9408 | Displays | EMRP1.2

Laboratory study of Hydraulic Fracture interaction with artificial interfaces.

Anna Shevtsova, Egor Filev, Maria Bobrova, Sergey Stanchits, and Vladimir Stukachev

Nowadays Hydraulic Fracturing (HF) is one of the most effective stimulation technique for hydrocarbon extraction from unconventional reservoirs, as well as enhanced geothermal applications. Practical applications of HF can have different aims. In one case, we need to stop cracks inside the host rock to avoid some HF breakthroughs into other formations and possible groundwater pollutions. The second situation is when we need to fracture several bedding planes in a reservoir which has a complex structure, especially in case of the presence of multiple natural fractures in unconventional reservoir. It is important to study hydraulic fracturing, its propagation and conditions of interaction with interfaces in laboratory conditions before expensive field application.

The present work demonstrates the results of a laboratory study designed to understand fracture interaction with artificial interfaces. For the first series of experiments, we used some natural materials such as shales, sandstones, dolomites and limestones with different porosity, permeability and mechanical properties. During these experiments we initiated hydraulic fracturing in homogeneous specimens with and without artificial surfaces, modelling natural fractures or bedding planes in unconventional reservoirs. For the second series of experiments, we used a combination of different materials to understand HF propagation in heterogeneous media, to study conditions of HF crossing or arrest at the boundaries between different types of rock. These laboratory experiments were done to create HF simulating natural processes in fractured and heterogeneous rocks or reservoirs.

Series of hydraulic fracturing experiments under uniaxial load conditions were conducted using the multifunctional system MTS 815.04. Before testing, samples were scanned by 3D CT System to characterize the rock fabric, and after testing, CT scanning was repeated to characterize 3D shape of created HF. The dynamics of HF initiation and propagation was monitored by Acoustic Emission (AE) technique, using piezoelectric sensors glued to the surface of the rock to record elastic waves radiated during the process of HF propagation. The experiments were made with different injection rates and fluid viscosities. Changes in radial strain, injection pressure and microseismic data over time were recorded.

As the result, these experiments indicate significant factors (rock heterogeneity, porosity, permeability, fluid viscosity and injection rate), influencing cracks initiation, propagation or arrest on the artificial interface. The fracture propagation and opening are characterized by measured radial deformation, fluid pressure and geometrical orientation in the sample volume. The experiments demonstrated, that fracture easily crossed artificial surface in the homogeneous limestone samples. And cracks initiated in limestone were arrested on the border with shale. In all cases combination of the AE and deformation monitoring allows to indicate fracture initiation, propagation and arrest.

How to cite: Shevtsova, A., Filev, E., Bobrova, M., Stanchits, S., and Stukachev, V.: Laboratory study of Hydraulic Fracture interaction with artificial interfaces., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9408, https://doi.org/10.5194/egusphere-egu2020-9408, 2020.

EGU2020-8736 | Displays | EMRP1.2

Density and magnetic susceptibility relationships in non-magnetic granites; a “wildcard” for modeling potential fields geophysical data.

Emilio L. Pueyo, Mª Teresa Román-Berdiel, Conxi Ayala, Francesca Loi, Ruth Soto, Elisabeth Beamud, Elena Fernandez de Arévalo, Ana Gimeno, Luis Galán, Stefanía Schamuells, Nuria Bach-Oller, Pilar Clariana, Félix M. Rubio, Antonio M. Casas-Sainz, Belén Oliva-Urcia, José Luis García-Lobón, Carmen Rey, and Joan Martí

Geophysical surveying (both gravity and magnetic) is of great help in 3D modeling of granitic bodies at depth. As in any potential-field geophysics study, petrophysical data (density [r], magnetic susceptibility [k] and remanence) are of key importance to reduce the uncertainty during the modeling of rock volumes. Several works have already demonstrated that ∂18O or [SiO2] display a negative correlation to density and to magnetic susceptibility. These relationships are particularly stable (and linear) in the so-called “non-magnetic” granites (susceptibilities falling within the paramagnetic range; between 0 and 500 10-6 S.I.) and usually coincident with calc-alcaline (CA) compositions (very common in Variscan domains). In this work we establish robust correlations between density and magnetic susceptibility at different scales in CA granites from the Pyrenees. Other plutons from Iberia were also considered (Veiga, Monesterio). The main goal is to use the available and densely sampled nets of anisotropy of magnetic susceptibility (AMS) data, performed during the 90’s and early 2000’s, together with new data acquired in the last few years, as an indirect measurement of density in order to carry out the 3D modelling of the gravimetric signal.

 

We sampled some sections covering the main range of variability of magnetic susceptibility in the Mont Louis-Andorra, Maladeta and Marimanha granite bodies (Pyrenees), all three characterized by even and dense nets of AMS sites (more than 550 sites and 2500 AMS measurements). We performed new density and susceptibility measurements along two main cross-sections (Maladeta and Mont Louis-Andorra). In these outcrops, numerous measurements (usually more than 50) were taken in the field with portable susceptometers (SM20 and KT20 devices). Density data were derived from the Arquimedes principle applied on large hand samples cut in regular cubes weighting between 0.3 and 0.6 kg (whenever possible). These samples were subsampled and measured later on with a KLY-3 susceptibility bridge in the laboratory. Additionally, some density data were derived from the geometry and weighting of AMS samples.

 

After the calibration of portable and laboratory susceptometers, density and magnetic susceptibility were plotted together. Regressions were derived for every granite body and they usually followed a linear function similar to: r = 2600 kg/m3 + (0.5 * k [10-6 S.I.]). As previously stated, this relationship is only valid in CA and paramagnetic granites, where iron is mostly fractioned in iron-bearing phyllosilicates and the occurrence of magnetite is negligible (or at least its contribution to the bulk susceptibility). These relationships allow transforming magnetic susceptibility data into density data helping in the 3D modelling of the gravimetric signal when density data from rock samples are scarce. Given the large amount of AMS studies worldwide, together with the quickness and cost-effectiveness of susceptibility measurements with portable devices, this methodology allows densifying and homogenizing the petrophysical data when modelling granite rock volumes based on both magnetic and gravimetric signal.

How to cite: Pueyo, E. L., Román-Berdiel, M. T., Ayala, C., Loi, F., Soto, R., Beamud, E., Fernandez de Arévalo, E., Gimeno, A., Galán, L., Schamuells, S., Bach-Oller, N., Clariana, P., Rubio, F. M., Casas-Sainz, A. M., Oliva-Urcia, B., García-Lobón, J. L., Rey, C., and Martí, J.: Density and magnetic susceptibility relationships in non-magnetic granites; a “wildcard” for modeling potential fields geophysical data., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8736, https://doi.org/10.5194/egusphere-egu2020-8736, 2020.

Testing the efficiency of ferrofluid impregnation in porous media – recommendations for future magnetic pore fabric studies

 

Michele Pugnetti*, Yi Zhou*, Andrea R. Biedermann*

* Institute of Geological Sciences, University of Bern, Baltzerstrasse 1+3, CH-3012 Bern, Switzerland (michele.pugnetti@geo.unibe.ch)

 

In AMS (anisotropy of magnetic susceptibility)-based pore fabric studies, the role of ferrofluid impregnation is crucial to ensure significant magnetic measurements. However, no standard methods to test the ferrofluid impregnation of porous media have been proposed so far. The details of fluid behaviour in porous media are important in many fields of natural sciences, but nanoparticle distribution in the fluid is particularly important for magnetic measurements. In this study methods to test the impregnation efficiency of ferrofluid in porous media, and nanoparticle distribution are proposed, using different materials: wood, agarose and TEOS (tetraethylorthosilicate) gel, and synthetic samples of given composition and grain size, as well as natural rocks. Magnetic pore fabric measurements are normally performed on natural porous samples to correlate the direction of maximum magnetic susceptibility with the direction of preferred pore elongation, and preferred flow direction. The advantage of using artificial samples is the possibility to control and adjust some physical parameters, including porosity and pore size, to keep them more uniform or fix them to a given value. This allows investigating the nanoparticle distribution in ideal samples without the influence of additional heterogeneities inherent to natural samples and to determine the lowest porosity value and smallest pore size that is possible to impregnate with ferrofluid. In particular, the agarose and TEOS gel have a uniform porous structure controlled by the gel concentration or chemical agents used in sample preparation. The wood has a wider range of porosity compared to rocks and a known intrinsically anisotropic structure. The synthetic samples have a uniform grain size, mineralogy and structure. First the porosity of the samples was measured, then to impregnate the samples different methods were developed and tested, (1) percolation, (2) standard vacuum impregnation, (3) flow-through impregnation, (4) diffusion process in gel structure. Impregnation efficiency was evaluated both optically and magnetically. Different impregnation methods provide different impregnation efficiency depending also on the investigated material; in particular porosity plays an important role in limiting the impregnation efficiency. Initial experiments indicate that in general, flow-through impregnation is more efficient than vacuum impregnation because it combines the effect of vacuum with the pressure applied to the fluid that is pushed through the sample. The best results on natural samples were obtained using calcarenites with relatively high porosity. These results and the methods proposed here will help advance magnetic pore fabrics studies and impregnation processes in general.   

 

How to cite: Pugnetti, M., Zhou, Y., and Biedermann, A.: Testing the efficiency of ferrofluid impregnation in porous media – recommendations for future magnetic pore fabric studies, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5434, https://doi.org/10.5194/egusphere-egu2020-5434, 2020.

Unconventional oils are emerging as an alternative hydrocarbon reserve since conventional oil is depleting nowadays. A kind of unconventional oil is bitumen, which is characterized by high density, high viscosity and API gravity less than 10° and these physical properties are temperature sensitive. Therefore, an accurate assessment of variation in petrophysical properties of bitumen as a function of temperature and pressure is interesting in oil exploration industry.

In this work we investigated the role of heavy hydrocarbons (HHC) in changing petrophysical properties of carbonate-bearing rocks of the Majella reservoir performing seismic wave velocity measurements at increasing temperature. The investigated lithology belongs to the Bolognano formation that outcrops naturally in saturated and unsaturated conditions in the northwest sector of Majella Mountain (in Central Italy).

We conducted ultrasonic measurements of compressional and shear wave velocities on HHC-bearing carbonate samples showing different bitumen content and porosity between 10% and 19%. Firstly, we characterized bitumen density by HCl dissolution of the hosting rock, that resulted to be included between 1.14 and 1.26 gr/cm3 at ambient temperature. Then, we calculated HHC content of our samples, spanning from 2% (low HHC-bearing sample) to 16% (high HHC-bearing sample). Our acoustic velocities point out an inverse relationship with temperature. P- and S-wave velocities depict a distinct trend with increasing temperature depending on the amount of HHC content. Indeed, samples with the highest HHC content show a larger gradient of velocity changes in the temperature range of about 60°-50° C, suggesting that bitumen can be in a fluid state. Conversely, below about 50° C the velocity gradient is lower because, at this temperature, bitumen can change its phase in a solid state. Currently, we are analysing the coupling effect of temperature and pressure on HHC-bearing carbonate samples to test the acoustic response of the investigated samples simulating the reservoir conditions.

Our preliminary results highlight a strongly temperature dependence for HHC-bearing carbonate properties and bitumen influences the acoustic response of carbonate rocks. Such petrophysical characterization would provide a better link between seismic parameters and the hydrocarbon properties with important implications for reservoir characterization from seismic data and for production monitoring. 

 

How to cite: Ruggieri, R. and Trippetta, F.: Combination effects of temperature and pressure on the petrophysical properties of bitumen-bearing carbonate rocks: insight for the Majella reservoir (Italy), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-14960, https://doi.org/10.5194/egusphere-egu2020-14960, 2020.

Carbonate rocks are common in many parts of the world including the Eastern Mediterranean where they host significant groundwater supplies and are used widely in engineering as building and ornamental stones. Porosity of carbonate rocks plays a critical role in fluid storage and retrieval. The pore structure connectivity, in particular, controls many properties of rocks, and the relationships between the characteristics of individual minerals and the gross behavior of the rock. To study the relationships between porosity, rock properties, pore structure, pore size, and their impact on reservoir characteristics, several carbonate rock samples were collected from four stratigraphic sections exposed near Sidon, south Lebanon. The studied carbonate rocks are related to marine deposits of different ages (e.g., Upper Cretaceous, Eocene and Upper Miocene). In order to understand the pore connectivity, the MICP (mercury injection capillary pressure) is conducted on ten representative samples. Results from the SEM analysis indicate the dominance of very fine and fine pore sizes with various categories ranging in diameter from 0.1 to10 µm. The MICP data revealed that the pore throat radii vary widely from 0.001 to 1.4µm, and that all samples are dominated by micropore throats. The grain size analysis indicated that the studied rocks have significant amounts of silt- and clay-size grains with respect to the coarser sand-size particles; suggesting a high proportion of microporosity. Obtained results such as the poorly-sorted nature of grains, high microporosity, and the high percentage of micropore throats justify the observed low mean hydraulic radius, the high entry pressure, and the very low permeability of the studied samples. These results suggest that the carbonate rocks near Sidon (south of Lebanon) are possibly classified as non-reservoir facies.

How to cite: Salah, M.: Pore Structure and Petrophysical Characterization of Carbonate Rocks from Southern Lebanon , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1920, https://doi.org/10.5194/egusphere-egu2020-1920, 2020.

Understanding the deformation-related thermomechanical state of reservoir rocks under in-situ conditions is essential for modelling the stress distribution and stability of subsurface structures, for example associated with aftershock activity and induced seismicity. Commonly, reservoir modelling approaches make use of the generalized friction criterion according to Byerlee, which distinguishes between depths below and above approximately 6 km. However, numerous studies have shown that thermomechanical rock properties under elevated pressure and temperature conditions differ significantly from those at the surface and among rock types. The significant influence of the geothermal gradient on elastic and inelastic rock properties has already been demonstrated for temperature variations as low as 150 °C. Studies on the effect of in-situ stress and temperature conditions on post-failure behaviour and frictional properties are completely lacking.

In our experimental study we determined the thermomechanical properties of porous Ruhr sandstone samples during conventional triaxial deformation tests to derive stress- and temperature-dependent failure and friction criteria. Effective confining pressures and temperatures applied in the tests cover the range of in-situ conditions equivalent to depths up to three kilometres. Simultaneously, ultrasonic P- and S-wave measurements were performed to determine properties of ultrasound wave propagation (i.e. dynamic elastic properties) as a function of in-situ conditions. Triaxial deformation experiments were conducted at various strain rates to investigate the deformation-rate dependence of the failure and friction criteria and the correlation between dynamic and static elastic properties.

How to cite: Ahrens, B., Duda, M., and Saenger, E. H.: Experimental investigations on the temperature and strain-rate dependence of failure and friction criteria for a porous sandstone, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15134, https://doi.org/10.5194/egusphere-egu2020-15134, 2020.

EGU2020-7273 | Displays | EMRP1.2

Fracture reactivation for permeability enhancement in geothermal systems

Alexandra Kushnir, Michael Heap, Patrick Baud, and Thierry Reuschlé

While the deep granitic basement in the Upper Rhine Graben is currently being exploited as a geothermal reservoir at numerous geothermal sites, the Permo-Triassic sandstones that lie directly above the granite are critical to continued regional hydrothermal convection. Here we investigate the propensity for variably sealed fractures to be reactivated during deformation and the role this fracture reactivation plays on permeability enhancement in geothermal reservoirs. We source un-fractured, bedded sandstones and the same bedded sandstones containing a single, variably-sealed fracture from a 400 m-thick unit of Permo-Triassic sandstone sampled from the EPS-1 exploration well near Soultz-sous-Forêts (France) in the Upper Rhine Graben.

31 cylindrical samples (20 mm in diameter and 40 mm long) were cored such that their dominant structural feature (i.e. bedding or natural fracture) was oriented parallel, perpendicular, or at 30° to the sample axis. The initial permeability of the un-fractured samples ranged between 2.5×10-17 and 5.6×10-16 m2 and between 3.6×10-16 and 3.3×10-14 m2 for naturally fractured samples. In un-fractured samples, permeability decreases as a function of increased bedding angle; fracture orientation, however, does not appear to have a discernable influence on permeability. Samples were water-saturated and deformed until failure under pressure conditions appropriate to the Soultz-sous-Forêts geothermal system - Peff of 14.5 MPa - and at a strain rate of 10-6 s-1. All samples developed through-going shear fractures, however, only in samples containing partially sealed fractures did the experimentally produced fractures take advantage of the pre-existing features. In samples containing a fully-sealed fracture, the experimentally induced fracture developed in a previously undeformed part of the sandstone matrix. Further, post-deformation permeability measurements indicate that while sample permeability increased by up to one order of magnitude for a given sample, this increase is generally independent of feature orientation.

Therefore, formations containing sealed fractures may not necessarily be weaker and, as a consequence, may not be more apt to significant permeability increases during stimulation than un-fractured formations. These data can contribute to the development and optimization of stimulation techniques used in the Upper Rhine Graben.

How to cite: Kushnir, A., Heap, M., Baud, P., and Reuschlé, T.: Fracture reactivation for permeability enhancement in geothermal systems, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7273, https://doi.org/10.5194/egusphere-egu2020-7273, 2020.

EGU2020-15484 | Displays | EMRP1.2

Impact of a partly sealed fault on hydro-mechanical properties of a granite reservoir

Guido Blöcher, Christian Kluge, Mauro Cacace, Harald Milsch, and Jean Schmittbuhl

The fluid flow in Enhanced Geothermal Systems (EGS) is dominated by hydraulically stimulated fractures and faults which are the key elements of their hydraulic performance and sustainability. At the fault scale, the flow performance is influenced by the aperture distribution which is strongly dependent on the fault roughness, the geological fault sealing, the relative shear displacement, and the amount of flow exchange between the matrix and the fault itself. On the mechanical side, stiffness and strength of partly sealed fault might alter or reinforced the mechanical behavior of the fault zone in particular with respect to new stimulations. In order to quantify the impact of chemical soft stimulation in EGS reservoir on the hydro-mechanical properties of a fault-rock system that includes fault-filling material, we conducted numerical flow through experiments of a granite reservoir hosting one single partly sealed fault of size 512x512 m². In order to mimic the chemical alteration of the fault-rock system we sequentially changed the distribution pattern of the fault-filling material by means of a hydro-poro-elastic coupled simulation. Navier-Stokes flow is solved in the 3-dimensional rough aperture and Darcy flow in the related poro-elastic matrix. By means of this model, an evaluation of the local channeling effect through the fault for various degrees of sealing was performed. Based on the obtained results, we derived a macroscopic change of the hydraulic-mechanical behavior of the fault-rock system, e.g. permeability change, fracture stiffness modulus.

How to cite: Blöcher, G., Kluge, C., Cacace, M., Milsch, H., and Schmittbuhl, J.: Impact of a partly sealed fault on hydro-mechanical properties of a granite reservoir, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15484, https://doi.org/10.5194/egusphere-egu2020-15484, 2020.

EGU2020-19657 | Displays | EMRP1.2

Permeability heterogeneity during sandstone compaction

Philip Meredith, Nicolas Brantut, and Patrick Baud

Compaction of porous sandstones is generally associated with a reduction in permeability. Depending on porosity and other microstructural characteristics, compaction may be diffuse or localised in bands. Compaction bands have been shown to act as barriers to fluid flow and therefore reduce permeability perpendicular to the band orentiation, and thus also introduce permeability anisotropy. Additionally, the localised nature of compaction bands should also introduce strong permeability heterogeneity. We present new experimental data on sandstone compaction combining acoustic emission monitoring and spatially distributed pore fluid pressure measurements, allowing us to establish how permeability heterogeneity develops during progressive compaction. Three sandstones were tested in the compactant regime: Locharbriggs sandstone, which is microstructurally heterogeneous with beds of higher and lower initial permeability; a low porosity (21%) Bleurville sandstone, which is microstructurally homogeneous and produces localised compaction bands; and a high porosity (24%) Bleurville sandstone, which is also homogeneous but produces compaction in a more diffuse pattern. At regular intervals during compactive deformation, a constant pore pressure difference was imposed at the upper and lower boundaries of the cylindrical samples, and steady-state flow allowed to become established. Following this, local pore pressure measurements were made at four locations, allowing us to derive estimates of the local permeability. In all samples, progressive compaction produced overall reductions in permeability. In addition, localised compaction also produced internal reorganisation of the permeability structure. Localised compaction bands caused local decreases in permeability, while more diffuse compaction produced a more homogeneous overall reduction in permeability.

 

How to cite: Meredith, P., Brantut, N., and Baud, P.: Permeability heterogeneity during sandstone compaction, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19657, https://doi.org/10.5194/egusphere-egu2020-19657, 2020.

EGU2020-2148 | Displays | EMRP1.2

New Predictive Model for Relative Permeability of Deformable Gas Hydrate-Bearing Sediments

Gang Lei, Qinzhuo Liao, and Patil Shirish

Global energy demand is expected to grow significantly as the world population and the standard of living increase in the coming decades. As a potential source of energy, gas hydrate, which is a crystalline compound of gas-water mixture formed in stable of high pressure and low temperature, has been intensively investigated in the past few decades. In this work, a new analytical model is derived to study the effect of hydrate saturation on stress-dependent relative permeability behavior of hydrate-bearing sediments. The proposed relative permeability model solves the steady-state Navier-Stokes equations for gas-water two-phase flow in porous media with hydrates. It considers water saturation, hydrate saturation, viscosity ratio and hydrate-growth pattern, and is adequately validated with the experimental results in existing literatures. The model demonstrates that gas-water relative permeability in wall coating hydrates (WC hydrates) is larger than that in pore filling hydrates (PF hydrates). For WC hydrates, water phase relative permeability monotonically decreases as gas saturation increases. However, for PF hydrates, water phase relative permeability firstly increases and then decreases with the increase of gas saturation, which can be explained by the “lubricative” effect of the gas phase that exists between the water phase and hydrates. This work constitutes a comprehensive investigation of stress-dependent relative permeability in deformable hydrate-bearing sediments, which is a key issue for sustainable gas production. It not only provides theoretical foundations for quantifying relative permeability in hydrate-bearing sediments, but also can be used to estimate pore-scale parameters and rock lithology of gas hydrate-bearing sediments using inverse modeling.

How to cite: Lei, G., Liao, Q., and Shirish, P.: New Predictive Model for Relative Permeability of Deformable Gas Hydrate-Bearing Sediments, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2148, https://doi.org/10.5194/egusphere-egu2020-2148, 2020.

EGU2020-16747 | Displays | EMRP1.2

Fluid flow along a rough fracture: impact on hydraulic diffusivity

Qinglin Deng, Jean Schimittbuhl, Guido Blocher, and Mauro Cacace

Fluid flow along fractures or in fractured rock is of great importance in Enhanced Geothermal System, since natural fracture networks generally affect the permeability of the reservoir rocks and therefore the hydraulic performance. The cubic law commonly estimates the permeability of a single fracture, which is only valid for the flow through two smooth parallel plates. In fact, the flow performance is strongly influenced by the aperture fluctuations, which are related to the fracture surface roughness, the fluid-rock interaction process, and the amount of flow exchange between the matrix and the fracture itself, etc.

To quantify the hydraulic performance and get the better knowledge of the more real fracture flow, we conduct numerical simulations of fluid flow in a fracture-rock system hosting one single rough fracture from laboratory to field scales. As an example, a 2D self-affine rough surface is synthetically generated (Candela et al, 2012), with two anisotropic roughness exponents H// = 0.6 along the slip direction, Hperp = 0.8 in the perpendicular direction and a RMS amplitude of 0.1m at the 512m scale. Based on this surface generation, the opening geometry of a rough fracture is obtained as an input structure for finite element mesh generation. On one hand, we apply a lubrication approximation and limit the fracture opening to spatially variable 2D features with lower-dimensional element embedded in a saturated porous. On the other hand, we consider the full 3D features of the fracture opening as the space between two surfaces symmetrical about the mean fracture plane. The simulations are performed in the framework of the Mutiphysics Object Oriented Simulation Environment (MOOSE) combined with a MOOSE-based application GOLEM dedicated to modeling coupled Thermal-Hydraulic-Mechanical (THM) process in fractured geothermal reservoirs.

For the lubrication case, the mass balance equation for a saturated porous medium is described in terms of volumetric averaged mass conservation equations for the fluid phase, with Darcy’s law governing the momentum conservation equation. For the 3D fracture case, the incompressible Navier-Stokes equation is solved for the dynamic pressure and the velocity field inside the fracture only.

We compare the 2D and 3D cases and assess the effects of the nonlinear inertial term (u•∇)u in 3D case especially when the Reynolds number is high. The objective is to evaluate the large-scale hydraulic diffusivity of the fractured domain and its anisotropy owing to the strong contrast between the fluctuating fracture opening, and the homogeneous bulk porosity. The results show that the long-range aperture variations significantly affect the fluid flow, like the channeling effect and the hydraulic diffusivity anisotropy (i.e., along and perpendicular to the fault), which may have strong implications on the spatial distribution of fluid-induced seismic events in faulted reservoir.

How to cite: Deng, Q., Schimittbuhl, J., Blocher, G., and Cacace, M.: Fluid flow along a rough fracture: impact on hydraulic diffusivity , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16747, https://doi.org/10.5194/egusphere-egu2020-16747, 2020.

EGU2020-5982 | Displays | EMRP1.2

Interfacial processes at dissimilarly charged mineral surfaces in contact – a surface forces apparatus study

Joanna Dziadkowiec, Hsiu-Wei Cheng, Anja Røyne, and Markus Valtiner

When two mineral surfaces are in close contact, nanometers to microns apart, the proximity of another surface can significantly influence the pathways of chemical reactions happening in the interfacial region. Apart from affecting the kinetics of dissolution and nucleation reactions in spatial confinement, the proximity of charged surfaces can lead to electrochemically induced recrystallization processes. The latter may happen in an asymmetric system, in which two surfaces have a dissimilar surface charge. The charge and mass transferred during electrochemical reactions can induce dissolution or growth of solids and can significantly affect the local topography of surfaces, causing them to smooth out or to roughen. In this work, we present the experimental study of reactive mineral interfaces, immersed in geologically relevant electrolyte solutions, obtained with the electrochemical surface forces apparatus (EC-SFA). EC-SFA setup consists of one mineral surface and one gold surface (working electrode), the surface charge of which is controlled by applying an electrical potential. EC-SFA can, therefore, monitor electrochemically induced surface recrystallization processes. As the SFA technique is based on white light interferometry measurements, the changes in mineral thickness during recrystallization can be determined with an accuracy better than a nanometer over micrometer-large contact regions. Moreover, SFA allows in situ measurement of surface forces acting between mineral surfaces, which can provide additional information about how the surface reactivity influences the cohesion between mineral surfaces by modifying adhesive and repulsive forces acting between them at small separations.

How to cite: Dziadkowiec, J., Cheng, H.-W., Røyne, A., and Valtiner, M.: Interfacial processes at dissimilarly charged mineral surfaces in contact – a surface forces apparatus study , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5982, https://doi.org/10.5194/egusphere-egu2020-5982, 2020.

EGU2020-20857 | Displays | EMRP1.2

Evolution of Modelling Zeta Potential: Impact of Brine Compositions and Concentration

Miftah Hidayat, Jan Vinogradov, Stefan Iglauer, and Mohammad Sarmadivaleh

Electrochemical interactions of calcite with brines in natural subsurface settings have received ample attention in the last decades due to the broad range of their applications. These interactions can be described by an electrical property termed the zeta potential. Many numerical simulation studies using surface complexation modelling (SCM) have been performed to investigate the relationship between the zeta potential and a wide range of salinities and complex brine compositions. Although most of the simulated results, especially in low salinity conditions, successfully match the experimentally measured zeta potential, the simulated zeta potential for high salinity conditions is still poorly understood.

In this study, we present a new approach of SCM to simulate the zeta potential by considering the actual molecular-scale phenomena at the calcite-brine interface. Unlike previous SCM studies, our model considers the hydrated diameter of ions as the distance of approach, which depends on salinity. We also consider the permittivity of the Stern layer as a function of salinity, which is consistent with previous unrelated studies. We calculate the capacitance for each salinity based on the relationship between the hydrated diameter of ions and the permittivity of the Stern layer. Moreover, all calcite-brine surface reactions are described by new equilibrium constants independent of salinity and composition of brines.

Our results show that the simulated zeta potential which is obtained from our SCM at a broad range of salinities is successfully matched with the published experimental data for two different carbonate rock samples as long as the salinity dependence of the hydration diameter and electrical permittivity is accounted for. We find that the potential determining ions (Ca2+, Mg2+, SO42-, HCO3-,CO32-) play a dominating role compared to the indifferent ions (Na+, Cl-) in the calcite-brine surface reactions. The Implications of our findings are significant for wettability evaluation, characterisation of shallow and deep aquifers and CO2 geological sequestration.

How to cite: Hidayat, M., Vinogradov, J., Iglauer, S., and Sarmadivaleh, M.: Evolution of Modelling Zeta Potential: Impact of Brine Compositions and Concentration, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20857, https://doi.org/10.5194/egusphere-egu2020-20857, 2020.

In this study, zeta potential has been measured by using the streaming potential method for the intact sandstone in contact with CaCl2 electrolytes. The experimental results show that a positive zeta potential has been observed for the first time for the intact Fontainebleau sandstone under high salinity of CaCl2, and its magnitude increases with increasing ionic strength. It cannot be explained by the Gouy-Chapman theory anticipating a constant potential for high salinities due to the collapse of the electrical double layer. Meanwhile, the brine effluents after the completion of the streaming potential measurements were collected and then pH and brine composition were analysed suggesting that those variations of pH and chemical composition are negligible and cannot explain the polarity change at high salinity. The anomalous positive potential of the intact Fontainebleau sandstone is due to that overcharge of calcium ions sorbed into the mineral surface, which is consistence with previous literature data.

How to cite: Li, S. and Jackson, M.: Critical role of the structure of the mineral-water interface in the zeta potential measured by streaming potential method, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22077, https://doi.org/10.5194/egusphere-egu2020-22077, 2020.

Seismoelectric signals result from an electrokinetic coupling phenomena that can be modeled through two approaches: the coupling coefficient or the effective excess charge density. The traditional approach is based on the frequency dependent coupling coefficient that can relate differences in pressure to differences in electrical potential. The second approach is more recent and is related to the description of the excess charge that is effectively dragged by the pore water displacement relatively to the mineral surface. In this contribution, we propose a new model to obtain the frequency dependent effective excess charge density. The electrokinetic coupling is mechanistically up-scaled considering the pore as a straight capillary. This approach, called flux-averaging, takes into account the inertial term of the Navier-Stokes equation to explain both the dynamic permeability and the effective excess charge density dependence with oscillation frequency. The frequency dependent coupling coefficient can then be calculated from this result. The model results are then successfully compared to previous models and published data. This work is a first step to predict seismoelectric electrokinetic coupling in much more complicated porous media in saturated and partially saturated conditions.

How to cite: Jougnot, D. and Solazzi, S.: Modeling the seismoelectric electrokinetic coupling: a new approach to up-scale the frequency-dependent effective excess charge density, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16880, https://doi.org/10.5194/egusphere-egu2020-16880, 2020.

EGU2020-1424 | Displays | EMRP1.2

Seismic attenuation and velocity dispersion due to squirt flow in cracks with rough walls

Simón Lissa, Nicolás D. Barbosa, Eva Caspari, Yury Alkhimenkov, and Beatriz Quintal

We numerically study the effects that roughness in the walls of cracks has on the P-wave modulus dispersion and attenuation due to squirt flow. We emulate the deformation caused by a seismic P-wave by applying an oscillatory relaxation test on numerical rock models having two perpendicular fluid-filled cracks interconnected and embedded in a cubic elastic background. The deformation caused by the P-wave induces a fluid pressure gradient and then, during the consequent fluid pressure diffusion process, the friction between fluid particles dissipate seismic wave energy. In this work, we consider P-wave deformation normal to one of the cracks. We first consider binary aperture distribution for the cracks to analyse where the energy dissipation process takes place. Then, more complex geometries for the roughness of the walls are also considered. In both cases, the cracks have finite length and square-shape and no contact areas between the walls of the cracks were allowed to occur. We show that the arithmetic mean of the apertures controls the P-wave modulus magnitudes at the low- and high-frequency limits. Additionally, two attenuation peaks and modulus dispersion regimes may occur associated with squirt flow. In general, at low-frequencies, the energy dissipation tends to happen inside the minimum aperture of the cracks, and consequently, the minimum aperture determines the frequency at which the low-frequency attenuation peak occurs. For the considered models, we observed that when the percentage of minimum aperture in the cracks is lower than 10$\%$, a second attenuation peak at high frequencies become dominant. The characteristic frequency of this attenuation process is controlled by an effective hydraulic aperture. Finally, we simulate an increase in confining pressure by reducing the crack apertures by a constant value, allowing for contact areas occurrence. In this scenario, the stiffness of the cracks can not longer be explained with the arithmetic mean of the aperture, as the stiffening effect of the distribution of the contact areas plays a much stronger role. In general, from the analysis of the local energy dissipation, different apertures seem to control the energy dissipation process at each frequency, which means that a frequency-dependent hydraulic aperture might be needed to describe the squirt flow process in cracks with rough walls.

How to cite: Lissa, S., Barbosa, N. D., Caspari, E., Alkhimenkov, Y., and Quintal, B.: Seismic attenuation and velocity dispersion due to squirt flow in cracks with rough walls, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1424, https://doi.org/10.5194/egusphere-egu2020-1424, 2020.

EGU2020-2902 | Displays | EMRP1.2

Poroelastic effects of the damaged zone on seismic fracture reflectivity

Edith Sotelo Gamboa, Santiago G. Solazzi, German J. Rubino, Nicolas D. Barbosa, and Klaus Holliger

The presence of fractures has a predominant influence on the hydraulic and mechanical behavior of rocks. These effects are particularly pronounced and relevant for otherwise largely impermeable and stiff formations. There is widespread evidence pointing to the ubiquitous presence of damaged zones surrounding fractures and faults. The enhanced permeability associated with these zones can promote fluid pressure diffusion in the vicinity of fractures when seismic waves travel through the corresponding subsurface volume. This process, together with the inherent mechanical weakness of damaged zones, is expected to affect the seismic reflectivity of fractures and faults. We investigate these effects based on Biot’s theory of poroelasticity. To this end, we consider a 1D layered representation of the fracture and the associated damaged zone in conjunction with embedding elastic and impermeable half-spaces. We compare a fully elastic fracture-background reference model with a model consisting of a poroelastic fracture and damaged zone enclosed within an elastic background. For these two models, we compute the normal incidence seismic P-wave reflectivities at the background-fracture and at background-damaged zone interfaces, respectively. We also include a model that represents the fracture-damaged zone poroelastic system as an equivalent viscoelastic layer. We aim to test the validity of this representation since it would imply that a similar correspondence is possible to establish when more realistic descriptions of the damaged zone are considered. For this additional model, the viscoelastic layer is characterized by its frequency-dependent P-wave modulus, estimated by applying White’s classical upscaling procedure for 1D poroelastic media composed of alternating layers. We test the validity of the elastic-viscolastic model by comparing its reflectivity against the corresponding results from the elastic-poroelastic model. In doing so, we find that the simplified elastic-viscoelastic model faithfully reproduces the reflectivity of its elastic-poroelastic counterpart up to a threshold frequency, at which resonances produced within the viscoelastic layer become dominant. Overall, our results show that, in the seismic frequency range, there is a substantial increase in seismic fracture reflectivity resulting from the combined effects of fluid pressure diffusion and mechanical weakening associated with the surrounding damaged zone. This, in turn, indicates that the seismic reflectivity of a fracture may indeed be dominated by the thickness and physical properties of its surrounding damaged zone rather than by the properties of the fracture sensu stricto.

How to cite: Sotelo Gamboa, E., Solazzi, S. G., Rubino, G. J., Barbosa, N. D., and Holliger, K.: Poroelastic effects of the damaged zone on seismic fracture reflectivity, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2902, https://doi.org/10.5194/egusphere-egu2020-2902, 2020.

EGU2020-14762 | Displays | EMRP1.2

Effects of fracture connectivity on Rayleigh wave velocity dispersion

Gabriel Quiroga, J. Germán Rubino, Santiago Solazzi, Nicolás Barbosa, and Klaus Holliger

The use of passive seismic techniques to monitor geothermal reservoirs allows to assess the risks associated with their exploitation and stimulation. One key characteristic of geothermal reservoirs is the degree of fracture connectivity and its evolution. The reason for this is that changes in the interconnectivity of the prevailing fractures affect the permeability and, thus, the productivity of the system. An increasing number of studies indicates that the Rayleigh wave velocity can be sensitive to changes in the mechanical and hydraulic properties of geothermal reservoirs. In this work, we explore the effects of fracture connectivity on Rayleigh wave velocity dispersion accounting for wave-induced fluid pressure diffusion effects. To this end, we consider a 1D layered model consisting of a surficial sandstone formation overlying a fractured and water-saturated granitic layer, which, in turn, is underlain by a compact granitic half-space. For the stochastic fracture network prevailing in the upper granitic layer, we consider varying levels of fracture connectivity, ranging from entirely unconnected to fully interconnected. We use an upscaling approach based on Biot’s poroelasticity theory to determine the effective properties associated with these scenarios. This procedure allows to obtain the frequency-dependent seismic body wave velocities accounting for fluid pressure diffusion effects. Finally, using these parameters, we compute the corresponding Rayleigh wave velocity dispersion. Our results show that Rayleigh wave phase and group velocities exhibit a significant sensitivity to the degree of fracture connectivity, which is mainly due to a reduction of the stiffening effect of the fluid residing in connected fractures in response to wave-induced fluid pressure diffusion. This suggests that time-lapse observations of Rayleigh wave velocity changes, which so far are commonly associated with changes in the fracture density, could also be related to changes in the interconnectivity of pre-existing fractures.

How to cite: Quiroga, G., Rubino, J. G., Solazzi, S., Barbosa, N., and Holliger, K.: Effects of fracture connectivity on Rayleigh wave velocity dispersion, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-14762, https://doi.org/10.5194/egusphere-egu2020-14762, 2020.

The O-ZNS observatory offers a unique geophysical support for characterization, at different scales (from nano- to metric scales) of the heterogeneous Beauce Limestones aquifer. Currently under development at an agricultural site in Villamblain (Centre Val de Loire, France), this observatory is based on an exceptional well (20 m-depth and 4 m-diameter) associated with many external boreholes on an area of around 2 400 m2. It will combine different geophysical techniques and innovative multi-geosciences sensors to image, monitor and understand fluid and heat transfers in the heterogeneous structure of the vadoze zone.

An initial geophysical characterization has been conducted with surface measurements (3D electrical resistivity imaging and 2D Magnetic Resonance Sounding) that gave interesting information on the lithology of O-ZNS site: a silty-clayed soil of a few meter thick, then a highly heterogeneous and karstified limestone and finally, the massive fractured limestone. Cross-hole radar measurements add to these information a description of the initial zone, the soil properties and the water content. Also, data from three boreholes and the collection of core samples as well as logging measurements completed and improved this initial characterization.

All these data have been used to develop a finite element numerical model representing both the study site and the well under Plaxis 2D. Through the realism of geotechnical engineering including deformation, stability and water flow, the idea, is to anticipate the effect of the digging and provide information about the induced damaged zone that will derive. We also look into describing the evolution of this damaged zone depending on the seasoning variation (i.e. from 3 to 5 m) of the groundwater level. All these characterizations will allow us to better focus our field geophysical investigations on monitoring the damaged zone.

The model consists of a description of the different soil layers from the boreholes that includes elastic, microstructural and transport properties, followed by a description of the interface between the soil and the well. The hydraulic conditions will take into account the time-variability of fluxes and the aquifer level. Furthermore, this model is coupled with the construction phasing from a civil engineering point of view. The results will give the evolution of stress and strain induced by the engineering development of O-ZNS well in the host rock as well as an estimate of the material displacement and its elasticity limits. The preliminary modelling generated a result stipulating a damaged zone of 1-2 m around the well at the surface. The magnitude of the damaged zone is reduced with depth. It seems that, at the bottom, the host rock is undamaged.

Undergoing development are focused on refining the model by providing more effective and updated estimations of the soil and structure properties in order to validate or improve the first results together with an estimation of the time evolution of the damaged zone with the water saturation state. Afterward, we will be able to compare and validate these results to pictures and measurements performed during the digging that will start in the spring 2020.

How to cite: Mallet, C., Jodry, C., Laurent, G., and Azaroual, M.: Geophysical estimation of the damage induced by an observatory digging in a limestone heterogeneous vadose zone – Beauce aquifer (France), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2411, https://doi.org/10.5194/egusphere-egu2020-2411, 2020.

EGU2020-18106 | Displays | EMRP1.2

Upscaling of elastic properties in carbonates: A modeling approach based on a multiscale geophysical data set

Jerome Fortin, Cedric Bailly, Mathilde Adelinet, and Youri Hamon

Linking ultrasonic measurements made on samples, with sonic logs and seismic subsurface data, is a key challenge for the understanding of carbonate reservoirs. To deal with this problem, we investigate the elastic properties of dry lacustrine carbonates. At one study site, we perform a seismic refraction survey (100 Hz), as well as sonic (54 kHz) and ultrasonic (250 kHz) measurements directly on outcrop and ultrasonic measurements on samples (500 kHz). By comparing the median of each data set, we show that the P wave velocity decreases from laboratory to seismic scale. Nevertheless, the median of the sonic measurements acquired on outcrop surfaces seems to fit with the seismic data, meaning that sonic acquisition may be representative of seismic scale. To explain the variations due to upscaling, we relate the concept of representative elementary volume with the wavelength of each scale of study. Indeed, with upscaling, the wavelength varies from millimetric to pluri-metric. This change of scale allows us to conclude that the behavior of P wave velocity is due to different geological features (matrix porosity, cracks, and fractures) related to the different wavelengths used. Based on effective medium theory, we quantify the pore aspect ratio at sample scale and the crack/fracture density at outcrop and seismic scales using a multiscale representative elementary volume concept. Results show that the matrix porosity that controls the ultrasonic P wave velocities is progressively lost with upscaling, implying that crack and fracture porosity impacts sonic and seismic P wave velocities, a result of paramount importance for seismic interpretation based on deterministic approaches.

Bailly, C., Fortin, J., Adelinet, M., & Hamon, Y. (2019). Upscaling of elastic properties in carbonates: A modeling approach based on a multiscale geophysical data set. Journal of Geophysical Research: Solid Earth, 124. https://doi.org/10.1029/2019JB018391

How to cite: Fortin, J., Bailly, C., Adelinet, M., and Hamon, Y.: Upscaling of elastic properties in carbonates: A modeling approach based on a multiscale geophysical data set, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18106, https://doi.org/10.5194/egusphere-egu2020-18106, 2020.

Elastic waves are commonly studied in geophysics. They are used for example for prospecting, to follow the exploitation of hydrocarbon reservoirs, to study the effect of fluid injection (CO2 storage)… However, the wave frequencies used in the field (sonic – seismic measurements) are not the same as the ones commonly used in the laboratory (ultrasonic measurements), and fluid-saturated rocks are known to be dispersive, i.e the P- and S- wave velocity in fluid-saturated rock change with frequency. The comparison between field and laboratory measurements is therefore not straightforward.

In the ENS facilities, it is possible to subject samples, under pressure (1 to 30 MPa) to forced - oscillations varying from 0.01 Hz to 1 kHz (field frequencies) and 1 MHz (ultrasonic frequencies) using a triaxial cell. Axial and radial strain gauges are installed to record the resulting strains on the sample. Forced-oscillation can be done on 1) confining pressure to get the bulk modulus as function of frequency or on 2) axial stress to get the Young modulus and Poisson ratio as function of frequency.  With this information, it is thus possible to deduce the P- and S- wave velocities with frequency.

The elastic properties were measured on different samples from the Libra oil field, for which logging measurements are available. Thus, the measurements obtained in the laboratory can be compared to the measurements in the field at the same frequency. In addition, the evolution of the velocity with frequency measured in the laboratory allows us to discuss the mechanisms at the origin of the dispersion.  

How to cite: Gallagher, A.: Comparison of the elastic properties of reservoir rocks in the field and the laboratory: link between seismic, sonic and ultrasonic measurements, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1335, https://doi.org/10.5194/egusphere-egu2020-1335, 2020.

EGU2020-2697 | Displays | EMRP1.2

Microstructural control on compaction localisation in granular materials

Lucille Carbillet, Michael J. Heap, Fabian B. Wadsworth, Patrick Baud, and Thierry Reuschlé

Field observations and laboratory experiments have demonstrated strain localisation can develop in porous rocks in response to an applied stress field. Shear fractures and compaction bands are strain localisation features that can form at relatively low confinement during brittle deformation and at higher confinement during shear-enhanced compaction, respectively. Previous experimental studies suggested that the formation and geometry of compaction bands also depends on the microstructural attributes of the rock.

We investigated the influence of microstructure on compaction localisation in porous rocks using sintered glass bead samples, which allowed for a tight control on grain size and shape and sample porosity. During the fabrication process, populations of solid glass microspheres of predetermined size and size distribution are heated above their glass transition temperature. Above this temperature, the glass beads act as viscous liquid droplets. Time-dependent coalescence of droplets that share contact then causes the bead-pack to evolve into a connected system, producing a porous granular material of known microstructural geometries and final porosity.

We previously conducted hydrostatic compaction and triaxial compression tests on synthetic samples of porosity ranging from 10 to 38% with a monodisperse grainsize (diameter ranging from 0.15 to 1.3 mm). Experimental results showed remarkable reproducibility for the same experimental conditions and concurrence with the phenomenology of mechanical behaviour of natural sandstones. After these validation tests, we conducted systematic experiments on monodisperse synthetic samples of 25 and 35% of porosity prepared using glass beads of mean diameter 0.25, 0.525 and 1.15 mm. Triaxial deformation tests were conducted on water-saturated samples, in drained conditions (with a fixed pore pressure of 10 MPa), at room temperature, at a constant strain-rate and at effective pressures corresponding to the regime of formation of compaction bands. Our mechanical data provide indirect evidence for compaction localisation. We have focused our attention on the influence of porosity and grain size on the formation and microstructural attributes (such as thickness, length and tortuosity) of the compaction bands.

How to cite: Carbillet, L., Heap, M. J., Wadsworth, F. B., Baud, P., and Reuschlé, T.: Microstructural control on compaction localisation in granular materials, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2697, https://doi.org/10.5194/egusphere-egu2020-2697, 2020.

EGU2020-2992 | Displays | EMRP1.2

Combined numerical and experimental study of microstructure and permeability in porous granular media

Philipp Eichheimer, Marcel Thielmann, Wakana Fujita, Gregor J. Golabek, Michihiko Nakamura, Satoshi Okumura, Takayuki Nakatani, and Maximilian O. Kottwitz

Fluid flow on different scales is of interest for several Earth science disciplines like petrophysics, hydrogeology and volcanology. To parameterize fluid flow in large-scale numerical simulations (e.g. groundwater and volcanic systems), flow properties on the microscale need to be considered. For this purpose experimental and numerical investigations of flow through porous media over a wide range of porosities are necessary. In the present study we sinter glass bead media with various porosities, representing shallow depth crustal sediments. The microstructure, namely effective porosity and effective specific surface, is investigated using image processing. We furthermore determine flow properties like hydraulic tortuosity and permeability using both experimental measurements and numerical simulations. By fitting microstructural and flow properties to porosity, we obtain a modified Kozeny-Carman equation for isotropic low-porosity media, that can be used to simulate permeability in large-scale numerical models. To verify the modified Kozeny-Carman equation we compare it to the numerically computed and experimentally measured permeability values.

How to cite: Eichheimer, P., Thielmann, M., Fujita, W., Golabek, G. J., Nakamura, M., Okumura, S., Nakatani, T., and Kottwitz, M. O.: Combined numerical and experimental study of microstructure and permeability in porous granular media, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2992, https://doi.org/10.5194/egusphere-egu2020-2992, 2020.

Scanning electron microscopy (SEM) and helium ion microscopy (HIM) are two of the fundamental tools in the study of the microstructures of shale. A comprehensive comparison of these two techniques in the application of organic pore structure characterization is presented in this work. Owing to the small wavelength of the helium ion, the spot size of the ion beam is not restricted by diffraction aberration, and the convergence angle of helium ion beam can be much smaller than of the electron beam. The microscopic images and reconstruction models indicate that HIM has higher spatial resolution and increased depth of field than SEM. The pores below 10 nm and inner structures of pore networks can be observed via HIM images. The advantages shown in the focused ion beam/helium ion microscopy (FIB/HIM) results are similar to the 2-D HIM images. Smaller pores whose size is beyond the resolution of focused ion beam/scanning electron microscopy (FIB/SEM) can be found, which suggests the connection possibility of the big pores. However, to get reliable pictures, the ion-induced damage on organic matters should be avoided. To lower the beam current and to shorten the dwell time are two effective ways to reduce the beam damage.

How to cite: Zhao, J., Zhang, W., and Zhang, D.: Organic pore structure characterization of shale: a comparison between scanning electron microscopy and helium ion microscopy, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12191, https://doi.org/10.5194/egusphere-egu2020-12191, 2020.

EGU2020-6706 | Displays | EMRP1.2

Digital rock physics and laboratory considerations on a high-porosity volcanic rock

Laura L. Schepp, Benedikt Ahrens, Martin Balcewicz, Mandy Duda, Mathias Nehler, Maria Osorno, David Uribe, Holger Steeb, Benoit Nigon, Ferdinand Stöckhert, Donald A. Swanson, Mirko Siegert, Marcel Gurris, and Erik H. Saenger

Microtomographic imaging techniques and advanced numerical simulations are combined by digital rock physics (DRP) to obtain effective physical material properties. The numerical results are typically used to complement laboratory investigations with the aim to gain a deeper understanding of physical processes related to transport (e.g. permeability and thermal conductivity) and effective elastic properties (e.g. bulk and shear modulus). The present study focuses on DRP and laboratory techniques applied to a rock called reticulite, which is considered as an end-member material with respect to porosity, stiffness and brittleness of the skeleton. Classical laboratory investigations on effective properties, such as ultrasonic transmission measurements and uniaxial deformation experiments, are very difficult to perform on this class of high-porosity and brittle materials.

Reticulite is a pyroclastic rock formed during intense Hawaiian fountaining events. The open honeycombed network has a porosity of more than 80 % and consists of bubbles that are supported by glassy threads. The natural mineral has a strong analogy to fabricated open-cell foams. By comparing experimental with numerical results and theoretical estimates we demonstrate the potential of digital material methodology with respect to the investigation of porosity, effective elastic properties, thermal conductivity and permeability

We show that the digital rock physics workflow, previously applied to conventional rock types, yields reasonable results for a high-porosity rock and can be adopted for fabricated foam-like materials. Numerically determined effective properties of reticulite are in good agreement with the experimentally determined results. Depending on the fields of application, numerical methods as well as theoretical estimates can become reasonable alternatives to laboratory methods for high porous foam-like materials.

How to cite: Schepp, L. L., Ahrens, B., Balcewicz, M., Duda, M., Nehler, M., Osorno, M., Uribe, D., Steeb, H., Nigon, B., Stöckhert, F., Swanson, D. A., Siegert, M., Gurris, M., and Saenger, E. H.: Digital rock physics and laboratory considerations on a high-porosity volcanic rock, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6706, https://doi.org/10.5194/egusphere-egu2020-6706, 2020.

EGU2020-3888 | Displays | EMRP1.2

Comparison of pore fabric based on high-resolution X-ray computed tomography and magnetic pore fabric in sedimentary rocks

Yi Zhou, Michele Pugnetti, Anneleen Foubert, Christoph Neururer, and Andrea R. Biedermann

Pore fabrics characterize pore geometry and network in rocks. The pore size, connectivity and elongation direction determine the permeation ability and preferred permeation direction. X-ray micro-tomography (XRCT) is a widely used technique to visualize the inner structure of rock samples. Based on XRCT data, digital rock models can be generated and analyzed to visualize and quantify pore shape distribution, pore sizes and the connectivity of pores. To measure the magnetic pore fabric (MPF), samples are impregnated with ferrofluid prior to measuring anisotropy of magnetic susceptibility. This technique could be complementary to existing techniques to capture smaller pores. Empirical relationships exist between pore fabric or permeability anisotropy and MPF, and the aim of this study is to quantitatively test these relationships. In this study, Upper Marine Molasse sandstone (OMM, Belpberg, Switzerland) with 10-20% porosity and relatively homogeneous pore structure, and Plio-Pleistocene calcarenite (Apulia, Italy) with ~50% porosity and complex pore structure, are tested. To understand the pore networks of these rock types, an integrated approach has been applied including standard pycnometer porosity measurements, MPFs, XRCT, and porosity and permeability simulations based on XRCT analyses. The average equivalent diameter of pores based on micro-CT is ~150 μm for the Molasse sandstone, and ~300 μm for calcarenite. XRCT data indicate preferential alignment of the long axes of the pores, and both MPFs and simulated permeabilities are anisotropic in these samples. For calcarenite with large pores, the direction of the maximum magnetic susceptibility coincides with the direction of the maximum grouping of long pore axes. Simulated permeability is affected by other factors in addition to the grouping of long pore axes, including porosity, pore size, connectivity and tortuosity of pores. Therefore, the next step of this study will compare laboratory-measured directional permeabilities with permeability simulations and with MPFs, to investigate their potential for predicting the preferred fluid flow direction in these samples. For the full understanding of MPFs, more types of sedimentary rocks will be analyzed. If MPFs prove a good and quantitative proxy for pore fabric characterization in hydrocarbon and geothermal studies, more measurements can be made in the future, making it possible to investigate regional-scale variations.

How to cite: Zhou, Y., Pugnetti, M., Foubert, A., Neururer, C., and R. Biedermann, A.: Comparison of pore fabric based on high-resolution X-ray computed tomography and magnetic pore fabric in sedimentary rocks, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3888, https://doi.org/10.5194/egusphere-egu2020-3888, 2020.

EGU2020-17144 | Displays | EMRP1.2

Using Gamma-Ray and X-Ray Computed Tomography for Porosity Quantification of Reservoir Analogue Rocks

Abraão Nova, Frederico Ribeiro, Pamalla Oliveira, Daniel Amancio, Cássia Machado, Alexandra Carolina, Marcio Paixão, Antonio Antonino, Enivaldo Barbosa, Antônio Barbosa, Maria Lourenço, Marcos Rodrigues, and Richard Heck

During the last few decades, X-ray micro-computed tomography (µCT) has been largely used to characterize rock properties and to create high-resolution 3D digital image volumes. It has allowed access to important information about porous systems in reservoir rocks. However, the reliable quantification of porosity of rocks which present porous volumes ranging from centimeter to nanometer scale remains a challenge. Assessment of nano scale porous volume is very difficult by image segmentation techniques, due to the intrinsic limits of the x-ray imaging method. Moreover, image processing for analysis of various types of porosity in the same sample, including microporosity could be computationally expensive. We present a method based in the Gamma-Ray computed tomography (axis attenuation) that can substantially improve the limits presented by conventional X-ray microtomography. This study compared the porosity values acquired by typical segmentation methods for microtomography images, and by the values obtained trough the proposed method of gamma-ray computed tomography to calculate the porosity. Results of both approaches were compared to porosity measurements obtained through experimental equipment (helium porosimeter). These analyses were performed in core samples of limestones and sandstones analogous of Brazilian oil reservoirs. The Gamma Ray Attenuation method (axis attenuation) presented a better correlation (R² = 0.9588) to the experimental measurements when compared to the image segmentation methods (R² = 0.9194). The results suggest that Industrial application of gamma ray tomography for precise evaluation of large number of core samples can be highly effective. Furthermore, the gamma ray data can be integrated with data provided by conventional µCT image processing to complement information regarding morphological aspects.

Keywords: Porous System, X-ray microtomography, Gamma Ray tomography,  Reservoir rocks

How to cite: Nova, A., Ribeiro, F., Oliveira, P., Amancio, D., Machado, C., Carolina, A., Paixão, M., Antonino, A., Barbosa, E., Barbosa, A., Lourenço, M., Rodrigues, M., and Heck, R.: Using Gamma-Ray and X-Ray Computed Tomography for Porosity Quantification of Reservoir Analogue Rocks, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17144, https://doi.org/10.5194/egusphere-egu2020-17144, 2020.

EMRP1.4 – Multiscale rock damage in geology, geophysics and geo-engineering systems

EGU2020-3316 | Displays | EMRP1.4

Predicting the proximity to system-scale rupture using fracture networks

Jessica McBeck, John Aiken, Joachim Mathiesen, Yehuda Ben-Zion, and Francois Renard

A fundamental challenge in geophysics is predicting the timing of large earthquakes. A key step in addressing this problem is constraining the factors that indicate the timing of the next large rupture. To isolate the factors that help predict the proximity of the next earthquake, we develop machine learning models to predict the stress distance to macroscopic failure in triaxial compression X-ray tomography experiments on rocks at the stress conditions of the upper crust. In these experiments, we apply increasing axial stress in steps, and acquire a 3D X-ray tomogram at each stress step while the rock is under constant load, revealing the 3D density distribution. Segmenting the density fields provide the locations of rock (voxels dominated by solid), and pores and fractures (voxels dominated by air). We train the machine learning models using the geometry and clustering properties of the fracture networks identified in the tomography scans. We develop extreme gradient boosting (XGBoost) models to predict the stress distance to failure. In experiments on Carrara marble, monzonite, and granite, the models predict the stress distance to failure with r2 values > 0.7. We examine the feature importance to identify the factors that provide the best predictive power of the distance to failure. Measurements of the fracture network clustering and the shape anisotropy of fractures tend to have the highest importance of the features, providing greater predictive information than the fracture volume, fracture length, fracture aperture, and fracture orientation relative to the maximum compression direction.

How to cite: McBeck, J., Aiken, J., Mathiesen, J., Ben-Zion, Y., and Renard, F.: Predicting the proximity to system-scale rupture using fracture networks, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3316, https://doi.org/10.5194/egusphere-egu2020-3316, 2020.

EGU2020-4092 | Displays | EMRP1.4

Fault (un-)stability and strain partitioning across the brittle-ductile transition

Jérôme Aubry, François Passelègue, Javier Escartín, Damien Deldicque, Julien Gasc, Samson Marty, Marine Page, and Alexandre Schubnel

In the lithosphere, the transition from brittle to ductile deformation corresponds to a regime where brittle fracturing and plastic flow coexist, called the semi-brittle deformation zone. Within these different regimes, a large fault slip spectrum has been observed, from fast to slow earthquakes. Studying the parameters controlling fault (un-)stability and strain partitioning across this transition is fundamental to understand how natural faults behave at varying crustal depths.

To investigate semi-brittle deformation and the conditions promoting it, we report here the results of experiments performed on Carrara marble saw-cut faults in triaxial conditions. We studied the influence of the confining pressure, axial loading rates and initial fault roughness on fault (un-)stability. From mechanical data, we performed strain partitioning calculations to infer elastic, frictional and plastic strain contributions during the deformation process.

We conclude that (laboratory) earthquakes may nucleate within a regime where homogeneous plastic deformation of the bulk and dynamic fault slip may coexist. The contribution of plastic strain is promoted with increasing confining pressure and fault roughness.

How to cite: Aubry, J., Passelègue, F., Escartín, J., Deldicque, D., Gasc, J., Marty, S., Page, M., and Schubnel, A.: Fault (un-)stability and strain partitioning across the brittle-ductile transition, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4092, https://doi.org/10.5194/egusphere-egu2020-4092, 2020.

EGU2020-13547 | Displays | EMRP1.4

Damage indicators and failure prediction in Focal Mechanism solutions

Sergio Vinciguerra, Thomas King, Philip Benson, and Luca De Siena

Acoustic Emissions (AE), the laboratory analogue to seismic events, recorded during conventional triaxial deformation tests allow for an unprecedented amount of information on the evolution of fractured media within a controlled environment. This study presents the results of a new and robust derivation of first motion polarity focal mechanism solutions (FMS). 4 x 10 cm cylindrical samples of Alzo Granite (AG) and Darley Dale Sandstone (DDS) underwent systematic triaxial deformation testing (5, 10, 20 and 40 MPa) in order to investigate the relationships between increasing confining pressure, deformation and failure mode and role of pre-existing microstructure. With an average of 11 of 12 waveforms picked using a neural network for each AE, high resolution datasets are obtained that can track the evolution of deformation structure through time. Focal mechanisms are solved using a least squares minimisation of the fit between projected polarity measurements and the deviatoric stress field induced by tensile, shearing and collapse/closing type sources. Results reveal a surprisingly limited dependency on the distribution of shear fracturing in the lead up to dynamic failure. Instead, deformation is driven by the competition between the opening and closure of fractures that is strongly related to the coupling of local stress fields with pre-existing damage.Spatio-temporal trends in mechanism type and AE amplitude allow for clear identification of: a) Fracture Enucleation. This phase is characterised by broadly distributed tensile fracturing that becomes preferentially aligned as confining pressure increases; b) Fracture Growth. The onset is characterized by a discrete increase in low amplitude shearing events and cyclic fracture development that evolves from a dominance of collapse to shearing followed by tensile fracturing which then returns to collapse type. Influences in mechanism dominance due to rock type are highlighted by increased tensile fracturing in AG, which is replaced by shearing in DDS. A reduction in low amplitude tensile events at 10 MPa in both rock types further reveals a switch from axial splitting to planar localisation as confinement increases; c) Crack Coalescence. The cyclic fracture growth prior to dynamic failure and the amount of strain of this phase share a positive log-linear relationship with confining pressure, allowing to identify the potential for real-time failure prediction; d) Dynamic Failure: High amplitude events characterize the propagation of fractures. Taken together results highlight that failure of the studied samples is the result of the complex interaction between distinct regions of dilatant and compactant deformation. Although planar localisation and preferentially aligned flaws play a more significant role at higher confining pressures, it is the initial heterogeneity or patchiness of the regions undergoing damage that control dynamic failure occurrence and the eventual fracture plane features.

How to cite: Vinciguerra, S., King, T., Benson, P., and De Siena, L.: Damage indicators and failure prediction in Focal Mechanism solutions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13547, https://doi.org/10.5194/egusphere-egu2020-13547, 2020.

EGU2020-10236 | Displays | EMRP1.4

An investigation into the controls on fracture tortuosity in rock sequences and the impact on fluid flow in the upper crust

Nathaniel Forbes Inskip, Tomos Phillips, Kevin Bisdom, Georgy Borisochev, Andreas Busch, and Sabine den Hartog

Fractures are ubiquitous in geological sequences, and play an important role in the movement of fluids in the earth’s crust, particularly in fields such as hydrogeology, petroleum geology and volcanology. When predicting or analysing fluid flow, fractures are often simplified as a set of smooth parallel plates. In reality, they exhibit tortuosity on a number of scales: Fine-scale tortuosity, or roughness, is the product of the small-scale (µm – mm) irregularities in the fracture surface, whereas large-scale (> mm) tortuosity occurs as a result of anisotropy and heterogeneity within the host formation that leads to the formation of irregularities in the fracture surfaces. It is important to consider such tortuosity when analysing processes that rely on the movement (or hindrance) of fluids flowing through fractures in the subsurface. Such processes include fluid injection into granitic plutons for the extraction of heat in Engineered Geothermal Systems, or the injection of CO2 into reservoirs overlain by fine-grained mudrocks acting as seals in Carbon Capture and Storage projects.

Although it is generally assumed that tortuosity is controlled by factors such as grain size, mineralogy and fracture mode, a systematic study of how these factors quantitatively affect tortuosity is currently lacking. Furthermore, in anisotropic rocks the fracture orientation with respect to any inherent anisotropy is also likely to affect tortuosity.

In order to address this gap, we have induced fractures in a selection of different rock types (mudrocks, sandstones and carbonates) using the Brazil disk method, and imaged the fracture surfaces using both a digital optical microscope and X-ray Computed Tomography. Using these methods we are able to characterise both the fine-scale (roughness) and large-scale tortuosity. In order to understand the effect of fracture orientation on tortuosity we have also analysed fractures induced at different angles to bedding in samples of a highly anisotropic mudrock taken from South Wales, UK. Results indicate that fine-scale tortuosity is highly dependent on the fracture orientation with regards to the bedding plane, with fractures normal to bedding being rougher than those induced parallel to bedding. Finally, in order to measure the effect of tortuosity on fluid flow, we have carried out a series of core flooding experiments on a subset of fractured samples showing that fracture transmissivity decreases with increasing tortuosity.

How to cite: Forbes Inskip, N., Phillips, T., Bisdom, K., Borisochev, G., Busch, A., and den Hartog, S.: An investigation into the controls on fracture tortuosity in rock sequences and the impact on fluid flow in the upper crust, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10236, https://doi.org/10.5194/egusphere-egu2020-10236, 2020.

EGU2020-14434 | Displays | EMRP1.4

Fluid-driven tensile fracture and fracture toughness in Nash Point Shale at elevated pressure.

Philip Benson, Stephan Gehne, Nathaniel Forbes Inskip, Philip Meredith, and Nick Koor

Fluid-driven fracturing is a key process in enhancing production in both the hydrocarbon and geothermal energy extraction industries. However, whilst a large number of studies have now developed laboratory methods to simulate the process in a range of settings, and across a number of different rock types, data relating the fundamental material parameters (such as fracture toughness) to the overall rock mechanics response as a function of parameters such as confining and pore pressure remain limited. Here we report a new analysis to recover fracture toughness across a range of effective pressures from hydraulic fracturing experiments that use a modified thick-walled cylinder sample mounted in a conventional triaxial deformation apparatus. We use samples that are 90mm in length and 40mm diameter, with a central, axially drilled borehole 12.6 mm in diameter. An array of 16 ports in the engineered, nitrile, sample jacket allows us to record radial strain (4 channels), acoustic emission output (11 channels) and borehole fluid pressure (1 channel) continuously throughout each test. The sample material was Nash Point shale (NPS) from the south coast of Wales, UK, with samples cored both normal and parallel to bedding in order to investigate the effect of anisotropy. Earlier, ambient pressure fracture toughness tests using the Semi-Circular Bend sample geometry had indicated significant anisotropy, values of 0.24 – 0.30 MPa.m1/2 in the Short-Transverse (ST) orientation, and 0.71 - 0.73 MPa.m1/2 in the Divider (DIV) orientation.

Here, we present results from a suite of 9 experiments, 6 with samples cored parallel to bedding (ST fracture orientation) and 3 with samples cored normal to bedding (DIV fracture orientation). We find that the fluid injection pressure required to fracture our annular shell samples is significantly higher for DIV samples than for ST samples, and increases significantly with increasing confining pressure in both orientations; ranging from 10 to 36 MPa for ST samples and 30 to 58 MPa for DIV samples as confining pressure is increased from 2.2 to 20.5 MPa. We note that the fluid injection pressure undergoes a number of oscillations between fracture nucleation and the fracture reaching the sample boundary. Such oscillations are more common in ST samples than in DIV samples, and in experiments at lower confining pressures. We use the magnitude of each pressure oscillation to estimate the associated increment of fracture extension via the proportion of AE energy generated relative to the total energy accumulated when the fracture reaches the sample boundary. This analysis produces fracture toughness values ranging from 0.36 to 2.76 MPa.m1/2 (ST orientation) and 2.98 to 4.05 MPa.m1/2 (DIV orientation) as confining pressure was increased from 2.2 to 20.5 MPa. We further find that the increase in fracture toughness increases essentially linearly with increasing effective pressure, and this trend appears to be independent of orientation and the material anisotropy.

How to cite: Benson, P., Gehne, S., Forbes Inskip, N., Meredith, P., and Koor, N.: Fluid-driven tensile fracture and fracture toughness in Nash Point Shale at elevated pressure., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-14434, https://doi.org/10.5194/egusphere-egu2020-14434, 2020.

A benchmark of low permeability measurements has been organized by the Geosciences and Environment Laboratory at University Cergy-Pontoise over the period 2015-2018. The objective of this benchmark was to measure or estimate through modelling the permeability of a single material, selected for its low permeability. A wide range of different approaches were covered, classified into (i) direct measurement methods, including steady-state, transient pulse and oscillatory techniques and (ii) models using microstructural data obtained from imaging or porosimetry techniques. At the beginning, 30 laboratories in 8 different countries volunteered to participate, and at the end results from 24 labs were collected which is remarkable.

The selected rock was the Grimsel granodiorite (Switzerland), so the benchmark was called “KG²B”, which means “K for Grimsel Granodiorite Benchmark”. Two fresh cores with diameter 85 mm and about one meter long each were provided by colleagues from ETH Zürich. The cores were drilled in the Swiss Grimsel test site, an underground research laboratory in hard rock, at a distance between 4 and 6 meters from the tunnel, away from the EDZ. The cores were cut into small pieces (between 2 and 10 cm long) and sent to the participants. The porosity of the Grimsel Granodiorite is less than 1%, and the permeability is in the 10-18 m² range.

The expected outcomes of the benchmark were: (i) to compare the results for each method separately and (ii) between the different methods/models, (iii) to assess the precision of each method, (iv) to study the influence of experimental conditions, especially sample size and the nature of pore fluid, (v) to gather information on the know-how in each laboratory, and finally (vi) to suggest good practice for low permeability measurements.

The benchmark was designed as a blind test, i.e. the results from each lab were not known by the other labs except for the organizers. A dedicated website [1] was constantly updated to allow the participants to follow the progression of the benchmark. It took about three years to manage the benchmark, collect all the data, complete the dataset analysis and publish the results [2,3]. The results collected allowed us to discuss the influence of pore-fluid, measurement method, sample size and pressure sensitivity, as well as the relevance of various models for permeability estimation. The most striking and unexpected result was that regardless of the method used, the mean gas permeability was higher than the mean liquid permeability by a factor approximately 2.

As an introduction to the session, our aim is to show how conducting such a benchmarking exercise can help to answer the questions raised by the session: - How repeatable are permeability measurements, and how dependent are they on the apparatuses and methods? - Which experimental pitfalls exist, what are the underlying assumptions and how might they impact permeability? - Can we define standard experimental procedures to improve permeability measurements in low permeability materials?

 

  • [1] https://labo.u-cergy.fr/~kg2b
  • [2] Geophys. J. Int., 215, 799-824, doi: 10.1093/gji/ggy304, 2018.
  • [3] Geophys. J. Int., 215, 825-843, doi: 10.1093/gji/ggy305, 2018.

How to cite: David, C.: KG²B, a world-wide inter-laboratory benchmark of low permeability measurement and modelling, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6010, https://doi.org/10.5194/egusphere-egu2020-6010, 2020.

EGU2020-20641 | Displays | EMRP1.4

Representativeness of volume investigated by high-resolution X-ray computed tomography in damaged fine-grained rocks

Hugo Saur, Charles Aubourg, Peter Moonen, Pascale Sénéchal, Tiphaine Boiron, and Hannelore Derluyn

We focus on calcareous homogenous shales featuring different degrees of damage along a km-long strain gradient, marked by cleavage development. In a previous study, we used high-resolution X-ray computed tomography (µCT) to document the evolution of the 3D fabric of the fine-grained shales along the strain gradient (Saur et al., JSG, 2020). Our conclusions were based on samples of ~ 2.5 mm3 containing over 10’000 quartz and calcite grains. The objective of the current study is to assess the representativeness of analyses on such small rock samples. To that extent, we first repeat the µCT analysis on multiple samples of the same, limited, volume and assess the variability of the results. These results are then compared to both macroscopic field observations and bulk fabric measurements obtained with AMS (Anisotropy of Magnetic Susceptibility) on larger samples (~ 10 cm3). AMS provides a statistical description of the magnetic susceptibility tensor, and particularly the confidence angle of axis orientation. Generally, this confidence angle is the result of matrix organization and rock magnetism. In this study, AMS is only controlled by the presence of illite particles which reflect the matrix organization. Finally, we perform a subvolume analysis on the µCT images to determine the smallest representative volume characterizing the fine-grained fabric. In light of these analyses we discuss the representativeness of investigated volume of fine grained shales, subjected to different degrees of deformation.

How to cite: Saur, H., Aubourg, C., Moonen, P., Sénéchal, P., Boiron, T., and Derluyn, H.: Representativeness of volume investigated by high-resolution X-ray computed tomography in damaged fine-grained rocks, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20641, https://doi.org/10.5194/egusphere-egu2020-20641, 2020.

EGU2020-2783 | Displays | EMRP1.4

Thermal influences on macroscale rock damage

Brian D. Collins, Greg M. Stock, Martha-Cary Eppes, Antoine Guerin, Michel Jaboyedoff, and Federica Sandrone

Fracture processes in rock have widespread implications in the geohazard, geomorphologic, and civil and mining engineering communities.  Propagation of fractures reduces overall rock mass strength, can lead to large-scale gravitational instabilities, and can cause significant hazard and damage to infrastructure.  The potential for critical fracture in the form of rock falls and rock bursts are often the primary driver for scientific investigations, civil work project planning, and mining investment outlays.  However, slower subcritical fracture from long-term monotonic and/or cyclic stress perturbations often control the eventual more rapid (and more catastrophic) response of rock.  These slower damage mechanisms may result from existing or perturbed tectonic stresses, stress relief from exhumation or excavation, or long-term environmental stressors such as thermal cycling and frost cracking.

Here we investigate the role of thermal cycling in generating subcritical stresses to which virtually all rock cliffs worldwide are exposed.  Our hypothesis – that diurnal and seasonal cycles of temperature can lead to substantial subcritical fracture propagation and eventual critical fracture – has led us to design several field and laboratory experiments to measure both the deformations and the stresses associated with environmental thermal forcing in rock.  Our studies focus on granitic exfoliation environments, common in many mountainous regions of the world, where relatively thin (centimeters to decimeters) exfoliation sheets are able to undergo a full thickness thermal response, and where exfoliation-related rock falls are common and in some places, well-documented.

In cliff environments located in Yosemite National Park (California, USA), our field studies using in-situ measurements (i.e., crackmeters and temperature sensors) have shown that diurnal and seasonal thermal cycles lead to cyclic stresses in the subcritical range, with resultant cumulative and seemingly permanent rock deformation outwards from the main cliff surface.  Additional field studies using thermal IRT (InfraRed Thermography) imaging identify the locations of rock bridges that likely serve as focal points for these thermally-induced stress concentrations.  Although we did not measure the critical fracture conditions that would result in a rock fall, we did, fortuitously, capture the deformation signals leading up to explosive fracture of a nearby granitic 100-m-diameter exfoliation dome during peak temperatures at the site (located ~60 km northwest from Yosemite), thereby proving the efficacy of thermal stresses in driving both long term – and catastrophic – rock damage.  These field studies are substantiated by analytical fracture mechanics solutions which show how rock may eventually fail under these conditions.  These studies therefore serve as proxies for understanding how some rock falls eventually occur under subcritical thermally-induced cyclic stress conditions, but also more generally for how thermal-stress conditions may affect rock damage in a multitude of environments.

How to cite: Collins, B. D., Stock, G. M., Eppes, M.-C., Guerin, A., Jaboyedoff, M., and Sandrone, F.: Thermal influences on macroscale rock damage, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2783, https://doi.org/10.5194/egusphere-egu2020-2783, 2020.

Geological and geophysical observations of fault zones reveal that fault cores are surrounded by regions of damaged rocks consist of fractures at a wide range of length scales with decaying intensity with distance from the fault core. The main mechanisms proposed for the development of off-fault damage include slip on faults with geometrical irregularities, migrating process zones, and dynamic damage from the passage of earthquake ruptures. Field observations of relatively deep exhumed fault zones have shown that fault damage zone width scales with the displacement on a fault. In this study, we combine such observations with numerical modeling to test what is the dominant mechanism producing off-fault damage at depth of several kilometres.

The field data [Faulkner et al., 2011] include measurements of micro-fracture damage zone width from small displacement fault zones within the Atacama fault zone in northern Chile that formed at ∼6 km depth within a dioritic protolith. An increase in damage zone width with displacement is clearly seen. We perform simulations of slip on synthetic faults, with roughness properties similar to that of natural faults, and examine how the total slip and roughness characteristics affect the extent and intensity of inelastic deformation to constrain the geometrical and frictional properties that could generate the observed damage. To accurately account for the effects of geometrical irregularities on the fault and allow slip that is large relative to the size the minimum roughness wavelength, we use the mortar finite element method, in which non-matching meshes are allowed across the fault and the contacts are continuously updated. Inelastic deformation of the bulk is modelled with Drucker–Prager viscoplasticity, which is a simple choice for describing cracked medium and is closely related to the Mohr–Coulomb model. Our results indicate that, for the depth and fault lengths in the field data, geometrical irregularities produce the scaling of damage zone width with displacement observed in the field and suggest that this, rather than the other mechanisms, produce most of the damage.

How to cite: Tal, Y. and Faulkner, D.: The effect of geometrical irregularities on damage zone width: Modeling and field observations , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5884, https://doi.org/10.5194/egusphere-egu2020-5884, 2020.

EGU2020-9982 | Displays | EMRP1.4

Differences between static and dynamic elastic moduli: Importance of experimental methods

Elisabeth Bemer, Noalwenn Dubos-Sallée, and Patrick N. J. Rasolofosaon

The differences between static and dynamic elastic moduli remain a controversial issue in rock physics. Various empirical correlations can be found in the literature. However, the experimental methods used to derive the static and dynamic elastic moduli differ and may entail substantial part of the discrepancies observed at the laboratory scale. The representativeness and bias of these methods should be fully assessed before applying big data analytics to the numerous datasets available in the literature.

We will illustrate, discuss and analyze the differences inherent to static and dynamic measurements through a series of triaxial and petroacoustic tests performed on an outcrop carbonate. The studied rock formation is Euville limestone, which is a crinoidal grainstone composed of roughly 99% calcite and coming from Meuse department located in Paris Basin. Sister plugs have been cored from the same quarry block and observed under CT-scanner to check their homogeneity levels.

The triaxial device is equipped with an internal stress sensor and provides axial strain measurements both from strain gauges glued to the samples and LVDTs placed inside the confinement chamber. Two measures of the static Young's modulus can thus be derived: the first one from the local strain measurements provided by the strain gauges and the second one from the semi-local strain measurements provided by the LVDTs. The P- and S-wave velocities are measured both through first break picking and the phase spectral ratio method, providing also two different measures of the dynamic Young's modulus.

The triaxial tests have been performed in drained conditions and the measured static elastic moduli correspond to drained elastic moduli. The petroacoustic tests have been performed using the fluid substitution method, which consists in measuring the acoustic velocities for various saturating fluids of different bulk modulus. No weakening or dispersion effects have been observed. Gassmann's equation can then be used to derive the dynamic drained elastic moduli and the solid matrix bulk modulus, which is otherwise either taken from the literature for pure calcite or dolomite samples, or computed using Voigt-Reuss-Hill or Hashin-Shtrikman averaging of the mineral constituents.

For the studied carbonate formation, we obtain similar values for static and dynamic elastic moduli when derived from careful lab experiments. Based on the obtained results, we will finally make recommendations, emphasizing the necessity of using relevant experimental techniques for a consistent characterization of the relation between static and dynamic elastic moduli.

How to cite: Bemer, E., Dubos-Sallée, N., and Rasolofosaon, P. N. J.: Differences between static and dynamic elastic moduli: Importance of experimental methods, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9982, https://doi.org/10.5194/egusphere-egu2020-9982, 2020.

The mechanical response of natural gypsum rock is relevant in a wide range of engineering applications (e.g. tunnel excavation, stability assessment of underground quarries, oil and gas accumulation). In particular, in underground quarry environments, static loading conditions insisting on the gypsum pillars during and after the exploitation activities (i.e. several decades) require a specific attention to the sub-critical time-dependent deformation of the rock. The short-term stability (referred to the possibility of a failure in consequence to the sudden application of the axial load) does not preclude the possibility of deformation or even failure in the long-term.

In addition, the underground drifts of gypsum quarries are often located below the static level of the groundwater table, requiring a continuous water pumping to allow for the accessibility of the drifts themselves. The end of the quarry activity, coinciding with the interruption of the de-watering operations and the re-assessment of the original level of water table, brings to the new water saturation of the gypsum body. The water fills the connected porosity of the rock, influencing the general stability of the underground voids.

For these reasons, the present work aims to investigate the mechanical response of gypsum rock in time-dependent regime, also considering the influence of water saturation. The study proposes an experimental investigation of the influence of water on the rheology of a natural gypsum facies (i.e. branching selenite gypsum), distinguishing between the mechanical effects of a saturating fluid (in relation to the internal pore pressure), that should also be observed with a non-reactive fluid such as oil, and the water-gypsum chemical interactions. This influence of water is investigated in uniaxial compression, under uniaxial creep conditions and conventional triaxial compression. The new mechanical data are accompanied by microstructural observations of the effects induced in the rock by the mechanical compression, aiming to propose a description of the mechanisms involved in the gypsum deformation process.

How to cite: Caselle, C., Bonetto, S., and Baud, P.: Time-dependent behaviour and water influence on the mechanical response of gypsum rock in underground quarry frameworks, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4138, https://doi.org/10.5194/egusphere-egu2020-4138, 2020.

EGU2020-16050 | Displays | EMRP1.4

Multiphysics of transient deformation processes leading to macroscopic instabilities in geomaterials

Antoine Jacquey, Klaus Regenauer-Lieb, Francesco Parisio, and Mauro Cacace

Material instabilities are critical phenomena which can occur in geomaterials at high stress and temperature conditions. They generally result in the degradation of the microstructure organisation, ultimately leading to material failure. These phenomena are relevant to a large variety of geoscientific and geotechnical applications including earthquake physics, fault mechanics, successful targeting of unconventional georesources and mitigation of induced seismicity. Quantifying and predicting the onset of material degradation upon instability remains a major challenge due to our lack of understanding of the physics controlling the behaviour of porous rocks subject to high temperature and pressure conditions.

In the laboratory, rocks gradually transition from a time-independent brittle behaviour to a transient semi-brittle, semi-ductile behaviour upon an increase in pressure and/or temperature. Furthermore, even when subject to constant subcritical stress conditions rocks have been observed to macroscopically fail due to growth of subcritical processes such as stress corrosion. Brittle creep is a phenomenon observed on a variety of rock types (volcanic and sedimentary) and shows a high sensitivity to temperature and stress conditions. In the field, such subcritical transient processes are difficult to detect and can jeopardise the safety of geothermal projects. Transient failure mechanisms in the reservoir have set back geotechnical projects through induced seismicity occurring days or even weeks after stimulation shut in as observed at the Basel geothermal site in Switzerland or at the Pohang geothermal project in South Korea. These observations demonstrate how conventional techniques fail at describing the physics responsible for fault reactivation, which is controlled by dynamic processes resulting from transient multiphysics coupling.

In this contribution, we detail the theory and procedure to develop a constitutive model for rate-dependent damage poro-elasto-plastic material behaviour suitable for porous rocks. To allow for a generic framework for assessing geomaterials instabilities, this model incorporates the potential for microstructure degradation and a path- and rate-dependence. To that purpose, we rely on thermodynamic principles to derive in the frame of the hyperplasticity theory a coupled hydro-mechanical rate-dependent plasticity and damage rheology. We present numerical examples of this new constitutive model at the laboratory scale using experimental data on brittle creep in sandstones and discuss the implications upon upscaling at the reservoir and lithosphere scale.

How to cite: Jacquey, A., Regenauer-Lieb, K., Parisio, F., and Cacace, M.: Multiphysics of transient deformation processes leading to macroscopic instabilities in geomaterials, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16050, https://doi.org/10.5194/egusphere-egu2020-16050, 2020.

EGU2020-19084 | Displays | EMRP1.4

To creep or to snap? How induced heat governs the brittleness of matter

Tom Vincent-Dospital, Renaud Toussaint, Stéphane Santucci, Loïc Vanel, Daniel Bonamy, Lamine Hattali, Alain Cochard, Eirik Grude Flekkøy, and Knut Jørgen Måløy

The growth of fractures within mechanically loaded materials often shows two different behaviors. When loaded below a particular threshold in energy release rate, cracks tend indeed to creep at very slow velocities, while the rupture becomes catastrophic beyond this threshold, with propagation velocities approaching that of the material mechanical waves. Understanding according to which of these two behaviors a material is prone to break is of paramount importance, notably in engineering, where the brittle rupture of structures can lead to unpredicted disasters. It is also fundamental in Earth science, as damaging earthquakes  are rather generated by abrupt ruptures in the crustal rocks than by their slow deformations. To explain both behaviors, we focus here on the thermal effects which are auto-induced by the growth of cracks. During their propagation, some of the system’s energy is indeed partly dissipated by Joule heating, which is arising from the friction in a damaged zone around the fracture fronts. The heat hence generated can in return have a significant impact on the physics of the propagation. For instance, the stability of faults is believed to be affected by the thermo-pressurization of their in situ fluids. Independently of this effect, we show, how statistical physics, as understood by an Arrhenius law that includes the dissipation and diffusion of heat around the fracture tip, can explain the full dynamics of cracks, from the slow creep to the fast rupture.

We indeed show that such a model can successfully describe most of the experimentally reported fracture rheology, quantified in terms of velocity / energy release rate relations, in two different types of polymers, acrylic glasses and pressure sensitive adhesives, over eight decades of crack velocities. In these two cases, it is sufficient to assume that these polymers are homogeneous to model their failure. Yet, we in addition illustrate how the thermal disorder, from both the ambient temperature and the propagation induced heat, should interact with the matter typical quenched disorder in fracture energy. Numerical simulations of planar cracks in heterogeneous media indeed show that such quenched disorder helps to trigger hot avalanches in the propagation of cracks, making the overall toughness of a material highly dependent on both its heterogeneities, as it is often reported in the literature, and its thermal properties.

How to cite: Vincent-Dospital, T., Toussaint, R., Santucci, S., Vanel, L., Bonamy, D., Hattali, L., Cochard, A., Flekkøy, E. G., and Måløy, K. J.: To creep or to snap? How induced heat governs the brittleness of matter, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19084, https://doi.org/10.5194/egusphere-egu2020-19084, 2020.

EGU2020-19783 | Displays | EMRP1.4

Assessment of the mechanism of fracture propagation of soft rock coastal cliffs by using non-local constitutive models

Piernicola Lollino, Nunzio Luciano Fazio, Michele Perrotti, Alessio Genco, Gaetano Elia, and Matteo Oryem Ciantia

The assessment of susceptibility to failure of soft rock coastal cliffs, along with the associated failure mechanism, is not a simple task. Equilibrium conditions depend on the combination of several factors such as structural setting, rock mechanical strength, weathering processes, the hydro-mechanical action of sea waves, the variation of the rock cliff geometry, to mention some of the most important ones. From a geomechanical perspective, the brittle - strain softening behaviour of the rocks plays a key role in the onset and propagation of failure (Ciantia & Castellanza 2015). In particular, the rapid strength reduction occurring after peak under mechanical loading leading to localised deformations within shear fractures is detrimental for rock cliffs. Taking rock brittleness into account in numerical simulations under the framework of continuum mechanics is not straightforward, due to the problems related to a strong dependence of the numerical results from the adopted mesh when strain-softening laws are implemented (Vermeer and Brinkgreve 1994). Nowadays, several regularization techniques are available to control the size of the localised region and prevent the mesh dependence. Within regularization techniques, the nonlocal integral type solution has the advantage of not changing the field equations which facilitates numerical implementation. In this approach, the chosen nonlocal variables are valuated from spatial averages of the field variables in a neighbourhood, and the constitutive model is updated by replacing a local variable with its nonlocal counterpart. Consequently, the constitutive response of a Gauss point is influenced by all the integration points within a neighbourhood, with the size determined through a characteristic length (Bažant and Jirásek 2002). This contribution addresses the problem of the stability of an ideal 2-D plane strain coastal cliff, 20-m high, by means of the use of a non-local constitutive model implemented in a commercial finite element code (Mánica et al. 2018). The numerical results show insights into the evolution of the strain field and the process of slip surface/fracture propagation in the rock cliff as well as highlight the importance of regularising the finite element solution in the presence of brittle materials.

How to cite: Lollino, P., Fazio, N. L., Perrotti, M., Genco, A., Elia, G., and Ciantia, M. O.: Assessment of the mechanism of fracture propagation of soft rock coastal cliffs by using non-local constitutive models, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19783, https://doi.org/10.5194/egusphere-egu2020-19783, 2020.

EGU2020-5397 | Displays | EMRP1.4

The onset of dilatancy in rocks

Sandra Schumacher and Werner Gräsle

The onset of dilatancy determines the start of critical fracture growth in rocks under increasing load. For various applications such as the construction of nuclear repositories or dams, a quantitative comprehensive knowledge on the critical conditions leading to dilatancy is required.

Thus, it is important to determine the parameters, which control the dilatant behaviour of rocks, and to analyse their interactions.

We conducted a series of undrained triaxial experiments on two consolidated, fully saturated Opalinus Clay samples from the Mont Terri underground research lab and one sample of Bunter Sandstone from southern Lower Saxony. By testing only a few samples but them extensively, we avoid that the natural material heterogeneity among multiple samples affected our results. Here we show that our approach allows identifying new correlations between different parameters with surprising clarity.

During the experiments, which can take years, the samples are repeatedly exposed to increases in differential stress (σ13) into the dilatant regime but always well below the point of failure. This we achieve by monitoring the pore pressure during the increase in differential stress. The onset of dilatancy becomes visible as clear drop in pore pressure with increasing differential stress.

In addition to the detection of the onset of dilatancy via the pore pressure evolution, pressure diffusion experiments are performed to determine the onset of dilatancy. For this, in the dilatant regime, the differential stress is kept constant and the pore pressure on one side of the sample is de- and increased repeatedly, while the reaction of the pore pressure on the other side of the sample is monitored. With the pore pressure pulse diffusing though our sample specimen, this controlled pore pressure variation induces a transition between dilatant and subdilatant regimes at constant differential stress.

The values for the onset of dilatancy derived by these two methods permit a comprehensive analysis of the dilatant behaviour not only of the Opalinus Clay samples, but also of the Bunter Sandstone sample. Our results show that dilatant behaviour of the tested materials is not governed by only one parameter but by an intricate interplay of several parameters.  Consequently, the development of an equation of state for the dilatant behaviour of different types of rock is achievable. However, due to the multiple parameter dependencies, it will be a time-consuming undertaking.

How to cite: Schumacher, S. and Gräsle, W.: The onset of dilatancy in rocks, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5397, https://doi.org/10.5194/egusphere-egu2020-5397, 2020.

EGU2020-10379 | Displays | EMRP1.4

Influence of the initial damage on fracture toughness and subcritical crack growth in a granite rock

Salvatore D'Urso, Lucas Pimienta, François Passelègue, Federica Sandrone, Sergio Vinciguerra, and Marie Violay

Fracture mechanics is an important tool to assess the slope stability, since this approach offers a methodology to study the fracture stress field in the neighborhood of the joint tips and accurately predict propagation of the joints over time. While the fracture toughness of material has been extensively studied in the past, low interest was given to the influence of initial damage on the subcritical crack growth, despite of its relevance to assess long term slope stability. Here we report new experimental results that address this question.

Starting from the real case of unstable rock mass of “Madonna del Sasso” (Colombero et al., 2015), a series of Cracked Chevron Notched Brazilian Disc (CCNBD) (Fowell, 1995) specimens were failed in a standard Mode I tensile test to investigate the effects of thermal damage on fracture toughness and subcritical crack growth on samples of granite of Alzo.

The degree of initial damage was imposed using slow heat treatment (1°C/min) up to 100, 200, 300 and 400°C, to emulate different levels of rock degradation. The samples were equipped with strain gauges close to the tips of the notch, an extensometer for the Crack Mouth Opening Displacement (CMOD) and twelve Acoustic Emission recorders.

Our results show that fracture toughness decreases with increasing thermal damage, in agreement with previous studies (Nasseri, Schubnel, & Young, 2007). The fracture toughness of undamaged granite is 1.04 MPa m1/2, but 0.65 MPa m1/2 after treatment up to 400°C.

Subcritical crack growth behaviour has been studied for samples treated from 100°C up to 400°C through creep tests on CCNBD specimens. The overall effect of heat treatment is to increase the crack growth rate in granite for a given stress intensity factor. The slopes of stress intensity factor versus crack velocity curves are sensitive to modifications of initial damage’s degree. Trend drops substantially with increase of the temperature of the heat treatment. This shows a substantial increase of the internal damage index n, and a decrease of the time to failure of the CCNBD specimens.

The study highlights the importance of considering both the time-dependent slope behaviour and effects of rocks internal damage, since these conditions could lead to an unexpected premature failure.

How to cite: D'Urso, S., Pimienta, L., Passelègue, F., Sandrone, F., Vinciguerra, S., and Violay, M.: Influence of the initial damage on fracture toughness and subcritical crack growth in a granite rock, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10379, https://doi.org/10.5194/egusphere-egu2020-10379, 2020.

EGU2020-10115 | Displays | EMRP1.4

Effect of a heterogeneity on tensile failure: interaction between fractures in a limestone

Anne Pluymakers, Richard Bakker, and Auke Barnhoorn

Not all rocks are perfect. Frequently heterogeneities will be present, either in the form of pre-existing fractures, or in the form of sealed fractures. Tensile strength and strength anisotropy of rocks has been investigated for strongly layered rocks, such as shales, sandstones and gneisses, but data is lacking on the effect of single planar heterogeneities, such as pre-existing fractures or stylolites. We have performed Brazilian Disc tests on limestone samples containing pre-existing fractures and stylolites, investigating Brazilian test Strength (BtS) and fracture orientation. We used Indiana limestone samples, pre-fractured with the Brazilian Disc method, and Treuchtlinger Marmor samples which contained central stylolites. All experiments were filmed. The planar discontinuity was set at different rotation angles of approximately 0–20–30–45–60–90⁰, where 90⁰ is parallel to the principal loading direction, and 0⁰ to the horizontal axis of the sample. Pre-fracturing Indiana limestone samples results in a cohesion-less planar discontinuity, whereas the stylolites in the Treuchtlinger Marmor samples are discontinuities which have some strength.

The results show that our imperfect samples with a planar discontinuity are always weaker than an intact sample. For the Indiana limestone, with a cohesion-less interface, there is 10 to 75% of weakening, which is angle-dependent. Once the angle is 30 or lower there is no influence from the initial fracture for the orientation of the new fracture. The stress-displacement pattern followed the expectation for Brazilian Disc testing. However, in the samples with a stylolite, strength is isotropic and between 25 and 65% of the strength of an intact sample. For all cases several new cracks appeared, of which the orientation is influenced by the orientation of the stylolite. The fracture pattern and associated stress drops are more complex for high angles. Interestingly, in the samples with stylolites, always more than one fracture was formed, whereas in the samples with a cohesionless interface usually only one new fracture formed, which for natural settings suggests a potential for higher fracture density when hydrofracturing a stylolite-rich interval.

A second difference between these datasets is the amplitude of the pre-existing interface. The effect of amplitude will be qualitatively investigated with a 2D Comsol model, to investigate the location of the first fracture occurring, which can then be compared to the camera data of the experiments.

How to cite: Pluymakers, A., Bakker, R., and Barnhoorn, A.: Effect of a heterogeneity on tensile failure: interaction between fractures in a limestone, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10115, https://doi.org/10.5194/egusphere-egu2020-10115, 2020.

EGU2020-21891 | Displays | EMRP1.4

Monitoring of damage processes in cemented granular materials with acoustic emissions and seismic velocity reduction

Vincent Canel, Xiaoping Jia, Michel Campillo, and Ioan R. Ionescu

Earthquakes or fault core sliding occur naturally in response to long-term deformation produced by plate tectonics. However, the way the damage or fracture process of rocks control the frictional slip is not well understood. It involves indeed materials in very different states: from granular-like materials near the shear band within the highly cracked fault core [1] to almost cohesive state in distant host rocks. To address this issue, we perform controlled laboratory experiments and new numerical simulations of damage in cemented granular materials to study the material evolution from cohesive to granular-like states under external loading. Our synthetic rocks (porous media) are made of cemented glass beads in which the packing density and the cement property (ductile or brittle) as well its content are tunable [2,3]. Two mechanical tests have been conducted: i) under oedometric load in a cylindrical cell with rigid wall; and ii) under triaxial load in a cell with elastic membrane (confined by atmospheric pressure). The fracture processes are monitored by acoustic waves, measuring the longitudinal ultrasound velocity (active detection) [4] and the acoustic emission (passive detection) [5].

More precisely, in the case (i) the fracture process is likely associated with the crack increase, spatially diffused without shear-band formation. For a rock sample cemented by a ductile bond, the damage induced by load appears likely as an anomalous deviation in the master curve of stress-strain whereas the combined acoustic detection provides a very clear evidence with an important sound velocity decrease. Upon cyclic unloading-reloading, we recover a power-law scaling of the sound velocity with the pressure similar to the law in purely granular media but with a finite velocity at vanishing pressure which depends on the residual cohesion of the damaged material. When the drop stress occurs intermittently in fractured samples cemented with brittle materials, we measure not only the sound velocity decrease but also acoustic emissions. In the case (ii) under a triaxial load, we observe the formation of shear-bands, i.e. fractures on the scale of the sample at a load much smaller than those applied in the oedometric loading (i). Again, there is a strong elastic softening (velocity decrease) [4]. Finally, we also compare these experiments with the finite-element modelling of damage and wave propagation in 2D dense cemented disk packings with various cement contents and elasto-visco-plastic properties. This numerical simulation allows to characterize the heterogeneous damage of the material at a microscopic scale.

 

References

[1] C. Marone, Laboratory-derived friction laws and their applications to seismic faulting, Annu. Rev. Earth Planet. Sci. 26 1998, 643-696.

[2] V. Langlois, X. Jia, Acoustic probing of elastic behavior and damage in weakly cemented granular media, Phys. Rev. E 89 2014, 023206.

[3] A. Hemmerle, M. Schröter, L. Goehring, A cohesive granular material with tunable elasticity, Scientific reports 2016.

[4] Y. Khidas, X. Jia, Probing the shear-band formation in granular media with sound waves, Phys. Rev. E 85 2012, 051302.

[5] P.A. Johnson et al., Acoustic emission and microslip precursors to stick-slip failure in sheared granular media, Geophys. Res. Lett. 40 2013, 5627-5631.

How to cite: Canel, V., Jia, X., Campillo, M., and Ionescu, I. R.: Monitoring of damage processes in cemented granular materials with acoustic emissions and seismic velocity reduction, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21891, https://doi.org/10.5194/egusphere-egu2020-21891, 2020.

EGU2020-4790 | Displays | EMRP1.4

Experimental Deformation of Sandy Opalinus Clay at Elevated Temperature and Pressure Conditions

Valerian Schuster, Erik Rybacki, Audrey Bonnelye, Anja Schleicher, and Georg Dresen

Studying the mechanical properties of argillaceous rocks is of major interest in geoscience. For example, these rocks are important in engineering applications such as being suitable cap-rocks for the geological storage of carbon dioxide and potential host rocks for the storage of nuclear waste. Furthermore, argillaceous rocks are encountered in different natural settings such as accretionary wedges or fault zones. As a result of their sedimentary and diagenetic history clay rich rocks are often characterised by multiscale textural anisotropy and compositional heterogeneity resulting in anisotropic mechanical and hydraulic properties.
Here, we studied the anisotropic deformation behaviour of Opalinus Clay, collected from the Mont Terri underground laboratory, which is the envisaged host rock formation for nuclear waste disposal in Switzerland. We used the sandy facies of Opalinus Clay, characterized by an irregular wavy lamination of quartz-rich and carbonate-cemented lenses with clay-rich interlayers. Unconsolidated-cylindrical samples cored at 0°, 45° and 90° to the macroscopically visible bedding were deformed in undrained constant strain rate experiments using a Paterson-type deformation apparatus. For each orientation, tests were performed at dry conditions varying either confining pressure (in the range of 50 - 100 MPa), temperature (25 - 200 °C) or strain rate (1*10-3 - 5*10-6 s-1) to study the influence of testing condition and sample orientation on the deformation behaviour. In addition, we deformed a set of back saturated samples at fixed conditions of 50 MPa, 100 °C and 5*10-4 s-1 to investigate the effect of water content on the material strength.
The results show semi-brittle deformation with low yield strength and strain weakening behaviour, in which strain is localized in sub-millimetre to millimetre-wide shear zones at all conditions. Increasing water content reduces, whereas increasing confining pressure increases the peak strength. Samples that were deformed parallel to bedding orientation exhibit the highest strength compared to samples with an orientation of 90° and 45° to bedding. Only for the latter orientations a weak correlation was found between temperature and failure behaviour. The variation of strain rate shows no clear influence for all orientations. Within this test series, there appears to be a potentially greater influence of the porosity on the peak strength for 45° and 90° oriented samples. Clay rich layers seem to have a strong influence on localization and formation of shear zones, in particular for samples oriented at 45° and 90° to bedding. This observation was confirmed by electron microscopy performed on broad ion beam polished surfaces of deformed sample material.
Our experiments reveal that water content, sample orientation with respect to bedding and confining pressure are the most important factors influencing the peak strength of the sandy facies of Opalinus Clay, whereas compositional heterogeneity is responsible for the localization behaviour.

How to cite: Schuster, V., Rybacki, E., Bonnelye, A., Schleicher, A., and Dresen, G.: Experimental Deformation of Sandy Opalinus Clay at Elevated Temperature and Pressure Conditions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4790, https://doi.org/10.5194/egusphere-egu2020-4790, 2020.

EGU2020-6001 | Displays | EMRP1.4

Poroelastic relaxation in thermally cracked and fluid-saturated glass

Abdulwaheed Ògúnsàmì, Jan Borgomano, Jérôme Fortin, and Ian Jackson

To test theoretical models of modulus dispersion and dissipation in fluid-saturated rocks, we have investigated the broadband mechanical properties of four thermally cracked glass specimens of simple microstructure with complementary forced-oscillation (0.004 -100 Hz) and ultrasonic techniques (~1MHz). Strong pressure dependence of moduli (bulk, Young’s, and shear), axial strain, and ultrasonic wave speeds for dry conditions, attests to essentially complete crack closure at a confining pressure of 15 MPa – indicative of ambient-pressure crack aspect ratios mainly < 2 ´ 10-4.Oscillation of the confining pressure reveals bulk modulus dispersion and a corresponding dissipation peak, near 2 mHz only at the lowest effective pressure (2.5 MPa) – attributed to the transition with increasing frequency from the drained to saturated-isobaric regime. The observations are consistent with Biot-Gassmann theory, with dispersion and dissipation adequately represented by a Zener model.  Above the draining frequency, axial forced-oscillation tests show dispersion of Young’s modulus and Poisson’s ratio, and an associated broad dissipation peak centred near 0.3 Hz, thought to reflect local ‘squirt’ flow and adequately modelled with a continuous distribution of relaxation times over two decades. Observations of Young’s and shear modulus dispersion and dissipation from complementary flexural and torsional oscillation measurements for differential pressure ≤ 10 MPa provide supporting evidence of the transition with increasing frequency from the saturated-isobaric to the saturated-isolated regime – also probed by the ultrasonic technique. These findings validate predictions from theoretical models of dispersion in cracked media and emphasize need for caution in the seismological application of laboratory ultrasonic data for cracked media.

How to cite: Ògúnsàmì, A., Borgomano, J., Fortin, J., and Jackson, I.: Poroelastic relaxation in thermally cracked and fluid-saturated glass, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6001, https://doi.org/10.5194/egusphere-egu2020-6001, 2020.

EGU2020-6743 | Displays | EMRP1.4

Digital rock physics: Segmentation of sub-resolution features

Martin Balcewicz and Erik H. Saenger

Digital rock physics (DRP) became a complementary part in reservoir characterization during the last two decades. Deriving transport, thermal, or effective elastic rock properties from a digital twin requires a three-step workflow: (1) Preparation of a high-resolution X-ray computed tomography image, (2) segmentation of pore and grain phases, respectively, and (3) solving equations due to the demanded properties. Despite the high resolution µ-CT images, the numerical predictions of rock properties have their specific uncertainties compared to laboratory measurements. Missing unresolved features in the µ-CT image might be the key issue. These findings indicate the importance of a full understanding of the rocks microfabrics. Most digital models used in DRP treat the rock as a heterogeneous, isotropic, intact medium which neglect unresolved features. However, we expect features within the microfabrics like micro-cracks, small-scale fluid inclusions, or stressed grains which may influence the elastic rock properties but have not been taken into account in DRP, yet. Former studies have shown resolution-issues in grain-to-grain contacts within sandstones or inaccuracies due to micro-porosity in carbonates, this means the micritic phase. Within the scope of this abstract, we image two different sandstone samples, Bentheim and Ruhrsandstone, as well as one carbonate sample. Here, we compare the mentioned difficulties of X-ray visualization with further analytical methods, this means thin section and focused ion beam measurements. This results into a better understanding of the rocks microstructures and allows us to segment unresolved features in the X-ray computed tomography image. Those features can be described with effective properties at the µ-scale in the DRP workflow to reduce the uncertainty of the predicted rock properties at the meso- and fieldscale.

How to cite: Balcewicz, M. and Saenger, E. H.: Digital rock physics: Segmentation of sub-resolution features, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6743, https://doi.org/10.5194/egusphere-egu2020-6743, 2020.

EGU2020-16134 | Displays | EMRP1.4

Elastic properties of rocks: Why shouldn’t they be constant?

Federica Sandrone, Lucas Pimienta, Laurent Gastaldo, and Marie Violay

How to cite: Sandrone, F., Pimienta, L., Gastaldo, L., and Violay, M.: Elastic properties of rocks: Why shouldn’t they be constant?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16134, https://doi.org/10.5194/egusphere-egu2020-16134, 2020.

EGU2020-10679 | Displays | EMRP1.4

Origin of the temporal evolution of elastic properties during laboratory seismic cycle.

Federica Paglialunga, François X. Passelègue, Mateo Acosta, and Marie Violay

Recent seismological observations highlighted that earthquakes are associated to drops in elastic properties around the fault zone (Brenguier et al., 2008). This drop is often attributed to co-seismic damage produced at the rupture tip, and can mostly be observed at shallow depths. However, it is known that in the upper crust, faults are surrounded by a zone of damage (Caine, Evans, & Forster, 1996). Because of this, the origin of the velocity change associated to earthquakes, as well as its recovery in the months following the rupture remains highly debated.

We conducted stick-slip experiments to explore the evolution of elastic waves velocities during the entire seismic cycle. The tests were run on saw-cut La Peyratte granite samples presenting different initial degrees of damage, obtained through thermal treatment. Three types of samples were studied: not thermally treated, thermally treated at 650 °C and thermally treated at 950 °C. Seismic events were induced in a triaxial configuration apparatus at different confining pressures ranging from 15 MPa to 120 MPa. Active acoustic measurements were carried through the whole duration of the tests and P-wave velocities were measured.

 

The evolution of P-wave velocity follows the evolution of the shear stress acting on the fault, showing velocity drops during dynamic slip events. The evolution of the P-wave velocity drops with increasing confining pressure shows two different trends; the largest drops can be observed for low confining pressure (15 MPa) and decrease for intermediate confining pressures (up to 45 MPa), while for confining pressures of 60 MPa to 120 MPa, drops in velocity slightly increase with confining pressure.

Our results highlight that at low confining pressures (15-45 MPa), the change in elastic velocity is controlled by the sample bulk properites (damage of the medium surrounding the fault), while for higher confining pressures (60-120 MPa), it might be the result of co-seismic damage.

These preliminary results bring a different interpretation to the seismic velocity drops observed in nature, attributed to co-seismic damage. In our experiments co-seismic damage is not observed, except for high confining pressures (laboratory equivalent for large depths), while the change in P-wave velocity seems to be highly related to combined stress conditions and initial damage around the fault for low confining pressures (laboratory equivalent for shallow depths).

How to cite: Paglialunga, F., Passelègue, F. X., Acosta, M., and Violay, M.: Origin of the temporal evolution of elastic properties during laboratory seismic cycle., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10679, https://doi.org/10.5194/egusphere-egu2020-10679, 2020.

Determining elastic wave velocities and intrinsic attenuation of cylindrical rock samples by transmission of ultrasound signals appears to be a simple experimental task, which is performed routinely in a range of geoscientific and engineering applications requiring characterization of rocks in field and laboratory. P- and S-wave velocities are generally determined from first arrivals of signals excited by specifically designed transducers. A couple of methods exist for determining the intrinsic attenuation, most of them relying either on a comparison between the sample under investigation and a standard material or on investigating the same material for various geometries.

Of the three properties of interest, P-wave velocity is certainly the least challenging one to determine, but dispersion phenomena lead to complications with the consistent identification of frequency-dependent first breaks. The determination of S-wave velocities is even more hampered by converted waves interfering with the S-wave arrival. Attenuation estimates are generally subject to higher uncertainties than velocity measurements due to the high sensitivity of amplitudes to experimental procedures. The achievable accuracy of determining S-wave velocity and intrinsic attenuation using standard procedures thus appears to be limited.

We pursue the determination of velocity and attenuation of rock samples based on full waveform modeling and inversion. Assuming the rock sample to be homogeneous - an assumption also underlying standard analyses - we quantify P-wave velocity, S-wave velocity and intrinsic P- and S-wave attenuation from matching a single ultrasound trace with a synthetic one numerically modelled using the spectral finite-element software packages SPECFEM2D and SPECFEM3D. We find that enough information on both velocities is contained in the recognizable reflected and converted phases even when nominal P-wave sensors are used. Attenuation characteristics are also inherently contained in the relative amplitudes of these phases due to their different travel paths. We present recommendations for and results from laboratory measurements on cylindrical samples of aluminum and rocks with different geometries that we also compare with various standard analysis methods. The effort put into processing for our approach is particularly justified when accurate values and/or small variations, for example in response to changing P-T-conditions, are of interest or when the amount of sample material is limited.

How to cite: Boxberg, M. S., Duda, M., Löer, K., Friederich, W., and Renner, J.: Determining P- and S-wave velocities and Q-values from single ultrasound transmission measurements performed on cylindrical rock samples: it’s possible, when…, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9178, https://doi.org/10.5194/egusphere-egu2020-9178, 2020.

The mechanical behavior of soft rocks is dominated by the mechanical properties of the rock itself. Because soft rocks have different physical properties to hard rocks, it is essential to understand the mechanical behavior of soft rocks when tunnels and huge structures are constructed in these. Strain softening is the mechanical behavior of soil and rock materials and is important in understanding soft rock foundation. To investigate the mechanical behavior of siltstone, a sedimentary soft rock, we performed the one-dimensional consolidation tests (hereafter called K0-consolidation test) using a constant strain-rate loading system. We also took high-resolution X-ray CT images of the test specimens before and after the consolidation tests to observe the consolidation deformation. Using Quaternary siltstones distributed in the Boso Peninsula, central Japan as specimens, strain softening in the consolidation process was confirmed in some formations using two test machines at Kyoto University and Nagoya Institute of Technology.

All specimens yielded and the consolidation curves showed over- and normal-consolidation areas. Some specimens’ stress decreased suddenly at increasing strain just before yielding, which can be regarded as a real strain softening because no strain localization could be confirmed within specimens. The stress at the time of the softening differed even for specimens taken from the same formation. Furthermore, the micro-focus X-ray CT images indicated that the specimens had no macro cracks inside. This suggests that strain softening is not due to brittle failure in local areas but due to the softening of the framework structure of the siltstone itself. The samples used in this study are siltstone taken from the Quaternary forearc basin, whose development is related not only to consolidation but also tectonic effects such as horizontal compaction accompanied by plate subduction. Therefore, it is possible that the strain softening confirmed in this study reflects the micro cracks and internal structure that developed during siltstone formation.

How to cite: Kamiya, N., Zhang, F., and Lin, W.: Strain softening of siltstones in consolidation process using a constant strain-rate loading system, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12851, https://doi.org/10.5194/egusphere-egu2020-12851, 2020.

EGU2020-902 | Displays | EMRP1.4

Microscale characterisation of damage evolution in curling stones used in international competition

Derek Leung, Florian Fusseis, and Ian Butler

The rocks used to produce curling stones for international competition are only sourced from two localities in the world: Ailsa Craig, Scotland and Trefor, Wales. Curling stones consist of two components: (1) the running band (the ring-shaped bottom surface of the stone which rests on the ice) and (2) the striking band (the convex band on the profile of stones which collides with those of other stones). With a focus on the striking bands, we aim to document the damage evolution of curling stones using synchrotron microtomography (3D characterisation of pristine samples and 4D damage evolution), optical and scanning electron microscopy (quantitative characterisation of pristine samples and microfracture characterisation of damaged striking bands), and petrophysical testing (fracture characteristics and comparative data). These data will be complemented by an on-ice experiment that will determine the mechanics (e.g., stress distribution, contact area, and velocity) of curling stone impacts. Out of four curling stone varieties (from Ailsa Craig and Trefor), we observe the striking bands of three varieties to show macroscopic, incipient to mature, curvilinear fractures. The curvature of these fractures is consistent and does not vary significantly between individual stones and between curling stone varieties. However, the degree of macroscopic fracture development differs between aged striking bands of curling stone types: Blue Trefor (macroscopic fractures not observed), Red Trefor (weakly incipient), Ailsa Craig Common Green (incipient to juvenile), and Ailsa Craig Blue Hone (juvenile to mature). Unfortunately, it is not possible to determine the degree of usage (age) of the selected samples and thus it is not possible to normalize these apparent differences in damage. Given that the striking band limits the lifetime of curling stones, understanding the damage evolution of curling stones can contribute valuable information to the maintenance of curling stones. The rock physics of curling stone impacts is linked to dynamic spalling and more broadly to rock failure, as these processes are ultimately related to the initiation and propagation of fractures.

How to cite: Leung, D., Fusseis, F., and Butler, I.: Microscale characterisation of damage evolution in curling stones used in international competition, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-902, https://doi.org/10.5194/egusphere-egu2020-902, 2020.

EGU2020-22562 | Displays | EMRP1.4

Quantitative characterization of fracture networks on Digital Outcrop Models obtained from avionic and terrestrial laser scanner

Gloria Arienti, Matteo Pozzi, Anna Losa, Federico Agliardi, Bruno Monopoli, Andrea Bistacchi, and Davide Bertolo

We present a semi-automatic workflow aimed at extracting quantitative structural data from point clouds obtained with avionic and terrestrial laser scanners (Lidar and TLS). The workflow is characterized by a calibration phase followed by an automatic data-collection phase. The large datasets of “fractures” mapped in this way are analysed with statistical methods allowing to define representative parameters of the fracture network.

In the first phase, the intervention of an expert interpreter with structural geology skills is fundamental to evaluate which features can be interpreted as fractures in the point clouds. In the second phase, an automatic segmentation and classification is performed, based on phase 1 calibration, that allows extracting very large fracture datasets. The main steps in phase 1 are: manual segmentation of facets representing fracture surfaces, orientation analysis and definition of fracture sets (possibly supported by kinematic analysis), definition of orientation parameters to be used for automatic segmentation. Phase 2 analysis proceeds with the automatic segmentation of subset point clouds that include just one fracture set. In these point clouds, facets representing fractures lying on different planes are well separated and disconnected, and this allows applying automatic vectorization techniques that extract individual facets representing single fractures on the outcrop surface. The datasets issued from this processing are analysed with automatic algorithms allowing to define fracture spacing and orientation statistics with a very large support, that would not have been allowed by other methodologies.

How to cite: Arienti, G., Pozzi, M., Losa, A., Agliardi, F., Monopoli, B., Bistacchi, A., and Bertolo, D.: Quantitative characterization of fracture networks on Digital Outcrop Models obtained from avionic and terrestrial laser scanner, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22562, https://doi.org/10.5194/egusphere-egu2020-22562, 2020.

This work proposed an available approach to analyze the property evolution of weak interlayers during immersion softening at micro and macro scales, which combining the advantages of nanoindentation tests and numerical modelling. The weak interlayers has significant impact on the failure process of natural slopes, however, their properties are difficult to be obtained using traditional triaxial compression tests. Because these weak interlayers are consist of clay and rock fragments which leads to the difficult to prepare intact samples. Additionally, the softening properties of these weak interlayers are strongly related to their fillings at micro scale. In this work, the weak interlayers is investigated using nano-scale micromechanical tests and upscaling methodologies, so only small rock fragments are required (see Fig.1). 
In northwestern Hubei China, the mountains often developed several layers of weak interlayers with major lithology as shale which is sedimentary rock with low strength and dense clay particles. We investigated these shale fragments in weak interlayers, which is prone to decrease in strength induced by precipitation erosion. The Gaussian mixture model was used to analyze a large amount of data obtained by statistical grid nanoindentation method. Then the Mori-Tanaka scheme was used to homogenize the elastic properties of the samples and upscale the nanoindentation data to the macroscale. The hardness values which obtain by Berkovich and Cube corner indenter were able to assess the cohesion and friction angle of shale. Finally, these achieved parameters were applied in numerical model, in order to analyze the slope failure caused by the softening of weak interlayers (see Fig. 2).
The results show that: (1) the chlorite and muscovite minerals, which are major proportion of shale, soften or dissolve with the increasing saturation time. The fine mineral particles are gradually stripped from micro structure. As a result, at microscale the compact shale samples sale became loose. The strength of these shale samples are also decrease because water seeped through pores and micro cracks. (2) After water immersion, the friction angle is almost constant, while the elastic modulus and cohesion decrease significantly with increasing saturation time. (3) The shear strength decrease so that the shearing creep occurs along the weak interlayers surface, then bottom sliding surface is cut, which leads to landslide.

How to cite: Xu, J., Feng, Y., and Tang, X.: Investigate the softening properties of weak interlayers in slope failure process using nanoindentation test and simulation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18394, https://doi.org/10.5194/egusphere-egu2020-18394, 2020.

EGU2020-12430 | Displays | EMRP1.4

Geotechnical characteristics of rocks around the King Sejong Station in Antarctica by Freeze-Thawing test
not presented

Jeongdu Noh and Seong-Seung Kang

The purpose of this study is to evaluate the rock in the extreme regions by conducting a field study and laboratory tests on three rocks of diorite, andesite, grano-diorite around Sejong Station in Antarctica. The King George Island, the research area, is mostly covered by glaciers and partially exposed bedrock along the coast. Around the coastal area and Sejong-bong, andesites, diorites, grano-diorites are distributed and were measured rebound values using Silver Schmidt hammer. This hammer, unlike conventional Schmidt value’s R, calculates Q values using input and output energy. As a result of field study, the average Q value of diorite was estimated 76, which is high compared others, and andesite was estimated 67, which is low compared others, grano-diorite was estimated 72, which is widely scattered. Freeze-Thawing test was performed based on ASTM C-666, KS F 2456. The temperature range of freeze-thawing test is from -20 ℃ to 20 ℃ referred to the published papers, and all rocks are completely saturated without humidity. The temperature holding time was set to 2 hours for temperature inside rock to -20 ℃ when the atmosphere temperature is -20 ℃. The freeze-thawing test was carried out every 20 cycles for porosity, absorption, and slaking durability. The laboratory tests were performed 200 times in total. As a result, up to 100 cycles, the porosity and absorption were not significantly different. Since then, they increased slightly. However, the slaking tended to increase gradually from the 0 cycle. In order to accurately assess the weathering of the three rocks, continuous freeze-thawing tests should be conducted.

 

Acknowledgement : This work was supported by the National Research Foundation of Korea(NRF) grant funded by the Korea government(MIST (2019R1F1A1048854)

How to cite: Noh, J. and Kang, S.-S.: Geotechnical characteristics of rocks around the King Sejong Station in Antarctica by Freeze-Thawing test, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12430, https://doi.org/10.5194/egusphere-egu2020-12430, 2020.

In situ stress state at shallow depths (<1 km) is important for designing underground systems for various projects such as nuclear waste disposal, carbon dioxide geological sequestration, and geo-resource development. Stress characterization for such projects rely largely on stress measurement data (such as hydraulic fracturing test data). We compile a large number of hydraulic fracturing test data measured in a total of 226 boreholes in South Korea, and attempt to characterize shallow crustal stress over the country. These data are measurements at depths down to 850 m, and classified mostly low-quality based on World Stress Map quality ranking scheme (B-quality: 7%, C: 42%, and D: 51%). We grid the country by 0.25°×0.25°, and find a circular bin size at each grid point using two statistical methods (weighted standard deviation and quasi interquartile range), by which the uniformity of stress orientation can be estimated. As many data are low-quality, we apply this process to two subsets of data (B-C and B-D) to find an optimal stress characterization. Our most optimal characterization results show that bin diameter in most of the country vary between 100 and 200 km, except for southeastern Korea. Bin diameters in southeastern Korea range between 0 and 60 km, which means that stress heterogeneity is especially significant in the region, where lithology varies markedly and several active faults are clustered. The stress orientations in the northeastern part of the country are characterized as intermediate stress uniformity (bin size of ~120 km in diameter) but a systematic horizontal stress rotation (up to ~60°) from that of the deep-seated regional stress. This region is mountainous with altitude as high as 1.4 km. To verify whether the stress rotation is a result of topographic effect, we model stress perturbation using the digital elevation model (DEM) data of the region, which yields stress rotation comparable to measurements. We find that lithology is a particularly important factor that affects stress magnitudes over the country, as the stress magnitudes at the same depth tend to be markedly smaller in sedimentary rocks than in crystalline rocks. Our study, although given data are of fairly low-quality, can provide a basis for shallow stress map of South Korea.

How to cite: Kang, M. and Chang, C.: Effects of topography and lithology variation on in situ stress at shallow depths in South Korea: results from statistical characterization of stress data, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8327, https://doi.org/10.5194/egusphere-egu2020-8327, 2020.

Self-organization is not a universal property of matter, it exists under certain internal and external conditions and this is not associated with a special class of substances. The study of the morphology and dynamics of migration of anomalous zones associated with increased stresses is of particular importance in the development of deep deposits, complicated by dynamic phenomena in the form of mountain impacts. An important tool for this study is geophysical exploration. To describe the geological environment in the form of an array of rocks with its natural and technogenic heterogeneity, one should use its more adequate description, which is a discrete model of the medium in the form of a piecewise inhomogeneous block medium with embedded heterogeneities of a lower rank than the block size. This nesting can be traced several times, i.e. changing the scale of the research, we see that heterogeneities of a lower rank now appear in the form of blocks for heterogeneities of the next rank. A simple averaging of the measured geophysical parameters can lead to distorted ideas about the structure of the medium and its evolution. We have analyzed the morphology of the structural features of disintegration zones before a strong dynamic phenomenon. The introduction of the proposed integrated passive and active geophysical monitoring into the mining system, aimed at studying the transient processes of the redistribution of stress-strain and phase states, can help prevent catastrophic dynamic manifestations during the development of deep-seated deposits. Active geophysical monitoring methods should be tuned to a model of a hierarchical heterogeneous environment. Iterative algorithms for 2-D modeling and interpretation for sound diffraction and a linearly polarized transversal elastic wave on the inclusion with a hierarchical elastic structure located in the J-th layer of the N-layer elastic medium are constructed. The case is considered when the inclusion density of each rank coincides with the density of the containing layer, and the elastic parameters of inclusion of each rank differ from the elastic parameters of the containing layer.

How to cite: Hachay, O. and Khachay, O.: Acoustic Monitoring of Anomalous Stressed Zones, Determination of their Positions, Surfaces, Evaluation of Catastrophic Risk., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1322, https://doi.org/10.5194/egusphere-egu2020-1322, 2020.

Due to the large burial depth of the Pliocene Red Layer in Qingyang, Gansu and its special historical causes, its engineering mechanical characteristics are quite different from those of the southern red clay. Lack of systematic data on the internal forces of the lining structure through the stratum tunnel. Therefore, this paper takes the Yinchuan-Xian High-speed Railway Qingyang Tunnel as the research object, through field measurement and finite element simulation to obtain the space-time distribution characteristics of the internal force of the lining structure, the surrounding rock pressure, the deep displacement of the surrounding rock from 5 to 10 m, and the convergent deformation of the support. The reasons for the stress state of the lining-surrounding rock composite structure reflected in the results are analyzed, and the ABAQUS software is used to simulate the tunnel excavation process to compare and verify the lining structure stressing law. Internal force characteristics. The results show that: 1) The physical and mechanical indicators of the Pliocene red layer in the Neogene in Qingyang, Gansu belong to the extremely hard soil-very soft rock critical category. Due to the long consolidation pressure and long consolidation history, it can be obvious on the saturated flooding fault surface. Observation of the characteristics of layered joints proves that this layer of red clay has a tendency of sedimentary diagenesis. 2) The quality of the surrounding rock of the stratum lining structure is good. The horizontal in-situ stress is twice that of the vertical in-situ stress. It can be optimized for the design of III-IV surrounding rock while increasing the side pressure coefficient. 3) The unclosed initial support cannot effectively limit the deformation of the surrounding rock, and the temporary stress can be used to improve the state of stress. The numerical simulation results are consistent with the field measurement laws. 4) This stratum with severe deformation is the cave diameter range of the excavation boundary to the surrounding rock. The deformation area is mainly concentrated in the vault. Delayed excavation of the inverted arch can effectively reduce the stress on the internal lining structure of the inverted arch.

How to cite: Xie, Z. and Wu, X.: Study on Deformation and Force Characteristics of Deep-buried Large Section Expansive Red Clay Tunnel, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2248, https://doi.org/10.5194/egusphere-egu2020-2248, 2020.

EGU2020-21768 | Displays | EMRP1.4

Issues with fracturing ice during an ice drilling project in Greenland (EastGRIP)

Ilka Weikusat, David Wallis, Steven Franke, Nicolas Stoll, Julien Westhoff, Steffen Bo Hansen, Trevor James Popp, Frank Wilhelms, and Dorthe Dahl-Jensen

Drilling an ice core through an ice sheet (typically 2000 to 3000 m thick) is a technical challenge that nonetheless generates valuable and unique information on palaeo-climate and ice dynamics. As technically the drilling cannot be done in one run, the core has to be fractured approximately every 3 m to retrieve core sections from the bore hole. This fracture process is initiated by breaking the core with core-catchers which also clamp the engaged core in the drill head while the whole drill is then pulled up with the winch motor.

 

This standard procedure is known to become difficult and requires extremely high pulling forces (Wilhelms et al. 2007), in the very deep part of the drill procedure, close to the bedrock of the ice sheet, especially when the ice material becomes warm (approximately -2°C) due to the geothermal heat released from the bedrock. Recently, during the EastGRIP (East Greenland Ice coring Project) drilling we observed a similar issue with breaking off cored sections only with extremely high pulling forces, but started from approximately 1800 m of depth, where the temperature is still very cold (approximately -20°C). This has not been observed at other ice drilling sites. As dependencies of fracture behaviour on crystal orientation and grain size are known (Schulson & Duval 2009) for ice, we thus examined the microstructure in the ice samples close to and at the core breaks.

 

First preliminary results suggest that these so far unexperienced difficulties are due to the profoundly different c-axes orientation distribution (CPO) in the EastGRIP ice core. In contrast to other deep ice cores which have been drilled on ice domes or ice divides, EastGRIP is located in an ice stream. This location means that the deformation geometry (kinematics) is completely different, resulting in a different CPO (girdle pattern instead of single maximum pattern). Evidence regarding additional grain-size dependence will hopefully help to refine the fracturing procedure, which is possible due to a rather strong grain size layering observed in natural ice formed by snow precipitation.

 

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Wilhelms, F.; Sheldon, S. G.; Hamann, I. & Kipfstuhl, S. Implications for and findings from deep ice core drillings - An example: The ultimate tensile strength of ice at high strain rates. Physics and Chemistry of Ice (The proceedings of the International Conference on the Physics and Chemistry of Ice held at Bremerhaven, Germany on 23-28 July 2006), 2007, 635-639

Schulson, E. M. & Duval, P. Creep and Fracture of Ice. Cambridge University Press, 2009, 401

How to cite: Weikusat, I., Wallis, D., Franke, S., Stoll, N., Westhoff, J., Hansen, S. B., Popp, T. J., Wilhelms, F., and Dahl-Jensen, D.: Issues with fracturing ice during an ice drilling project in Greenland (EastGRIP), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21768, https://doi.org/10.5194/egusphere-egu2020-21768, 2020.

EGU2020-18041 | Displays | EMRP1.4

Coupled processes in clay during tunnel excavation

Antonio Pio Rinaldi, Yves Guglielmi, Alba Zappone, Florian Soom, Michelle Robertson, Paul Cook, Maria Kakurina, Quinn Wenning, Dorothee Rebscher, and Christophe Nussbaum

Tunnel excavations are known to perturb the hosting rock mass at long distances, with changes in the hydrogeological flow affecting, as well as deforming the rock mass, inducing subsidence in a zone above the tunnel. During the extension of the Mont Terri Underground Rock Laboratory, we had the unique opportunity to monitor the final part of the excavation of Gallery18 and the final breaktrough.

The joint effort of two experiments (CS-D lead by ETH Zurich and FS-B lead by LBNL) allowed for a detailed characterization of the poro-elastic response of the rock mass and the Mont Terri Main Fault Zone to the excavation. Geophysical, geomechanical, and hydrogeological monitoring include: (1) pressure monitoring in several borehole intervals; (2) deformation at a chain potentiometer and fiber optics grouted in boreholes (normal to bedding and parallel to fault zone), and platform-tilmeters installed at the tunnel floor, as well as detailed 3D displacement at the SIMFIP probe.

All monitoring systems detected major perturbations starting from 15 days before the breakthrough and continuing for several days after it. We summarize the observations and will combine numerical modelling and observed trend to conceptualized the pattern of poro-elastic deformation. The results of the analysis could help shedding light on the poro-elastic behaviour of clay, providing interesting hints for the modeling community and helping in planning of future nuclear waste repositories in such material.

How to cite: Rinaldi, A. P., Guglielmi, Y., Zappone, A., Soom, F., Robertson, M., Cook, P., Kakurina, M., Wenning, Q., Rebscher, D., and Nussbaum, C.: Coupled processes in clay during tunnel excavation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18041, https://doi.org/10.5194/egusphere-egu2020-18041, 2020.

EGU2020-2348 | Displays | EMRP1.4

3 failure limits to relief

Anne Voigtländer, Rachel C. Glade, and Jens M. Turowski

Reaching the top of a high mountain is a great experience, yet there seem to be several limits. One is the relief of the mountain itself, which constitutes the driving stress consisting of the height, h, and density, ρ of the mass, accelerated by gravity, g and modulated by the slope, α. The material strength required to balance this stress defines the limit to relief. There are three failure modes in which the material strength can be surpassed: shear, compression, and tension. Failure criteria established for shear and compression have been demonstrated to be useful in certain settings, but don’t hold in steep (50-90°), hard and rocky landscapes. For those, we propose a tensile strength limit criterion (TSL). Due to the Poisson effect of normal stress (σn), indirect tensile stresses (σt) arise near free surfaces. The magnitude of these stresses is defined by the Poisson’s ratio (ν) of the lithology and the relief. First-order estimates of different lithologies and their material properties are in good agreement with the height of cliffs and slopes of the same lithology. Similar to the approach by Schmidt and Montgomery (1995) predicting bulk, slope scale material properties from relief, we can invert the tensile strength limit criterion. By this, we can infer material tensile strength and Poisson’s ratio from the maximum slope heights and angle on Earth, and beyond!

In terms of dynamics, the tensile strength limit criterion (TSL) predicts critical yielding at the foot of the slope, causing surface parallel fractures that would lead to further critical yielding and failure slope upward. This pattern of progressive rock failure has been observed in steep rock walls, like El Capitan or Half Dome in Yosemite National Park.

We propose this solely geometrically and stress-controlled criterion not contrary but in addition to existing limit criteria. Implications of the three failure limits to relief are that, (i) over-steepening doesn’t necessarily exist, as there is not only a threshold slope angle but also a threshold height, (ii) there is a transition from one dominant limit and failure mechanism to the other, shifting from shear failure and sliding to toppling and fall, and (iii) internal material property changes, due to chemical and/or mechanical weathering, and subcritical crack growth can evoke a progressive reorganisation of yielding and potential rock failure without external triggering events.

How to cite: Voigtländer, A., Glade, R. C., and Turowski, J. M.: 3 failure limits to relief, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2348, https://doi.org/10.5194/egusphere-egu2020-2348, 2020.

EGU2020-4831 | Displays | EMRP1.4

Petrological constraints on ultra-high pressure metamorphism and frictionite formation in a catastrophic rockslide: The Koefels event (Eastern Alps).

Diethard Sanders, Bastian Joachim-Mrosko, Jürgen Konzett, Julian Lanthaler, Marc Ostermann, and Peter Tropper

The P-T conditions in extremely-rapid gravity-driven rockslides are difficult to constrain from the descended rock mass itself. Here, we report mineralogical observations from the Koefels rockslide and their interpretation. The Koefels event – happened between 9527-9498 cal BP – comprises 3.9 km3 mainly of muscovite + biotite-bearing orthogneiss, and is one of the few large rockslides in silicate-bearing rocks worldwide. Detached by collapse of a valley flank, the rockslide impacted the opposite valley flank: While the lower part of the mass was sharply stopped, the overriding part propagated farther. This led to shear localization along discrete planes and, in consequence, to transient melting by frictional heating. The resulting frictionites comprise thin glassy levels with floating crystal fragments. The bulk composition of the glassy melt corresponds to the composition of the orthogneiss.

            In the frictionites, ultra-high pressure metamorphosed quartz (UPQ) occurs next to unaffected quartz in a glassy matrix. Micro-Raman spectroscopy of unaffected quartz yielded an intense A1 Raman mode at 464 cm-1 ; UPQ shows a shift of this band down to 460cm-1, with some grains showing an internal gradient of up to 3 cm-1 from the core (463cm-1) to the rim (460 cm-1). Some UPQ are rimmed by lechatelierite (SiO2 glass), which never surrounds unaffected quartz grains. Until now lechatelierite formation in frictionites was considered to be a function of temperature only (Heuberger et al. 1984). Because lechatelierite only rims UPQ with outward decreasing band numbers, we interpret lechatelierite formation to be mainly pressure-driven. The completely molten matrix and the lack of glassy rims at the edges of normal quartz indicates minimum temperatures of 900°C. Experimental investigations have shown that the shifted A1 mode of UPQ equilibrates to 464 cm-1 at 1100°C, thus giving an upper limit of the temperature range. The Raman shift of the A1 mode and the presence of lechatelierite strongly suggest that a pressure >23 GPa was attained (cf., McMillan et al. 1991, Fritz et al. 2011, Kowitz et al. 2013).

            The UPQ and lechatelierite rims formed by grain collisions during initial shear localization, when the shear plane was relatively cool. Next, upon rapid frictional heating the glassy frictionite matrix formed and became locally injected into lechatelierite rims. Once formed, the melt prevented high-energy grain collisions. Unaffected quartz (which nevertheless may have seen pressures up to 22 GPa) in the frictionites perhaps escaped UHP overprint due to position in local pressure shadows and/or was sheared out from the adjacent caciritic rock mass into the melt. Our results help to better constrain numerical simulations of P-T-conditions in rockslides. Since our investigation only provides limiting estimates the actual P-T conditions in deep shear levels of rockslides exceeding the volume of the Koefels event might be even higher.

 

References:

Fritz et al. 2011: International Journal of Impact Engineering, 38:440

Heuberger et al. 1984: Mountain Research and Development, 4:345

Kowitz et al. 2013: Earth and Planetary Science Letters, 384:17

McMillan et al. 1992: Physics and Chemistry of Minerals, 19:71

How to cite: Sanders, D., Joachim-Mrosko, B., Konzett, J., Lanthaler, J., Ostermann, M., and Tropper, P.: Petrological constraints on ultra-high pressure metamorphism and frictionite formation in a catastrophic rockslide: The Koefels event (Eastern Alps). , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4831, https://doi.org/10.5194/egusphere-egu2020-4831, 2020.

EGU2020-5125 | Displays | EMRP1.4

P-wave velocity anisotropy in an active methane venting pockmark: The Scanner Pockmark, northern North Sea

Gaye Bayrakci, Timothy A. Minshull, Jonathan M. Bull, Timothy J. Henstock, Giuseppe Provenzano, Hamza Birinci, Calum Macdonald, and Robert Dunn

Scanner pockmark is an active and continuous methane venting seafloor depression of ~ 900 x 450 m wide and 22 m deep. It is located in the northern North Sea, within the Witch Ground basin where the seafloor and shallow sediments are heavily affected by pockmarks and paleo-pockmarks of various sizes. A seismic chimney structure is present below the Scanner pockmark. It is expressed as a near-vertical column of acoustic blanking below a bright zone of gas-bearing sediments. Seismic chimneys are thought to host connected vertical fractures which may be concentric within the chimney and align parallel to maximum compression outside it. The crack geometry modifies the seismic velocities, and hence, the anisotropy measured inside and outside of the chimney is expected to be different.

 

We carried out anisotropic P-wave tomography with a GI-gun wide-angle dataset recorded by the 25 Ocean Bottom Seismometers (OBSs) of the CHIMNEY experiment (2017). Travel times of more than 60,000 refracted phases propagating within a volume of 4 x 4 x 2 km were inverted for P-wave velocity and the direction and degree of P-wave anisotropy. The grid is centred on the Scanner Pockmark and has a y-axis parallel to -34o N. The horizontal node interval is denser in the zone covered by the OBSs and the vertical node interval is denser near the seabed. A 3 iteration inversion leads to a chi2 misfit value of 1 and a root-mean-square misfit of <10 ms. The results show a maximum P-wave anisotropy of 5%, and higher degrees of anisotropy correlates well with higher velocities. The fast P-wave velocity orientation, a proxy for fracture orientations, is 46o N. The top of the chimney possibly links a bright spot mapped at 270 ms in two way travel time using RMS amplitudes of MCS data, to the surface gas emission. The bright spot corresponds to low tomographic P-wave velocity and anisotropy, suggesting that gas is located in a zone with unaligned fractures or porosity. This observation is in good agreement with early multi-channel seismic data interpretations which suggested that the gas is trapped within a sandy clay layer, the Ling Bank Formation, capped by an upper clay layer, the Coal Pit Formation. In the next step, we will invert the travel-times of reflected phases in order to increase the image resolution.  

How to cite: Bayrakci, G., Minshull, T. A., Bull, J. M., Henstock, T. J., Provenzano, G., Birinci, H., Macdonald, C., and Dunn, R.: P-wave velocity anisotropy in an active methane venting pockmark: The Scanner Pockmark, northern North Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5125, https://doi.org/10.5194/egusphere-egu2020-5125, 2020.

EGU2020-6669 | Displays | EMRP1.4

Fracture characterisation using frequency-dependent shear-wave splitting analysis of azimuthal anisotropy: application to fluid flow pathways at the Scanner Pockmark area, North Sea

Adam Robinson, Gaye Bayracki, Calum MacDonald, Ben Callow, Giuseppe Provenzano, Timothy Minshull, Mark Chapman, Timothy Henstock, and Jonathan Bull

Scanner pockmark, located in the Witch Ground Graben region of the North Sea, is a ~900 m by 450 m, ~22 m-deep elliptical seafloor depression at which vigorous and persistent methane venting is observed. Previous studies here have indicated the presence of chimney structures which extend to depths of several hundred meters, and which may represent the pathways along which upwards fluid migration occurs. A proposed geometry for the crack networks associated with such chimney structures comprises a background pattern outside the chimney with unconnected vertical fractures preferentially aligned with the regional stress field, and a more connected, possibly concentric fracture system within the chimney. The measurement of seismic anisotropy using shear-wave splitting (SWS) allows the presence, orientation and density of subsurface fracture networks to be determined. If the proposed model for the fracture structure of a chimney feature is correct, we would expect, therefore, to be able to observe variations in the anisotropy measured inside and outside of the chimney.

Here we test this hypothesis, using observations of SWS recorded on ocean bottom seismographs (OBS), with the arrivals generated using two different air gun seismic sources with a frequency range of ~10-200 Hz. We apply a layer-stripping approach based on observations of SWS events and shallow subsurface structures mapped using additional geophysical data to progressively determine and correct for the orientations of anisotropy for individual layers. The resulting patterns are then interpreted in the context of the chimney structure as mapped using other geophysical data. By comparing observations both at the Scanner pockmark and at a nearby reference site, we aim to further contribute to the understanding of the structures and their role in governing fluid migration. Our interpretation will additionally be informed by combining the field observations with analogue laboratory measurements and new and existing rock physics models.

This work has received funding from the NERC (CHIMNEY; NE/N016130/1) and EU Horizon 2020 programme (STEMM-CCS; No.654462).

How to cite: Robinson, A., Bayracki, G., MacDonald, C., Callow, B., Provenzano, G., Minshull, T., Chapman, M., Henstock, T., and Bull, J.: Fracture characterisation using frequency-dependent shear-wave splitting analysis of azimuthal anisotropy: application to fluid flow pathways at the Scanner Pockmark area, North Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6669, https://doi.org/10.5194/egusphere-egu2020-6669, 2020.

EGU2020-4707 | Displays | EMRP1.4

How many plates?

Tiantian Chen, Chun‘an Tang, and Yongyi Wang

The spacing of opening-mode fractures in layered materials, such as certain sedimentary rocks and laminated engineering materials, is often proportional to the thickness of fractured layers. Bai, Pollard & Gao (2000) investigated the full stress distribution between such fractures, from which they show that the spacing initially decreases as extensional strain increases in the direction perpendicular to the fractures. But at a certain ratio of spacing to layer thickness, no new fractures form and the additional strain is accommodated by further opening of existing fractures: the spacing then simply scales with layer thickness, which is called fracture saturation. Their conclusion is in marked contrast to existing theories of fracture, such as the stress-transfer theory, which predict that spacing should decrease with increasing strain ad infinitum. Here we show that the principle for 2D equal spaced fracture problem also applies to the 3D polygonal fracture problem. By using 3D mechanical modeling on a spherical shell model under interior expansion, we found that the modeled plate mosaic exactly follows the same principle that the size of formed plates is also proportional to the thickness of the fractured shell. By using a spherical shell model with isotropic, elastic two-layers, we numerically load the shell to fail under a quasistatical, slowly increasing interior pressure in a displacement controlling manner (induced, e.g., by gradual thermal expansion). The fractures only occur in the surface layer. The value at which a particular element breaks is random, but fixed at the start of the fragmentation process (i.e., the disorder is quenched). The probability distribution (PD) of breakdown thresholds is a material property and is known from the start. We account for this local randomness by assigning to each element a failure threshold taken from a Weibull probability distribution (PD), with a parameter defines the degree of material homogeneity, called the homogeneity index. We use a three-dimensional finite element code named RFPA (Rock Failure Process Analysis) to solve the problem. The modeling results show that, under conditions of uniform expansion force from inside the shell, the cracking pattern also follows a global scale law in terms of the thickness of the fractured layer. The numerical modeling demonstrates an important observation that, under conditions of uniform and layer-parallel tension induced by thermal expansion within the spherical shell, surface cracks spontaneously self-organize into quasi-hexagonal tessellations, following the mechanical principle that the hexagonal pattern relieves the greatest strain energy for the least work invested in nucleation and propagation of fractures. If this applies to the problems of Earth tessellations, called Platonics (Anderson, 2002), it implies that the thermal expanded Earth may breakup to form plate-like network as a consequence of thermal-expansion induced rift rather than mantle convective or plutonic causes, and the plate size may be proportional to the thickness of lithosphere. This provides a new explanation on how the plate number should be, and whether there is a pattern in the plate mosaic, issues related to the optimal sizes and shapes of plates in terms of fracture spacing.

How to cite: Chen, T., Tang, C., and Wang, Y.: How many plates?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4707, https://doi.org/10.5194/egusphere-egu2020-4707, 2020.

EMRP2.2 – Observing Earth with Swarm: Results from Six Years in Orbit and Future Perspectives

EGU2020-9616 | Displays | EMRP2.2

Core-mantle boundary flows obtained purely from Swarm secular variation gradient information

Kathy Whaler, Magnus Hammer, Chris Finlay, and Nils Olsen

The Swarm constellation provides information on both along- and across-track magnetic field gradients. Spatial changes of the magnetic vector field elements are described by a magnetic field gradient tensor, whose elements and their uncertainties can be estimated using the Virtual Observatory (VO) concept, whereby data within a cylinder centred on the VO with axis perpendicular to the Earth’s surface are reduced to a central point at satellite altitude. Recent experiments have shown that analysing data collected over a 4 month window provides the best compromise between reducing bias from the way the satellite orbits sample each VO cylinder and preserving information on temporal changes of the field, and that the data provide spatial information sufficient to resolve 300 non-overlapping VOs. We invert annual first differences of the 5 independent gradient tensor elements (providing estimates of secular variation, SV, gradients) at these 300 VOs over the Swarm era for advective velocity at the core-mantle boundary, forcing the flow to have minimal acceleration while providing an adequate fit to the data. We obtain flows similar to those from previous SV inversions but purely from the gradient information. The resolution of the SV gradients is higher than that of the SV itself, resulting in a ~30% increase in the number of effective flow parameters; this is thought to be because the gradients are less affected by long period external signals that are difficult to remove from the data, resulting in an improved signal to noise ratio. Although very little temporal change in the flow is required to reproduce even rapid changes in the magnetic field, we are able to isolate some robust flow changes, in particular regarding changes in the azimuthal flow acceleration, associated with the geomagnetic impulse in the Pacific region in around 2016.

How to cite: Whaler, K., Hammer, M., Finlay, C., and Olsen, N.: Core-mantle boundary flows obtained purely from Swarm secular variation gradient information, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9616, https://doi.org/10.5194/egusphere-egu2020-9616, 2020.

EGU2020-7879 | Displays | EMRP2.2

Mapping 3-D mantle electrical conductivity using Swarm, Cryosat-2 and ground observatory data

Alexey Kuvshinov, Alexander Grayver, Lars Tøffner-Clausen, and Nils Olsen

In this contribution, we report on our recent attempts to detect lateral variations of the electrical conductivity at mid mantle depths (400­ – 1600 km) using 6 years of Swarm, Cryosat-2 and observatory magnetic data. The approach involves a three-dimensional (3-D) inversion of matrix Q-responses. These responses relate spherical harmonic coefficients of external (inducing) and internal (induced) parts of the magnetic potential, derived for geomagnetic variations at periods longer than 1 day and hence mainly describing signals of magnetospheric origin (i.e. external also to satellites, as required). In addition to the inversion results, we discuss potential ways to improve the recovery of 3-D conductivity structures in the mantle.

How to cite: Kuvshinov, A., Grayver, A., Tøffner-Clausen, L., and Olsen, N.: Mapping 3-D mantle electrical conductivity using Swarm, Cryosat-2 and ground observatory data , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7879, https://doi.org/10.5194/egusphere-egu2020-7879, 2020.

EGU2020-4651 | Displays | EMRP2.2

Oceanic tidal signals in satellite magnetic data: quo vadis?

Alexander Grayver, Nils Olsen, Chris Finlay, and Alexey Kuvshinov

The continuous high-quality geomagnetic field measurements delivered by the Swarm satellite constellation trio have enabled reliable global mapping of the magnetic signature of ocean tides for several tidal constituents. These signals provide geophysical constraints on the average electrical conductivity profile of the upper mantle below the oceans. In principle, these signals can also sense lateral variations of the electrical conductivity in the oceanic upper mantle, although the amplitude of these effects is small. Additionally, the long-term changes in the climatology of the ocean can be potentially detected by the magnetic satellite signals. Both applications put additional demands on the accuracy and resolution of the extracted signals. This contribution discusses potential ways to meet the required demands and evaluates the feasibility of using the magnetic signature of ocean tides for studying these effects.

How to cite: Grayver, A., Olsen, N., Finlay, C., and Kuvshinov, A.: Oceanic tidal signals in satellite magnetic data: quo vadis?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4651, https://doi.org/10.5194/egusphere-egu2020-4651, 2020.

EGU2020-13612 | Displays | EMRP2.2

The magnetic signatures of oceanic tides in satellite data: A virtual-observatory approach

Jakub Velímský, Magnus D. Hammer, and Christopher C. Finlay

The magnetic signatures of the M2, and more recently also the N2, and O1 oceanic tides have been successfully extracted from satellite observations (Grayver & Olsen, 2019). The traditional method uses the spatial representation of the tidal signals by spherical harmonics. Here we present an alternative approach based on the concept of virtual observatories, motivated by similar development in the analysis of the core field (Mandea & Olsen 2006). All quiet-time, night-side vector magnetic field values observed by the satellite(s) in the proximity of a selected virtual observatory are parameterized by a scalar magnetic potential represented by a cubic harmonic polynomial in a local Cartesian coordinate system. The time-dependence of the polynomial coefficients is constrained by selected tidal frequency, taking into account also the phase and amplitude corrections. The local approach offers several advantages over the use of the global spherical-harmonic base. The disturbances from external field in the polar areas have no impact on the inversion at lower latitudes, and local error estimates can be also provided. In this initial report, we will explore the possibilities of the new technique in terms of resolution, the combination of datasets from multiple satellites and the use of NS and EW field differences from the Swarm A-C pair.

How to cite: Velímský, J., Hammer, M. D., and Finlay, C. C.: The magnetic signatures of oceanic tides in satellite data: A virtual-observatory approach, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13612, https://doi.org/10.5194/egusphere-egu2020-13612, 2020.

Analysing ionospheric electron density and magnetic field data from several years of the Swarm three-satellite mission we define a dataset of anomalies statistically.  We then use a superposed epoch approach to study the possible relation with a corresponding dataset of earthquakes occurred in the same space-time domain. Two statistical quantities d and n are then established comparing the statistics of the real analyses with simulations to assess the effectiveness of the largest concentrations of anomalies as ionospheric precursors. In detail, d would show how much the real maximum concentration is above the expected typical maximum concentration of a random anomaly distribution; while n value measures how much the largest concentration deviates with respect a typical random deviation: the larger are the d and n values, the more the results of the analysis applied to real data deviate from randomness. The best cases for which the real analyses are well distinct from random simulations are selected when d≥1.5, because the anomaly density is equal to or larger than 50% of random distribution, and n≥4, because the probability to be random is equal to or less than 0.1%.  This is the case of Y magnetic field component with a search in the Dobrovolsky area around each considered earthquake epicentre. The electron density is slightly less effective in the correlation with earthquakes, but still better than a homogeneous random distribution of anomalies.

How to cite: De Santis, A. and the SAFE Team: Statistical analysis of Swarm satellite data for assessing the effectiveness of ionospheric precursors of earthquakes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13140, https://doi.org/10.5194/egusphere-egu2020-13140, 2020.

EGU2020-10018 | Displays | EMRP2.2 | Highlight

Reconstructing the propagation of Whistlers observed in ELF during ASM burst sessions from the lightning strikes to their detection and validation of IRI model

Pierdavide Coïsson, Vladimir Truhlik, Janusz Mlynarczyk, Gauthier Hulot, Rémi Madelon, Olivier Bonnot, Pierre Vigneron, Dalia Burešová, Jaroslav Chum, Pawel Rzonca, and Andzej Kulak

New sessions of burst-mode acquisition of the Absolute Scalar Magnetometers (ASM) onboard Swarm satellites have been conducted during 2019 , with the aim of acquiring events covering various geophysical conditions, in terms of geomagnetic latitude, spacecraft Local Time and season, to better understand the conditions under which the ELF component of whistlers is excited and can be detected at satellite altitude and to provide an additional ionospheric monitoring.

Among all candidate events detected using an automatic algorithm specifically designed for that purpose, a selection of remarkable whistler events have been further studied. Firstly, from the estimation of the whistler dispersions, the origin times of the lightning discharge have been estimated and validated with ground data from the World ELF Radiolocation Array (WERA), providing the locations of the lightning strikes and their intensity in the ELF spectral band. These locations have also been validated using data from the World Wide Lightning Location Network (WWLLN) providing measurements.

Subsequently, to reconstruct the propagation path inside the ionosphere of the ELF component of the whistler, a dedicated ray-tracing algorithm has been designed. It uses a background ionosphere model of electron and ions based on the International Reference Ionosphere. For the purposes of producing a ionospheric representation as close as possible to the experimental conditions, the update of the main ionospheric parameters based on worldwide ionosonde data IRTAM has been applied, validating it by using ionosonde data available in the vicinity of specific whistler events. The in-situ electron density measurements of the Electric Field Instrument (EFI) of Swarm satellite have also been used to constrain the model in the topside ionosphere.

We present the recent results obtained during some of these burst sessions, and discuss the possibility offered by this new dataset to validate global ionospheric models and provide a new avenue in ionospheric research, that could be also pursued by the NanoMagSat mission.

How to cite: Coïsson, P., Truhlik, V., Mlynarczyk, J., Hulot, G., Madelon, R., Bonnot, O., Vigneron, P., Burešová, D., Chum, J., Rzonca, P., and Kulak, A.: Reconstructing the propagation of Whistlers observed in ELF during ASM burst sessions from the lightning strikes to their detection and validation of IRI model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10018, https://doi.org/10.5194/egusphere-egu2020-10018, 2020.

EGU2020-11130 | Displays | EMRP2.2

Turbulence and Plasma Inhomogeneity Observed by Swarm Constellation

Paola De Michelis, Giuseppe Consolini, Georgios Balasis, and Jerome Bouffard and the INTENS Team

The ionospheric environment is a complex system where dynamic phenomena, such as turbulence (fluid and magnetohydrodynamics) and plasma instabilities generally occur as a consequence of the coupling processes among solar wind, magnetosphere and ionosphere. It has been suggested that the turbulent character of the ionospheric plasma density also enters into the formation and dynamics of ionospheric inhomogeneities and irregularities, which essentially characterize the active equatorial, mid-latitude and polar regions. The ionospheric turbulence indirectly plays an important role also in the framework of space weather when due to the arrival of solar perturbations the plasma, the energetic particle distributions, the electric and magnetic fields within the magnetosphere and ionosphere are deeply modified thus paving the way for an increase in the ionospheric turbulence. Recent findings within the ESA funded project “Characterization of IoNospheric TurbulENce level by Swarm constellation (INTENS)” permitted us to investigate the role played by the turbulence on scales from hundreds of kilometers to a few kilometers in generating multi-scale plasma structures and inhomogeneities in the ionospheric environment at different latitudes. This presentation reports on the most promising results of the INTENS project regarding the investigation of turbulence and plasma conditions in the topside ionosphere using Swarm data.

How to cite: De Michelis, P., Consolini, G., Balasis, G., and Bouffard, J. and the INTENS Team: Turbulence and Plasma Inhomogeneity Observed by Swarm Constellation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11130, https://doi.org/10.5194/egusphere-egu2020-11130, 2020.

EGU2020-3760 | Displays | EMRP2.2

Integrated Science Operations of CASSIOPE e-POP with the Swarm Constellation for New Studies of Magnetosphere-Ionosphere Coupling

Andrew Yau, Andrew Howarth, H. Gordon James, David Knudsen, Richard Langley, and David Miles

The CASSIOPE Enhanced Polar Outflow Probe (e-POP) was originally envisioned as a low-cost, short-lifetime (18-month) small-satellite mission for investigating polar ion outflows and related magnetosphere-ionosphere coupling phenomena. However, e-POP is currently in its seventh year of continuing operation, as an addition to and as the fourth component of the Swarm constellation of satellites, under the European Space Agency Third Party Mission Programme.

Since 2017, the increased operation duty-cycle of e-POP has enabled the routine extension of its science operations to its full altitude range and to all latitudes, and made possible several new studies of important mid- and low-latitude topside ionospheric phenomena. In addition, the integrated e-POP and Swarm operation takes advantage of the synergy between the orbit characteristics and unique instrument capabilities between e-POP and Swarm, to enable or enhance a host of coordinated studies of magnetosphere-ionosphere coupling: including the Earth’s magnetic field and related current systems, auroral and upper atmospheric dynamics, and ionosphere-thermosphere and ionosphere-plasmasphere coupling processes. We present an overview of these new studies, focusing on their results on the effects of space weather in the ionosphere and upper atmosphere such as anomalous satellite orbit drag and ionospheric scintillation.

How to cite: Yau, A., Howarth, A., James, H. G., Knudsen, D., Langley, R., and Miles, D.: Integrated Science Operations of CASSIOPE e-POP with the Swarm Constellation for New Studies of Magnetosphere-Ionosphere Coupling, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3760, https://doi.org/10.5194/egusphere-egu2020-3760, 2020.

The Thermal Ion Imagers on Swarm A-C, and the Suprathermal Electron/Ion Imager on ePOP (now “Swarm-E”) provide a unique view of charged particle distribution functions in the ionosphere at high time resolution (up to 100 images/s). Through high resolution, CCD-based imaging (~3000 pixels/image), ion drift velocity is derived from these images at a resolution of 20 m/s or better, and in general agreement with velocities derived from ground based radars [1] and an empirical convection model [2]. This talk reviews recent scientific applications of this technique, which are wide-ranging and include mechanisms of ion heating and upflow [3,4], M-I coupling via Alfven waves [5,6], electron acceleration and heating by Alfven waves [7,8, 9], intense plasma flows associated with “Steve” [10,11], and electrodynamics of large-scale FAC systems[ 12], among others. In addition, future opportunities made possible by these data will be discussed.

[1] Koustov et al. (2019), JGR, https://doi.org/10.1029/2018JA026245

[2] Lomidze et al. (2019), ESS, https://doi.org/10.1029/2018EA000546

[3] Shen and Knudsen (2020a), On O+ ion heating by BBELF waves at low altitude, JGR, in revision.

[4] van Irsel et al. (2020), Highly correlated ion upflow and electron temperature variations in the high latitude topside ionosphere, submitted to JGR.

[5] Pakhotin et al. (2020), JGR, https://doi.org/10.1029/2019JA027277

[6] Wu et al. (2020a), Swarm survey of Alfvenic fluctuations and their relation to nightside field-aligned current and auroral arcs systems, JGR, in revision.

[7] Liang et al. (2019), JGR, https://doi.org/10.1029/2019JA026679

[8] Wu et al. (2020b), e-POP observations of suprathermal electron bursts in the ionospheric Alfven resonator, GRL, submitted.  

[9] Shen and Knudsen (2020b), Suprathermal electron acceleration perpendicular to the magnetic field in the topside ionosphere, JGR, in press.

[10] Archer et al. (2019), JGR, https://doi.org/10.1029/2019GL082687

[11] Nishimura et al. (2019), JGR, https://doi.org/10.1029/2019GL082460

[12] Olifer et al (2020), Swarm observations of dawn/dusk asymmetries between Pedersen conductance in upward and downward FAC regions, submitted to JGR.

 

How to cite: Knudsen, D.: Recent scientific findings based on high-resolution core plasma imaging of the ionosphere with Swarm and ePOP, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12192, https://doi.org/10.5194/egusphere-egu2020-12192, 2020.

EGU2020-20871 | Displays | EMRP2.2

Using Swarm to study ionosphere-thermosphere coupling

Johnathan Burchill

Properties and dynamics of ionosphere-thermosphere coupling may be investigated using observations from the Swarm electric field instruments (EFI). We illustrate this claim using measurements of vertical ion drift and electron temperature made by the EFIs, within the context of ambipolar diffusion parallel to the geomagnetic field. The associated ambipolar electric field is difficult to measure directly. Rather, under conditions where the ambipolar electric field is assumed to be specified, the ion-neutral momentum transfer collision frequency may be derived from the EFI measurements. In this talk, the theory, measurements and methodology of this approach are presented. Statistical analysis reveals highly-correlated ion upflow and electron temperature. Derived collision frequencies are found to be within an order of magnitude of empirical estimates at Swarm altitudes. We speculate on the feasibility of using this technique to examine the dynamics of ionosphere-thermosphere coupling using Swarm.

How to cite: Burchill, J.: Using Swarm to study ionosphere-thermosphere coupling, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20871, https://doi.org/10.5194/egusphere-egu2020-20871, 2020.

EGU2020-11721 | Displays | EMRP2.2 | Highlight

Steve: The optical signature of subauroral ion drifts

William Archer, Bea Gallardo-Lacour, Gareth Perry, Jean-Pierre St.-Maurice, Stephan Buchert, and Eric Donovan

Little is currently known about the optical phenomenon known as Steve. The first scientific publication on the subject suggests that Steve is associated with an intense subauroral ion drift (SAID). However, additional inquiry is warranted as this suggested relationship as it is based on a single case study. Here we present eight occurrences of Steve with coincident or near‐coincident measurements from the European Space Agency's Swarm satellites and show that Steve is consistently associated with SAID. When satellite observations coincident with Steve are compared to that of typical SAID, we find the SAID associated with Steve to have above average peak ion velocities and electron temperatures, as well as extremely low plasma densities.

How to cite: Archer, W., Gallardo-Lacour, B., Perry, G., St.-Maurice, J.-P., Buchert, S., and Donovan, E.: Steve: The optical signature of subauroral ion drifts, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11721, https://doi.org/10.5194/egusphere-egu2020-11721, 2020.

EGU2020-19206 | Displays | EMRP2.2

Longitudinal Gradients in Field-Aligned Currents as Observed by Swarm

Octav Marghitu, Adrian Blăgău, and Joachim Vogt

Field-aligned currents (FACs) are closely related to aurora and a key component of the magnetosphere-ionosphere-thermosphere system. Large scale FAC structures, like Region 1 / Region 2, threading the whole auroral oval, as well as smaller scale FACs, associated with auroral arcs, are often assumed to consists of upward / downward current sheet pairs, uniform in longitudinal direction. While such a uniformity is consistent with the prevalent 1D symmetry of the auroral arcs and oval, longitudinal gradients may develop at times, for example when the 1D symmetry prepares to break, during the growth phase of auroral substorms. The Swarm mission provides optimum conditions to explore systematically longitudinal gradients in FACs, namely a proper spacecraft configuration, with the Swarm A / Swarm C pair lining up periodically with Swarm B at auroral latitudes, and high quality magnetic field data. The present report concentrates on a set of auroral events observed by the Swarm satellites, in this suitable configuration, during the first six months of the mission operational phase. At that time, the distance between Swarm A / Swarm C and Swarm B was in the range of a few 100 km, comparable to the length scale of electrojet currents associated with auroral arcs. Not surprising, longitudinal gradients in FACs are occasionally significant, a feature which is discussed with respect to the location, activity level, and substorm phase of the event.

How to cite: Marghitu, O., Blăgău, A., and Vogt, J.: Longitudinal Gradients in Field-Aligned Currents as Observed by Swarm, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19206, https://doi.org/10.5194/egusphere-egu2020-19206, 2020.

EGU2020-1166 | Displays | EMRP2.2

Multi-scale Analysis of Electromagnetic Energy Input using Swarm: Quantifying Key Scales in Magnetosphere-Ionosphere Coupling

Ivan Pakhotin, Ian Mann, Kai Xie, David Knudsen, and Johnathan Burchill

Electromagnetic energy transfer in magnetosphere-ionosphere coupling (MIC) is an inherently multiscale process, where the relative contributions of various scale sizes, linked to various auroral phenomena, are largely unknown. While work in previous decades has largely focused on large scales, in recent years with the development of new instrumentation smaller scale electromagnetic disturbances have once again come into focus. Recent work by the authors has demonstrated evidence that small-scale processes appear to be so important as to potentially account for a global interhemispheric asymmetry in ionospheric energy input. This study attempts to statistically quantify the contribution of energy at the small and mesoscales using Poynting flux, calculated using the unprecedented ESA Swarm mission dataset of simultaneous electric and magnetic field measurements at 16 Hz, with statistics now spanning several years. We find important contributions at small scales to the total energy budget, while at the same time noting that there appears to be a limit above which energy content tends to drop off. In the context of previous observations from other spacecraft this may shed light on key small-scale processes happening in and around the auroral acceleration region, in particular discrete arcs and Alfvén wave reflection from the ionosphere, which are important in forming inputs to coupled magnetosphere-ionosphere-thermosphere modelling studies.

How to cite: Pakhotin, I., Mann, I., Xie, K., Knudsen, D., and Burchill, J.: Multi-scale Analysis of Electromagnetic Energy Input using Swarm: Quantifying Key Scales in Magnetosphere-Ionosphere Coupling, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1166, https://doi.org/10.5194/egusphere-egu2020-1166, 2020.

EGU2020-6554 | Displays | EMRP2.2 | Highlight

Recent achievements from the Swarm mission on the low latitude space environment and combinations with other satellite missions

Claudia Stolle, Juan Rodríguez-Zuluaga, Chao Xiong, Yosuke Yamazaki, Guram Kervalishvili, and Lucas Schreiter

The Swarm three-satellite constellation mission provides high resolution and high-quality observations of the Earth’s magnetic field and of multiple parameters of the ionosphere, which lead to new knowledge on the Earth’s interior and space environment and help to investigate space weather effects on space technology. Several findings would otherwise not have been possible and demonstrate that missions like Swarm are indispensable for Earth and space exploration. In addition, aspects of longterm variations or enhanced understanding in temporal and spatial resolution on regional scales could be gained in combination with other missions. This presentation  focuses on recent achievements on the low latitude ionosphere. Examples include an empirical model of the occurrence of post-sunset equatorial plasma irregularities derived in combination with ten years of CHAMP geomagnetic data, an enhanced description of the Swarm irregularity observations together with regional maps of the South Atlantic ionosphere from GOLD, and the identification of differing GPS scintillation characteristics evoked by the irregularities in comparison with the lower orbit GOCE data. Equatorial electrojet and plasma data from Swarm also helped to empirically prove that Antarctic sudden stratospheric warming events, such as in September 2019, couple to the low latitude ionosphere through modified planetary waves.

How to cite: Stolle, C., Rodríguez-Zuluaga, J., Xiong, C., Yamazaki, Y., Kervalishvili, G., and Schreiter, L.: Recent achievements from the Swarm mission on the low latitude space environment and combinations with other satellite missions , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6554, https://doi.org/10.5194/egusphere-egu2020-6554, 2020.

EGU2020-8072 | Displays | EMRP2.2

Earth's core magnetic field model Mag.num and the IGRF 13 candidate

Martin Rother, Monika Korte, Jürgen Matzka, Achim Morschhauser, Claudia Stolle, and Foteini Vervelidou

The Earth's core magnetic field model Mag.num was the parent model for the GFZ IGRF 13 candidate submission. The model is based on geomagnetic ground observatory and Swarm satellite observations. Epochs 2020.0 and beyond were not covered by the data available at the time of submission and our results were based on predictions. In this study, we investigate the effect of the more recent available data on our results of the 2020.0 epoch and the predicted secular variation by generating an updated Mag.num version. We especially focus on the spatial and temporal patterns of the local geomagnetic field minimum of the South Atlantic Anomaly (SAA). Recently, global geomagnetic field models have shown that an additional, although shallow, secondary minimum at Earth's surface has developed since around 2005. The location and significance of the secondary minimum and of the saddle point between the two minima are assessed also in view of the respective differences among the candidate models.

How to cite: Rother, M., Korte, M., Matzka, J., Morschhauser, A., Stolle, C., and Vervelidou, F.: Earth's core magnetic field model Mag.num and the IGRF 13 candidate, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8072, https://doi.org/10.5194/egusphere-egu2020-8072, 2020.

EGU2020-9775 | Displays | EMRP2.2

The new WMM2020 and IGRF-13 models, and a retrospective analysis of IGRF secular variation

William Brown, Ciarán Beggan, Grace Cox, and Susan Macmillan

2020 marks the start of a new 5-year cycle and updated releases of the World Magnetic Model (WMM) and International Geomagnetic Reference Field (IGRF). These models provide a reference for the up-to-date internal geomagnetic field in 2020, and a prediction of its secular variation for the next 5 years, to 2025. While similar in some aspects, the two models have different specifications and many different users across diverse fields. They provide references to be used primarily for navigation (WMM) and geomagnetic coordinate systems (IGRF).

BGS produces the WMM in collaboration with the US’ NOAA/NCEI, while the IGRF is produced by an IAGA Div. V-MOD task force, this time consisting of fifteen teams across nine nations, including BGS. Here we present a summary of the production of the updated WMM2020 and IGRF-13, and BGS efforts to enable access to these models.

We also present a retrospective analysis of the predictive components of the candidate models for previous IGRF epoch’s secular variation. Recent epochs have seen notable geomagnetic jerks and the acceleration of the North magnetic dip pole, features not well represented by the constant SV format of models such as the IGRF. We assess the range of candidate models submitted for previous IGRF epochs, assess the accuracy of physically derived predictions versus mathematical extrapolations, and discuss the implications given the range of candidate models submitted for IGRF-13 secular variation over the next five years.

How to cite: Brown, W., Beggan, C., Cox, G., and Macmillan, S.: The new WMM2020 and IGRF-13 models, and a retrospective analysis of IGRF secular variation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9775, https://doi.org/10.5194/egusphere-egu2020-9775, 2020.

Conventional methods of seismic tomography, surface topography and gravity data analysis constrain distributions of seismic velocity and density at depth, all depending on temperature and composition of the rocks within the Earth. WINTERC-grav, a new global thermochemical model of the lithosphere-upper mantle constrained by state-of-the-art global waveform tomography, satellite gravity (geoid and gravity anomalies and gradiometric measurements from ESA's GOCE mission), surface elevation and heat flow data has been recently released. WINTERC-grav is based upon an integrated geophysical-petrological approach where all relevant rock physical properties modelled (seismic velocities and density) are computed within a thermodynamically self-consistent framework allowing for a direct parameterization of the temperature and composition variables. In this study, we derive a new three dimensional distribution of the electrical conductivity in the Earth's upper mantle combining WINTERC-grav's thermal and compositional fields along with laboratory experiments constraining the conductivity of mantle minerals and melt. We test the derived conductivity model over oceans by simulating a tidally induced magnetic field. Here, we concentrate on the simulation of M2 tidal magnetic field induced by the ocean M2 tidal flow that is modelled by two different assimilative barotropic models, TPXO8-atlas (Egbert and Erofeeva, 2002) and DEBOT (Ein\v spigel and Martinec, 2017). We compare our synthetic results with the M2 tidal magnetic field estimated from 5 years of Swarm satellite observations and CHAMP satellite data by the comprehensive inversion of Sabaka et al. (2018).

How to cite: Martinec, Z., Fullea, J., and Velimsky, J.: Probing the oceanic upper mantle using M2 tidal magnetic field, waveform tomography, satellite gravity field, surface elevation and heat flow data, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3869, https://doi.org/10.5194/egusphere-egu2020-3869, 2020.

EGU2020-8277 | Displays | EMRP2.2

Detection of Magnetic Signals from Ocean Circulation with Satellite Altimetry and Magnetometer

Aaron Hornschild, Jan Saynisch-Wagner, Christopher Irrgang, Johannes Petereit, and Maik Thomas

Electrically conducting sea-water moves through Earth's magnetic field and generates electromagnetic signals itself. These signals can be detected by space borne Earth observation technologies, like the Swarm satellite magnetometer mission. In contrast to already successfully detected ocean tidal magnetic signatures, the magnetic signals from ocean circulation are still unidentified in observations. However, the electromagnetic signals from the ocean circulation would be an additional, interesting source of information.
We propose, that satellite altimetry can be helpful in order to detect magnetic signals from ocean circulation. Sea surface height measurements allow to estimate depth-integrated current velocities by using the geostrophic approximation, which describes a balance between sea surface height gradients and horizontal currents. With the resulting integrated electric current density, the magnetic signals from ocean circulation can be calculated using an electromagnetic induction solver. In a further step, the estimations are a basis for the  separation of magnetometer observations and for data assimilation.
Therefore, it is necessary that the geostrophic approach reflects the realistic time behavior of electromagnetic signals from ocean circulation. Ocean model data allows to verify this approach with respect to the identification of magnetic signals from ocean circulation in satellite magnetometer observations. We present this analysis and report about the feasibility of this approach regarding the Swarm mission and possible future missions.

How to cite: Hornschild, A., Saynisch-Wagner, J., Irrgang, C., Petereit, J., and Thomas, M.: Detection of Magnetic Signals from Ocean Circulation with Satellite Altimetry and Magnetometer, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8277, https://doi.org/10.5194/egusphere-egu2020-8277, 2020.

EGU2020-13913 | Displays | EMRP2.2

Geomagnetic Virtual Observatories: Global monitoring of geomagnetic secular variation with Swarm data

Magnus Hammer, Christopher Finlay, Ciaran Beggan, William Brown, and Grace Cox

The ESA Swarm DISC Geomagnetic Virtual Observatories (GVO) project aims to apply the virtual observatory concept to Swarm magnetic field measurements. The Virtual Observatory concept is a data processing method which mimics the behavior of magnetic monthly-mean time-series measured at ground observatories but at fixed locations on a uniform global grid at satellite altitude instead. Here we present several new GVO data products consisting of the average time-series of vector magnetic field values, regularly distributed in space and time which are suitable for monitoring the geomagnetic field. The GVO products consist of an equal-area grid with separation spacing of 300 km and cadence of either 1 month or 4 months. Various levels of processing are applied to remove the effects of altitude change and satellite local-time differences to produce a consistent time series. It is known that monthly time-series can have strong local-time artifacts which are removed with four-monthly averages, though with a loss of temporal resolution. The GVO products are designed to make Swarm magnetic data more accessible to researchers studying the physics of the core dynamo process, and related phenomenon such are secular variation, geomagnetic jerks and rapid core dynamics. In addition, the GVO data products also provide valuable information for investigating magnetospheric and ionospheric magnetic signals on timescales of months and longer.

How to cite: Hammer, M., Finlay, C., Beggan, C., Brown, W., and Cox, G.: Geomagnetic Virtual Observatories: Global monitoring of geomagnetic secular variation with Swarm data, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13913, https://doi.org/10.5194/egusphere-egu2020-13913, 2020.

EGU2020-9957 | Displays | EMRP2.2

Denoising Swarm Geomagnetic Virtual Observatories using principal component analysis

Grace Cox, Will Brown, Ciaran Beggan, Magnus Hammer, and Chris Finlay

Geomagnetic Virtual Observatories (GVOs) use satellite measurements to provide estimates of the mean internally-generated magnetic field (MF) over a specified period (usually one or four months) at a fixed location in space, mimicking the mean values obtained at ground-based observatories (GOs). These permit secular variation (SV) estimates anywhere on the globe, thereby mitigating the effects of uneven GO coverage. Current GVO estimates suffer from two key contamination sources: first, local time sampling biases due to satellite orbital dynamics, and second, MFs generated in regions external to the Earth such as the magnetosphere and ionosphere. Current methods to alleviate this contamination have drawbacks:Averaging over four months removes the local time sampling bias at the cost of reduced temporal resolution

  1. Stringent data selection criteria such as night-time, quiet-time only data greatly reduce, but do not entirely remove, external MF contamination and result in a small subset (<5%) of the available data being used
  2. Removing model predictions for external MFs from the measurements also reduces noise, however such parameterisations cannot fully describe these physical systems and some of their signal remains in the data.

Here we present an alternative approach to denoising GVOs that uses principal component analysis (PCA). This method retains monthly resolution, uses all available vector satellite data and removes contamination from orbital effects and external MFs. We present an application of PCA, implemented in an open-source Python package called MagPySV, to new GVOs calculated as part of a Swarm DISC project.  The denoised data will be incorporated into a new GVO data set that will be available to the geomagnetism community as an official Swarm product.  

How to cite: Cox, G., Brown, W., Beggan, C., Hammer, M., and Finlay, C.: Denoising Swarm Geomagnetic Virtual Observatories using principal component analysis, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9957, https://doi.org/10.5194/egusphere-egu2020-9957, 2020.

EGU2020-18876 | Displays | EMRP2.2

Analysis of Swarm Electric Field Data in View of Tsunami Events and related Earthquakes

Wojciech Jarmolowski, Pawel Wielgosz, Anna Krypiak-Gregorczyk, and Beata Milanowska

Three Swarm satellites are equipped with Langmuir Probes (LP) measuring in-situ electron density of Earth electric field and POD GNSS receivers determining topside total electron content (TEC) in the upper ionosphere. It is proved that different events on the Earth and in its atmopshere have their own impact on Earth electric field, and the earthquakes are in this group. Many strong earthquakes induce tsunamis, which are also suspected as contributing to the gravity waves having an impact on the ionospheric TEC. These reasons encourage to the study on the sensitivity of Swarm LP and POD GNSS data to the abovementioned phenomena. Referring to the sensitivity of TEC data derived from GNSS stations to Earthquakes, sensitivity of GNSS and LP data at around 500 km high orbit is analyzed here. A similar orbital height can be found in case of many LEO missions equipped at least with GNSS POD receivers, which makes Swarm especially interesting data acquisition platforms.

The investigation of Swarm data in view of Tsunamis and earthquakes is difficult due to several factors. There are only three satellites, the two of which fly almost together, which gives in fact only two points of the survey. The orbital repetition period is long, which seriously limits the number of comparable observations in terms of the location and time of the day. Finally, the number of large earthquakes and tsunami events in time of Swarm science mission is low, and many Earthquakes do not coincide sufficiently with Swarm passes in time and space. All these factors, however, doesn’t exclude an opportunity of analyzing of Swarm data passes above the earthquakes of magnitude nearby 8, linked with the tsunamis reaching several decimeters.

Swarm LP data is detrended and analyzed before the earthquakes and also during the earthquakes and resulting tsunami events. The GNSS POD topside TEC from Swarm is analyzed together as a background for LP data. In-situ electron density disturbances occurring during a pass close to the earthquake is compared to selected STEC measurements between LEO and GNSS satellites. Additionally absolute STEC values from selected nearby ground stations are analyzed in order to  find existing correlations for detected disturbances in the electric and magnetic fields. All the observations are sparse in time and space, and therefore, leave some unanswered questions and uncertainties. However, several interesting perturbations over earthquake/tsunami events are observable in both Swarm LP data and GNSS TEC data.

How to cite: Jarmolowski, W., Wielgosz, P., Krypiak-Gregorczyk, A., and Milanowska, B.: Analysis of Swarm Electric Field Data in View of Tsunami Events and related Earthquakes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18876, https://doi.org/10.5194/egusphere-egu2020-18876, 2020.

EGU2020-4981 | Displays | EMRP2.2

Dynamical Complexity of Magnetic Storms at Swarm Altitudes Using Entropy Measures

Constantinos Papadimitriou, Georgios Balasis, Adamantia-Zoe Boutsi, Omiros GIannakis, Anastasios Anastasiadis, Ioannis A. Daglis, Paola De Michelis, and Giuseppe Consolini

Recently, many novel concepts originated in dynamical systems or information theory have been developed, partly motivated by specific research questions linked to geosciences, and found a variety of different applications. This continuously extending toolbox of nonlinear time series analysis highlights the importance of the dynamical complexity to understand the behavior of the complex solar wind – magnetosphere – ionosphere - thermosphere coupling system and its components. Here, we propose to apply such new approaches, mainly a series of entropy methods to the time series of the Earth's magnetic field measured by the Swarm constellation. Swarm is an ESA mission launched on November 22, 2013, comprising three satellites at low Earth polar orbits. The mission delivers data that provide new insight into the Earth's system by improving our understanding of the Earth's interior as well as the near-Earth electromagnetic environment. We show successful applications of methods originated in information theory to quantitatively studying complexity in the dynamical response of the topside ionosphere, at Swarm altitudes, focusing on the most intense magnetic storms of the present solar cycle.

How to cite: Papadimitriou, C., Balasis, G., Boutsi, A.-Z., GIannakis, O., Anastasiadis, A., Daglis, I. A., De Michelis, P., and Consolini, G.: Dynamical Complexity of Magnetic Storms at Swarm Altitudes Using Entropy Measures, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4981, https://doi.org/10.5194/egusphere-egu2020-4981, 2020.

EGU2020-5070 | Displays | EMRP2.2

Complex system perspectives of geospace electromagnetic environment research

Georgios Balasis, Michael A. Balikhin, Sandra C. Chapman, Giuseppe Consolini, Ioannis A. Daglis, Reik V. Donner, Juergen Kurths, Milan Palus, Jakob Runge, Bruce Tsurutani, Dimitris Vassiliadis, Simon Wing, Rune Floberghagen, Jesper W. Gjerloev, Jay Johnson, Massimo Materassi, Tommaso Alberti, Adamantia Zoe Boutsi, Constantinos Papadimitriou, and Anja Strømme

Learning from successful applications of methods originating in statistical mechanics or information theory in one scientific field (e.g. atmospheric physics or weather) can provide important insights or conceptual ideas for other areas (e.g. space sciences) or even stimulate new research questions and approaches. For instance, quantification and attribution of dynamical complexity in output time series of nonlinear dynamical systems is a key challenge across scientific disciplines. Especially in the field of space physics, an early and accurate detection of characteristic dissimilarity between normal and abnormal states (e.g. pre-storm activity vs. magnetic storms) has the potential to vastly improve space weather diagnosis and, consequently, the mitigation of space weather hazards. This presentation reports on the progress of a largely interdisciplinary International Team, combining expertise from both space physics and nonlinear physics communities, which was selected for funding by the International Space Science Institute (ISSI) in 2019. The Team attempts to combine advanced mathematical tools and identify key directions for future methodological progress relevant to space weather forecasting using Swarm, SuperMAG, and other space/ground datasets. By utilizing a variety of complementary modern complex systems based approaches, an entirely novel view on nonlinear magnetospheric variability is obtained. Taken together, the multiplicity of recently developed approaches in the field of nonlinear time series analysis offers great potential for uncovering relevant yet complex processes interlinking different geospace subsystems, variables and spatio-temporal scales. The Team provides a first-time systematic assessment of these techniques and their applicability in the context of geomagnetic variability.

How to cite: Balasis, G., Balikhin, M. A., Chapman, S. C., Consolini, G., Daglis, I. A., Donner, R. V., Kurths, J., Palus, M., Runge, J., Tsurutani, B., Vassiliadis, D., Wing, S., Floberghagen, R., Gjerloev, J. W., Johnson, J., Materassi, M., Alberti, T., Boutsi, A. Z., Papadimitriou, C., and Strømme, A.: Complex system perspectives of geospace electromagnetic environment research, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5070, https://doi.org/10.5194/egusphere-egu2020-5070, 2020.

EGU2020-8796 | Displays | EMRP2.2

A Deep Learning Technique for Automated Detection of ULF Waves in Swarm Time Series

Alexandra Antonopoulou, Constantinos Papadimitriou, Georgios Balasis, Adamantia Zoe Boutsi, Konstantinos Koutroumbas, Athanasios Rontogiannis, and Omiros Giannakis

Ultra-low frequency (ULF) magnetospheric plasma waves play a key role in the dynamics of the Earth’s magnetosphere and, therefore, their importance in Space Weather studies is indisputable. Magnetic field measurements from recent multi-satellite missions (e.g. Cluster, THEMIS, Van Allen Probes and Swarm) are currently advancing our knowledge on the physics of ULF waves. In particular, Swarm satellites, one of the most successful mission for the study of the near-Earth electromagnetic environment, have contributed to the expansion of data availability in the topside ionosphere, stimulating much recent progress in this area. Coupled with the new successful developments in artificial intelligence (AI), we are now able to use more robust approaches devoted to automated ULF wave event identification and classification. The goal of this effort is to use a deep learning method in order to classify ULF wave events using magnetic field data from Swarm. We construct a Convolutional Neural Network (CNN) that takes as input the wavelet spectra of the Earth’s magnetic field variations per track, as measured by each one of the three Swarm satellites, and whose building blocks consist of two convolution layers, two pooling layers and a fully connected (dense) layer, aiming to classify ULF wave events in four different categories: 1) Pc3 wave events (i.e., frequency range 20-100 MHz), 2) non-events, 3) false positives, and 4) plasma instabilities. Our primary experiments show promising results, yielding successful identification of more than 95% accuracy. We are currently working on producing larger training/test datasets, by analyzing Swarm data from the mid-2014 onwards, when the final constellation was formed, aiming to construct a dataset comprising of more than 50000 wavelet image inputs for our network.

How to cite: Antonopoulou, A., Papadimitriou, C., Balasis, G., Boutsi, A. Z., Koutroumbas, K., Rontogiannis, A., and Giannakis, O.: A Deep Learning Technique for Automated Detection of ULF Waves in Swarm Time Series, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8796, https://doi.org/10.5194/egusphere-egu2020-8796, 2020.

EGU2020-7122 | Displays | EMRP2.2

Update on scales and energetics of auroral field-aligned currents as observed by Swarm

Leonie Pick, Joachim Vogt, Adrian Blagau, and Nele Stachlys

Auroral field-aligned currents (FACs) are of key importance for the electromagnetic coupling and the energy transport in the magnetosphere-ionosphere system. We use Swarm multi-spacecraft magnetic and electric field measurements from a selection of auroral oval crossing events to advance our understanding of the spatial scales and the electromagnetic energy flux (Poynting flux) associated with sheets of auroral FACs. Our study comprises the derivation of a scale-dependent correlation function based on dual-satellite vectorial magnetic field perturbation time series, in order to identify and analyze planar current structures. Applying concepts from multi-point boundary crossing analysis to data from Swarm-A and Swarm-C, a correlation measure is constructed using the mean square deviation of the observed magnetic perturbations and an empirical pattern function. Peak correlations indicate the positions and the scales of auroral FAC sheets, which we contextualize with the magnetic local time, the geomagnetic latitude, and geomagnetic activity indices (e.g., AL). In a parallel strand of work, we estimate the associated Poynting flux from the combination of the magnetic field perturbations and those of the electric field as deduced from the observed cross-track ion drift velocity. We assess the quality of our Swarm-based estimate by a comparison to the Poynting flux given by the “Cosgrove-PF” empirical model, which is based on FAST data from 1996 to 2001 and available from NASA’s Community Coordinated Modeling Center. Connecting both strands of work, we check to what degree this data-model comparison depends on the current sheets’ spatial scale. Throughout the study, we adopt a framework for describing planar magnetic structures that facilitates error analysis and accommodates not only boundary analysis, but also single-spacecraft polarization techniques.

How to cite: Pick, L., Vogt, J., Blagau, A., and Stachlys, N.: Update on scales and energetics of auroral field-aligned currents as observed by Swarm, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7122, https://doi.org/10.5194/egusphere-egu2020-7122, 2020.

EGU2020-9644 | Displays | EMRP2.2

Hemispheric asymmetry in field-aligned and ionospheric horizontal currents from the Swarm satellite measurements

Abiyot Workayehu, Heikki Vanhamäki, and Anita Aikio

We present statistical investigation of the high-latitude ionospheric current systems in the Northern hemisphere (NH) and Southern hemisphere (SH) during low (Kp < 2) and high (Kp ≥ 2) geomagnetic activity levels. Nearly four years of vector magnetic field measurements are analyzed from the two parallel flying Swarm A and C satellites using the spherical elementary current system (SECS) method. The ionospheric horizontal and field-aligned currents (FACs) for each auroral oval crossing are calculated. The mean values of FACs, as well as the horizontal curl-free (CF) and divergence-free (DF) currents in 1o magnetic latitude by 1 h magnetic local time grid cells, are calculated for each hemisphere and activity level. To estimate the NH/SH current ratios for the two activity levels, we remove seasonal bias in the number of samples and in the Kp distribution by bootstrap resampling.

Averaging over all seasons, we found that for the low activity level the currents in the NH are stronger than in the SH by 12 ± 4 % for FAC, 9 ± 2% for the horizontal CF current and 8 ± 2% for the horizontal DF current. During the high activity level, the hemispheric differences are not statistically significant.

When making the statistical analysis for the four seasons separately, we find a seasonal dependence in the hemispheric asymmetry. During low Kp conditions, both FACs and horizontal currents are larger in the NH than SH with the largest difference observed in winter. In winter, the currents in the NH are larger than the SH by 21 ± 5 %  for FAC, 14 ± 3% for the horizontal CF current and 10±3%  for the horizontal DF current. During the high activity level, the asymmetry is smaller compared to the low activity level with the largest and smallest hemispheric differences observed in autumn and summer, respectively. In autumn, the currents in the NH are larger than the SH by 8 ± 5%  for FAC, 9 ± 2%  for the horizontal CF current and 8 ± 3%  for the horizontal DF current. Interestingly, during high Kp conditions, the NH/SH ratio of horizontal current is >1 in autumn and <1 in spring.

The physical mechanism producing the hemispheric asymmetry is not known. One hypothesis is that the local ionospheric conditions, such as magnetic field strength or daily variations in insolation may play a role. We present preliminary results indicating that only a small part of the seasonal dependence in the NH/SH total current ratios can be explained by variations in the background conductances caused by solar irradiance and affected by local hemispheric values of the magnetic field.

 

 

 

How to cite: Workayehu, A., Vanhamäki, H., and Aikio, A.: Hemispheric asymmetry in field-aligned and ionospheric horizontal currents from the Swarm satellite measurements, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9644, https://doi.org/10.5194/egusphere-egu2020-9644, 2020.

It has long been a goal of active experiments to understand the response of the ionosphere to the high-power high-frequency (HF) radio wave pumping. The altitudes of 400-500 km are of particular interest since they correspond to the transition from the region, in which the most intense plasma heating and artificial ionospheric turbulence are observed, to the region where the disturbed plasma escapes to the magnetosphere. No observational data on the properties of plasma turbulence induced by the high-power HF pumping at this altitudinal range existed, until the emergence of a multi-satellite low-orbiting SWARM mission.

A series of experiments were conducted with a conjunction between the midlatitude SURA ionospheric heating facility and the SWARM satellites. We present the first observations made by SWARM on the plasma perturbations and electric currents induced in the F2 region ionosphere by the O‐mode radio wave pumping. In the heated region, significant effects include a localized increase of the electron temperature accompanied by stratification of the electron density and the magnetic signatures of field‐aligned currents (FAC) of 0.01-0.02 μA/m2 densities. The upward FAC is confined within the central part of the artificially perturbed magnetic flux tube, while the return downward current flows through the ambient plasma adjoining to the boundary of the HF-disturbed region. The spatial structure and amplitude of FACs indicate the current system is likely associated with the unipolar diffusion and excitation of eddy electric currents in the topside ionosphere. Similar effects are revealed in the laboratory experiment but not previously observed in space. The spaceborne experimental information is being accumulated and further analysis is underway.

This study was supported by the Russian Foundation for Basic Research, grant 20-05-00166. 

How to cite: Lukianova, R., Frolov, V., and Ryabov, A.: SWARM observations of the artificial ionospheric plasma disturbances and field‐aligned currents induced by the SURA power HF heating, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11746, https://doi.org/10.5194/egusphere-egu2020-11746, 2020.

EGU2020-21247 | Displays | EMRP2.2

Hemispheric and seasonal variations in the cold plasma outflow source region: polar cap ionosphere electron density at 350–500 km

Spencer Hatch, Stein Haaland, Karl Magnus Laundal, Therese Moretto Jørgensen, Andrew Yau, Lindis Bjoland, Jone Peter Reistad, Anders Ohma, and Kjellmar Oksavik

The polar cap ionosphere (here defined as the region above 80° magnetic latitude) is the primary source region of cold plasma outflows observed in the magnetosphere. The two factors controlling cold plasma outflows are the availability of plasma in the polar cap ionosphere, and transport from the ionosphere to the magnetosphere. Some statistical studies have indicated that the former of these two factors, availability of cold plasma, is the limiting factor. We use 15 years of electron density measurements made by Swarm and CHAMP spacecraft, corrected for variations in observation altitude and solar activity, to investigate how variations in solar wind driving and local hemispheric season affect the polar cap ionosphere electron density Ne. We show that the dependence of Ne on the By component of the interplanetary magnetic field is apparently antisymmetric in the two hemispheres, that Nestatistically decreases with decreasing Dst index (i.e., increasing geomagnetic activity) and that Ne is apparently insensitive to the AE index. We also show that Ne distributions around March and September equinoxes display weak evidence of hemispheric asymmetry. We show that during local summer, observed Ne distributions under high solar wind driving conditions are relatively lower than Ne distributions under low solar wind driving conditions. During local winter the reverse is true, with Ne distributions under low solar wind driving conditions being relatively lower than Ne distributions under high solar wind driving conditions. Thus solar wind driving and seasonal effects may apparently both constructively and destructively interfere. Altitude variation in Swarm and CHAMP Nemeasurements is accounted for via an empirical scale height derived from 1687 conjunctions between Swarm B and either Swarm A or Swarm C during 2013–2019. The approximately linear dependence of Ne on F10.7 measurements is also accounted for. Swarm Ne measurements are additionally corrected using the Lomidze et al. (2018) calibrations.

How to cite: Hatch, S., Haaland, S., Laundal, K. M., Jørgensen, T. M., Yau, A., Bjoland, L., Reistad, J. P., Ohma, A., and Oksavik, K.: Hemispheric and seasonal variations in the cold plasma outflow source region: polar cap ionosphere electron density at 350–500 km, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21247, https://doi.org/10.5194/egusphere-egu2020-21247, 2020.

EGU2020-14042 | Displays | EMRP2.2

Derivation of the full current density vector in the Earth's ionosphere low- and mid-latitude F region using ESA's Swarm satellites

Martin Fillion, Gauthier Hulot, Patrick Alken, Arnaud Chulliat, and Pierre Vigneron

A new multi-spacecraft method to recover estimates of the average three-dimensional current density in the Earth's ionosphere is presented. It is demonstrated using the ESA's Swarm satellite constellation and by taking advantage of the favorable geometrical configurations during the early phase of the mission. The current density vector is calculated inside prisms whose vortices are defined by the satellite positions. The mathematical formalism differs from previous approaches such as the one known as the ”curlometer”. It makes use of the well-known curl-B technique and involves an inverse problem which allows for error propagation through the calculation. Data from the vector field magnetometers of the three satellites are used and special care is taken to characterize the errors on these data. The method is applied in the low- and mid-latitude F-region on 15 February 2014. It provides latitudinal profiles of the full current density vector together with the associated error bars in the morning and evening sectors. We observe several dynamical features such as clear signatures of field-aligned interhemispheric currents, potential signatures of the wind dynamo current system as well as mid-latitude east-west currents.

How to cite: Fillion, M., Hulot, G., Alken, P., Chulliat, A., and Vigneron, P.: Derivation of the full current density vector in the Earth's ionosphere low- and mid-latitude F region using ESA's Swarm satellites, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-14042, https://doi.org/10.5194/egusphere-egu2020-14042, 2020.

EGU2020-10515 | Displays | EMRP2.2

On the self-calibrated absolute vector data produced by the ASM absolute magnetometers on board the Swarm satellites, results and prospect

Gauthier Hulot, Pierre Vigneron, Jean-Michel Léger, and Thomas Jager

Satellites of the ESA Swarm mission carry Absolute Scalar Magnetometers (ASM) that provide the nominal 1 Hz scalar data of the mission and allow the calibration of the nominal fluxgate vector magnetometry payload. ASM instruments, however, also provide independent 1 Hz experimental self-calibrated ASM-V vector data. More than six years of such data have been produced since the launch of the mission in November 2013. They allow the construction of global geomagnetic field models fully capable of capturing the fast temporal evolution of the core field, illustrating the ability of the ASM instruments to operate as a stand-alone instrument for advanced geomagnetic investigations. In this presentation we will provide the latest update on the ASM-V data (soon to be released as a new Swarm product), report on our ongoing efforts to further use these data to improve the nominal data of the mission, and discuss the prospect offered by the planned use of a miniaturized version of this ASM on board the satellites of the NanoMagSat constellation. This nano-satellite project is currently undergoing a 6 months consolidation study funded by the ESA Scout mission program. With a launch planned in 2024, it aims at forming the basis of a low-cost constellation for permanent long-term monitoring of the geomagnetic field and ionospheric environment from space.

How to cite: Hulot, G., Vigneron, P., Léger, J.-M., and Jager, T.: On the self-calibrated absolute vector data produced by the ASM absolute magnetometers on board the Swarm satellites, results and prospect, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10515, https://doi.org/10.5194/egusphere-egu2020-10515, 2020.

EGU2020-9850 | Displays | EMRP2.2

Six years of Swarm: instruments and data quality status

Enkelejda Qamili, Jerome Bouffard, Filomena Catapano, Christian Siemes, Jan Miedzik, Lars Tøffner-Clausen, Stephan Buchert, Lorenzo Trenchi, Anja Stromme, and Pierre Vogel

The European Space Agency (ESA) Swarm mission, launched in November 2013, continue to provide the best ever survey of the geomagnetic field and its temporal evolution. These high quality measurements of the strength, direction and variation of the magnetic field, together with precise navigation, accelerometer, electric field, plasma density and temperature measurements, are crucial for a better understanding of the Earth’s interior and its environment. This paper will provide an overview of the Swarm Instruments and data quality status and product evolution after six years of operations, focusing on the most significant payload investigations to improve science quality, data validation activities and results along with future validation/calibration plans.

How to cite: Qamili, E., Bouffard, J., Catapano, F., Siemes, C., Miedzik, J., Tøffner-Clausen, L., Buchert, S., Trenchi, L., Stromme, A., and Vogel, P.: Six years of Swarm: instruments and data quality status, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9850, https://doi.org/10.5194/egusphere-egu2020-9850, 2020.

EGU2020-13403 | Displays | EMRP2.2

Swarm Langmuir Probe measurements : analysis and characterization of the data quality

Filomena Catapano, Stephan Buchert, Igino Coco, Ewa Slominska, Enkelejda Qamili, Lorenzo Trenchi, and Jerome Bouffard

Swarm is a three-satellite constellation mission launched by ESA in 2013 flying at an altitude of about 510 km for Swarm Bravo, and 460 km for Alpha and Charlie. The three satellites carry identical instruments continuously collecting ground-breaking data on the various components of the magnetic field and on the near-Earth environment and their dynamics. The Electric Field Instrument (EFI)  is composed by the Thermal Ion Imager (TII) and two Langmuir Probes (LPs) which measure the electron density, temperature and spacecraft potential with the cadence of 2Hz. The scope of this work is to provide an updated status of the L1B data derived from LP measurements, describing some of anomalies affecting the data products as well the outcomes of recent investigations aiming at further improving the science quality of the LP-based Swarm data.

How to cite: Catapano, F., Buchert, S., Coco, I., Slominska, E., Qamili, E., Trenchi, L., and Bouffard, J.: Swarm Langmuir Probe measurements : analysis and characterization of the data quality, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13403, https://doi.org/10.5194/egusphere-egu2020-13403, 2020.

EGU2020-22239 | Displays | EMRP2.2

Earth inner drift shells as observed by the Advanced Stellar Compass on Swarm

Jose M G Merayo, John L Joergensen, Peter S Joergensen, Matija Herceg, Mathias Benn, and Troelz Denver

Since launch in November 2013, the Swarm constellation of three satellites provides detailed measurements of the magnetic field of the Earth. To ensure the high accuracy of magnetic vector observation by Vector Field Magnetometer (VFM), the Swarm inertial attitude is determined by the micro Advanced Stellar Compass (μASC). Besides its primary function of attitude determination, the µASC is also capable of detecting particles with energies high enough to penetrate its camera shielding, where particles passing the focal plane CCD detector leave detectable ionization tracks. The typical shielding employed requires the minimum energy to penetrate >15MeV for electrons, > 80MeV for protons and >~GeV for heavier elements.

The signature of passing particle will only persist in one frame time, but the signature differs between electrons and protons. To ensure full attitude performance operations even during the most intense CMEs, the signatures are removed before star tracking. By counting the signatures, and using a model for the flux transport through the shielding, an accurate measure of the instantaneous high energy particle flux is achieved at each update cycle (250ms).

With this feature installed on all three Swarm spacecrafts, a hitherto unprecedented accurate mapping of the proton population around Earth is achieved at two distances, 450 and 530km.

The superrelativistic protons measured by the μASC (g>>1), travel at speeds very close to c, and bouncing between the North and South Earth sphere, encounters complex field structures for at least some of the time. The bounce period is much smaller than the Earth rotation period, and an east-west drift component is caused by the magnetic field gradient.

We will present observations of the trapped proton fluxes and show how the magnetic field affects their motion shells. Slightly deformed particle drift shells due to the magnetic field structure (for orbits with L>1.07) and the differential east-west drift as measured by the Swarm Alpha and Charlie satellites will be discussed.

How to cite: Merayo, J. M. G., Joergensen, J. L., Joergensen, P. S., Herceg, M., Benn, M., and Denver, T.: Earth inner drift shells as observed by the Advanced Stellar Compass on Swarm, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22239, https://doi.org/10.5194/egusphere-egu2020-22239, 2020.

EGU2020-22445 | Displays | EMRP2.2

Recent results from scientific ESA Swarm projects

Lorenzo Trenchi, Jerome Bouffard, Anja Stromme, Octav Marghitu, Kirsti Kauristie, Adrian Blăgău, Joachim Vogt, Balázs Heilig, and Péter Kovács

This presentation illustrates the recent results obtained in the context of scientific ESA Swarm projects. The project “Swarm data quality Investigation of Field-Aligned Current products, Ionosphere, and Thermosphere system” (SIFACIT) has been recently extended in order to achieve two additional objectives: To provide to users an open-source program package to estimate Field Aligned Current (FAC) density and quality indicators, using single- and multi-s/c methods from Swarm data; To study the Joule heating of the ionosphere–thermosphere system on multiple scales, using Swarm data, together with conjugate ground information and simulations.

The other project illustrated here is EPHEMERIS (nEw sPace weatHER inforMation Exploited from the SwaRm observatIonS). This project is investigating the Midlatitude Ionospheric Trough (MIT) with Swarm data, and will also develop a new MIT Swarm data product based on Swarm L1b Langmuir Probe (LP) data. The second part of the project will develop a quasi-real-time intermittency index (IMI) for the detection of ionosphere plasma irregularities along the Swarm orbit, which can be responsible for errors and loss of lock in GPS signals. A statistical comparison of the IMI index with GPS signal from ground based receivers will be performed, in order to identify the ionospheric irregularities at Swarm altitude responsible for scintillations in GPS signals.

How to cite: Trenchi, L., Bouffard, J., Stromme, A., Marghitu, O., Kauristie, K., Blăgău, A., Vogt, J., Heilig, B., and Kovács, P.: Recent results from scientific ESA Swarm projects, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22445, https://doi.org/10.5194/egusphere-egu2020-22445, 2020.

EGU2020-19908 | Displays | EMRP2.2

VirES for Swarm – Virtual Research Environment

Martin Pačes, Daniel Santillan Pedrosa, and Ashley Smith

VirES for Swarm [1] is a data manipulation and retrieval interface for the ESA Swarm constellation mission data products. It includes tools for studying various geomagnetic models by comparing them to the Swarm satellite measurements at given space weather and ionospheric conditions.

The list of the provided Swarm products is growing and it currently includes MAG (both, LR and HR), EFI, IBI, TEC, FAC, EEF, and IPD products as well as the collection of L2 SHA Swarm magnetic models, all synchronized to their latest available versions.

VirES provides access to the Swarm measurements and models either through an interactive visual web user interface or through a Python-based API (machine-to-machine interface). The latter allows integration of the users' custom processing and visualization.

The API allows easy extraction of data subsets of various Swarm products (temporal, spatial or filtered by ranges of other data parameters, such as, e.g., space weather conditions) without needing to handle the original product files. This includes evaluation of composed magnetic models (MCO, MLI, MMA, and MIO) and calculation of residuals along the satellite orbit.

The Python API can be exploited in the recently opened Virtual Research Environment (VRE), a JupyterLab based web interface allowing writing of processing and visualization scripts without need for software installation. The VRE comes also with pre-installed third party software libraries (processors and models) as well as the generic Python data handling and visualization tools.

A rich library of tutorial notebooks has been prepared to ease the first steps and make it a convenient tool for a broad audience ranging from students and enthusiasts to advanced scientists.

Our presentation focuses on the introduction of the new Virtual Research Environment and recent VirES evolution.

[1] https://vires.services

How to cite: Pačes, M., Santillan Pedrosa, D., and Smith, A.: VirES for Swarm – Virtual Research Environment, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19908, https://doi.org/10.5194/egusphere-egu2020-19908, 2020.

EMRP2.3 – Magnetic Observations from the Ground and Space: Opportunities and Challenges in the Space Weather Era

EGU2020-2309 | Displays | EMRP2.3

Telluric currents play a big role in interpreting geomagnetic variations

Ari Viljanen and Liisa Juusola

Fast geomagnetic variations of periods from seconds to hours and days are primarily produced by currents in the ionosphere and magnetosphere. There is always an associated secondary (internal, telluric) current system induced in the conducting ground and contributing to the total variation field measured by ground magnetometers. Mathematically, it is possible to fully explain the variation field by two equivalent current systems, one at the ionospheric altitude and another just below the ground. In practice, this separation is feasible using dense magnetometer networks.

A common way in space physics has been to implicitly neglect the internal part and interpret the ground field only in terms of ionospheric currents. As known from previous studies, this is often a reasonable assumption, since a typical internal contribution is about 30%. However, the situation is much different when the time derivative of the magnetic field (dB/dt) is considered. For the north European IMAGE magnetometer network, the internal part exceeds the external one nearly at all stations. The largest effects due to telluric currents occur at coastal sites close to highly-conducting ocean water and at inland locations close to highly-conducting near-surface anomalies.

This finding gives a new viewpoint for studies of geomagnetically induced currents (GIC), which are closely related to dB/dt. One key question is to understand which are the ionospheric drivers of big GIC events. We will demonstrate how the telluric currents can strongly modify field variations and especially dB/dt, and how this is correspondingly seen in equivalent current patterns. Consequently, it is recommended that the field separation is performed whenever it is feasible, i.e. a dense observation network is available.

How to cite: Viljanen, A. and Juusola, L.: Telluric currents play a big role in interpreting geomagnetic variations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2309, https://doi.org/10.5194/egusphere-egu2020-2309, 2020.

EGU2020-437 | Displays | EMRP2.3

Preliminary investigation of the possibility of GIC development in Greece

Adamantia Zoe Boutsi, Georgios Balasis, and Ioannis A. Daglis

Geomagnetically Induced Currents (GIC) constitute an integral part of the space weather research and a subject of ever-growing attention for countries located in the low and middle latitudes. A series of recent studies highlights the importance of considering GIC risks for the Mediterranean region. The HellENIc GeoMagnetic Array (ENIGMA) is a network of 4 ground-based magnetometer stations in the areas of Thessaly, Central Greece, Peloponnese and Crete in Greece that provides geomagnetic measurements for the study of pulsations, resulting from the solar wind - magnetosphere coupling. ENIGMA magnetometer array enables effective remote sensing of geospace dynamics and the study of space weather effects on the ground (i.e. GIC). ENIGMA contributes data to SuperMAG, a worldwide collaboration of organizations and national agencies that currently operate approximately 300 ground-based magnetometers. In this presentation, we exploit ENIGMA data in order to study the spatio-temporal variations of the geomagnetic field that emanate during active geospace conditions. Moreover, we investigate the possibility that these variations produce hazardous currents and provide an estimation of their intensity, focusing on the most intense magnetic storms of the present solar cycle.

How to cite: Boutsi, A. Z., Balasis, G., and Daglis, I. A.: Preliminary investigation of the possibility of GIC development in Greece, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-437, https://doi.org/10.5194/egusphere-egu2020-437, 2020.

EGU2020-19917 | Displays | EMRP2.3

The geoelectric structure of the Romanian underground and its contribution to the geoelectric hazard during the solar cycle 23

Venera Dobrica, Dumitru Stanica, Crisan Demetrescu, and Cristiana Stefan

A recent model of the Romanian lithosphere electric properties, based on magnetotelluric transects carried out in the past 50 years across main tectonic units, is used to assess the geoelectric hazard represented by geomagnetically induced currents (GICs) for certain space weather events. Based on the geomagnetic field recordings and on information regarding the underground electric conductivity, the surface geoelectric field associated to geomagnetic variations during several large geomagnetic storms of the solar cycle 23 (1986-1996) is determined using the plane wave approximation for the depth propagation of the geomagnetic disturbance. A comparison to the territory of the European continent is done as well.   

How to cite: Dobrica, V., Stanica, D., Demetrescu, C., and Stefan, C.: The geoelectric structure of the Romanian underground and its contribution to the geoelectric hazard during the solar cycle 23 , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19917, https://doi.org/10.5194/egusphere-egu2020-19917, 2020.

EGU2020-19203 | Displays | EMRP2.3

Field-aligned current ordering in ground and space measurements

Malcolm Dunlop, Junying Yang, Xiangcheng Dong, Mervyn Freeman, Neil Rogers, Jim Wild, Colin Forsyth, Jinbin Cao, Hermann Lühr, and Chao Xiong

The orientation of field-aligned current sheets (FACs) can be inferred from dual-spacecraft correlations of the FAC signatures between two Swarm spacecraft (A and C), using the maximum correlations obtained from sliding data segments. Statistical analysis of both the correlations and the inferred orientations shows clear trends in magnetic local time (MLT) which reveal behaviour of both large and small scale currents. The maximum correlation coefficients show distinct behaviour in terms of either the time shift, or the shift in longitude between Swarm A and C for various filtering levels. The lower-latitude FACs show the strongest correlations for a broad range of MLT centred on dawn and dusk, with a higher correlation coefficient on the dusk-side and lower correlations near noon and midnight, and broadly follow the mean shape of the auroral boundary for the lower latitudes corresponding to Region 2 FACs (and are most well-ordered on the dusk side). Individual events sampled by higher altitude spacecraft (e.g. the 4 Cluster spacecraft), in conjunction with Swarm (mapping both to region 1 and 2), also show two different domains of FACs: time variable, small-scale (10s km), and more stationary large-scale (>100 km) FACs. We investigate further how these FAC regimes are dependent on geomagnetic activity, focusing on high activity events. Both the statistical trends, and individual conjugate events, show comparable effects seen in the ground magnetometer signals (dH/dt) during storm/substorm activity and show distributions that are similar.

How to cite: Dunlop, M., Yang, J., Dong, X., Freeman, M., Rogers, N., Wild, J., Forsyth, C., Cao, J., Lühr, H., and Xiong, C.: Field-aligned current ordering in ground and space measurements, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19203, https://doi.org/10.5194/egusphere-egu2020-19203, 2020.

EGU2020-19826 | Displays | EMRP2.3

Electrojet estimates from mesospheric magnetic field measurements

Karl Laundal, Jesper Gjerloev, Sam Yee, Slava Merkin, Heikki Vanhamäki, Liisa Juusola, and Jone Reistad

The auroral electrojet is traditionally measured remotely with magnetometers on ground or in low Earth orbit. The long distance, more than 100 km, means that smaller scale sizes are not detected. Because of this, the spatiotemporal characteristics of the electrojet are not known. Recent advances in measurement technology give hope of remote detections of the magnetic field in the mesosphere, very close to the electrojet. We present a prediction of the magnitude of these disturbances, inferred from the spatiotemporal characteristics of magnetic field-aligned currents. We also discuss how a constellation of small satellites carrying the Microwave Electrojet Magnetogram (MEM) instrument (Yee et al., 2020), could be used to essentially image the equivalent current at unprecedented spatial resolution. 

How to cite: Laundal, K., Gjerloev, J., Yee, S., Merkin, S., Vanhamäki, H., Juusola, L., and Reistad, J.: Electrojet estimates from mesospheric magnetic field measurements, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19826, https://doi.org/10.5194/egusphere-egu2020-19826, 2020.

EGU2020-20938 | Displays | EMRP2.3

Remote Sensing of Magnetic Fields Induced by Electrojets From Space: Measurement Techniques and Sensor Design Requirements

Jeng-Hwa Yee, Jesper Gjerloev, Viacheslav Merkin, and Karl Laundal

The Zeeman effect of the O2 118 GHz spectral radiance measurements can be utilized to remotely measure the magnetic field perturbations at altitudes close to the auroral electrojets. The technique has been demonstrated using the measurements provided by the Microwave Limb Souncer onboard the Aura spacecraft.  The derived current-induced magnetic field perturbations were found to be highly correlated with those coincidently obtained by ground magnetometers and to be consistent with the well-known auroral electrojet current distribution thereby providing a strong argument for the validity of the technique. With today's technology, a 118 GHz instrument, can be miniaturized allowing it to fly on small satellites such as CubeSats.  A constellation of small satellites with each one carrying a number of these identical mini-radiometers would have the ability to provide simultaneous multipoint measurement of the magnetic field perturbations at altitudes close to the electrojet, thereby greatly advancing our understanding of the ionospheric current system.  In this paper, we present the Zeeman magnetic field sensing technique, the requirements and specifications of the instrument, and an example of a cost effectively cubesat mission that provides unprecedented measurements of the evolution and structure of the auroral electrojet system.

How to cite: Yee, J.-H., Gjerloev, J., Merkin, V., and Laundal, K.: Remote Sensing of Magnetic Fields Induced by Electrojets From Space: Measurement Techniques and Sensor Design Requirements, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20938, https://doi.org/10.5194/egusphere-egu2020-20938, 2020.

EGU2020-11647 | Displays | EMRP2.3

Short timescale magnetic field fluctuations and their impact on space weather forecasting

Lucia Santarelli, Paola De Michelis, and Giuseppe Consolini

The features of the horizontal intensity of the geomagnetic field fluctuations during a geomagnetically disturbed period are analyzed. The Empirical Mode Decomposition (EMD) method is applied to separate short timescale (T<200 min) and long timescale (T>200 min) magnetic field fluctuations, which have been suggested to be related to different physical processes. The magnetic fluctuations at long timescales (T>200 min) seem to show a large degree of correlation between solar wind parameters and magnetospheric dynamics proxies, while the magnetic field fluctuations at short timescales (T<200 min) seem to be essentially related to internal magnetospheric processes and not directly driven by interplanetary changes.

Daily maps of the short timescale magnetic field fluctuations during a selected period are analyzed in order to investigate their contribution to the total magnetic signal. The aim is to evaluate the role that the internal magnetospheric processes have on the magnetic signal recorded on the ground and to investigate their dependence on the geomagnetic activity level. A comparison between the two hemispheres is also shown. The obtained results can be useful in the Space weather framework. They show the magnetic field fluctuation forecasting requires the development of models that take into account not only the solar wind parameters but also the internal dynamics of the magnetosphere that although triggered by changes of the interplanetary conditions is not directly driven by solar wind/interplanetary magnetic field.

How to cite: Santarelli, L., De Michelis, P., and Consolini, G.: Short timescale magnetic field fluctuations and their impact on space weather forecasting , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11647, https://doi.org/10.5194/egusphere-egu2020-11647, 2020.

Temporal variations of the electric field in near-surface layer of the Earth are determined by many factors, among which strong disturbances of the magnetic field should be especially noted. Magnetic storms cause an increase in the ionospheric electric field, which leads to variations in the gradient of the electric field potential near the Earth's surface. We consider the effect of magnetic storms in variations in the electrical characteristics of the atmosphere at Geophysical observatory «Mikhnevo» of Sadovsky Institute of Geosphere Dynamics of Russian Academy of Sciences and at Center for geophysical monitoring of Moscow of Sadovsky Institute of Geosphere Dynamics of Russian Academy of Sciences. We used data from the continuous monitoring of three components of the magnetic field, vertical components of the atmospheric electric field and atmospheric current carried out in fair weather. Experimental data processing and analysis show that accompanying magnetic storms with geomagnetic K index more or equal 5 increased variations in the electric field and vertical atmospheric current are characterized by different morphological structures. It is currently difficult to interpret the data. Nevertheless, the research results can be of great help in the development and verification of theoretical and computational models for generating variations in the electric field as a result of strong geomagnetic disturbances.

How to cite: Riabova, S. and Spivak, A.: Variations of electrical characteristics of near-surface atmosphere of the Earth during magnetic storm, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20128, https://doi.org/10.5194/egusphere-egu2020-20128, 2020.

Geomagnetic field (GMF) variations from external sources are classified as regular diurnal or occurring during periods of disturbances. The most significant regular variations are the quiet solar daily variation (Sq) and the disturbance daily variation (SD). These variations have well recognized daily cycles and need to be accounted for before the analysis of the disturbed field. Preliminary analysis of the GMF variations shows that the principal component analysis (PCA) is a useful tool for extraction of regular variations of GMF; however the requirements to the data set length, geomagnetic activity level etc. need to be established.

Here we present preliminary results of the PCA-based Sq extraction procedure based on the analysis of the Coimbra Geomagnetic Observatory (COI) measurements of the geomagnetic field components H, X, Y and Z between 2007 and 2015. The PCA-based Sq curves are compared with the standard ones obtained using 5 IQD per month. PCA was applied to data sets of different length: either 1 month-long data set for one of 2007-2015 years or data series for the same month but from different years (2007-2015) combined together. For most of the analyzed years the first PCA mode (PC1) was identified as SD variation and the second mode (PC2) was identified as Sq variation.

How to cite: Morozova, A., Rebbah, R., and Pais, M. A.: Separation of the daily quiet variation from the geomagnetic field observations with the principal component analysis, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3423, https://doi.org/10.5194/egusphere-egu2020-3423, 2020.

The geomagnetic field shows a regular diunal variation at the middle and low latitudes during geomagnetic quiet time, which is called as solar quiet daily variation (Sq). It is mainly generated from the ionosphere dynamic current system in the E-region of ionosphere, which is controlled by the ionospheric diunal and semi-diunal tidal wind field. The variation of the Sq field is greatly related to the latitude and the local time, and its amplitude and the phase vary very slowly in the whole year. Furthermore, a significant day-to-day (DTD) variation is usually seen in the amplitude and the phase of the Sq. It is greatly related to many factors such as the conductivity and the wind field in the ionosphere, and states of the magnetosphere.

This work is primarily to investigate the seasonal variation of the amplitude of the Sq field on both north-and-south sides of the Sq current, by using of the hourly data of the geomagnetic horizontal field from 75 observatories at mid-and-low latitudes. The result indicates that there is a significant seasonal variation in the amplitude of Sq(H) at all observatories, which shows a great enhancement during equinoxes months. However, a notable latitudinal asymmetry is clearly seen between the northside and southside observatories. The amplitude of Sq(H) reaches the maximum value in autumn at northside observatories, but in spring at southside observatories. This latitudinal asymmetry is most likely to reflect the tilt of the ionosphere current vortex.

 

How to cite: Yingyan, W.: The latitudinal asymmetry of the seasonal variation of the amplitude of the Sq field, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13424, https://doi.org/10.5194/egusphere-egu2020-13424, 2020.

EGU2020-5276 | Displays | EMRP2.3

Solar quiet daily (Sq) geomagnetic variation during minimum of solar cycle 23/24

Anatoly Soloviev and Artem Smirnov

The most regular of all daily geomagnetic field variations is the so-called solar quiet, or Sq, variation. It is attributed to the two current vortices flowing in the E-region of the dayside ionosphere. We present an investigation of the time-dependent parameters of Sq variation for the historical minimum of solar activity in 2008. We apply "Measure of Anomalousness" algorithm to detection of magnetically quiet days. The global maps of seasonal Sq amplitudes of the three orthogonal components are derived using 75 INTERMAGNET and 46 SuperMAG stations at low and middle latitudes. The global Sq amplitudes are compared to the previous Coupled Magnetosphere-Ionosphere-Thermosphere (CMIT) model simulations and show good agreement. Significant variability was found in Sq(X) and Sq(Y) based on the solar activity and latitude, while almost no difference is observed in Sq(Z) for across all latitudes and seasons. We analyze equivalent Sq current system using observatory data from the Australian mainland and narrow European-African latitudinal segment. Sq current system also strongly depends on solar activity, as current vortices are strongest in the local summer-hemisphere and disintegrate during local winter. The dynamics of Sq variation along the solar cycles 23 and 24 is also discussed and compared to Swarm-based spherical harmonic Sq model.

How to cite: Soloviev, A. and Smirnov, A.: Solar quiet daily (Sq) geomagnetic variation during minimum of solar cycle 23/24, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5276, https://doi.org/10.5194/egusphere-egu2020-5276, 2020.

EGU2020-17741 | Displays | EMRP2.3

Cleaning magnetometer data using multi sensor configuration

Dragos Constantinescu, Hans-Ulrich Auster, Magda Delva, Olaf Hillenmaier, Werner Magnes, and Ferdinand Plaschke

Measuring the in situ magnetic field using space borne instruments requires either a magnetically clean platform and/or a very long boom for accommodating magnetometers sensors at a large distance from the spacecraft body. This significantly drives up the costs and time for building the spacecraft. Here we present an alternative sensor configuration and an algorithm allowing for ulterior removing of the spacecraft generated disturbances from the magnetic field measurements, thus lessening the need for a magnetic cleanliness program.

The Service Oriented Spacecraft Magnetometer (SOSMAG) onboard the Korean Geostationary Satellite GEO-KOMPSAT-2A (GK-2A) uses for the first time a multi-sensor configuration for onboard data cleaning. To remove the AC disturbances, a combination of the measurements from sensors placed at different positions from the disturbance sources is processed onboard. Sensor biases due to daily temperature variations are also removed using the specific SOSMAG sensor arrangement. 

 

How to cite: Constantinescu, D., Auster, H.-U., Delva, M., Hillenmaier, O., Magnes, W., and Plaschke, F.: Cleaning magnetometer data using multi sensor configuration, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17741, https://doi.org/10.5194/egusphere-egu2020-17741, 2020.

EMRP2.4 – Advancements in magnetic field studies and natural resources exploration

EGU2020-6860 | Displays | EMRP2.4

Sequential modelling of the Earth magnetic field

Vincent Lesur and Guillaume Ropp

Geomagnetic field models derived from satellite data cover now more than twenty years and are obtained through the processing and analyses of a massive amount of vector magnetic data. As our understanding of the geomagnetic field progresses, these models have to describe contributions of more and more sources with rather complex mathematical representation. In order to handle this increasing complexity and amount of available data, we use a sequential modelling approach (a Kalman filter), combined with a correlation based modelling step (Holschneider et al; 2016). In order to reach high temporal resolution for the core field, a sequence of snapshot models, 3-months apart, has been built. The main characteristics of the derived series of Gauss coefficients are the same as those of recently released field models based on classic modelling techniques. The results we obtained show the importance of a careful calibration of the Kalman prediction steps as well as applying Kalman smoother at the end of the modelling. We identify also the induced fields as the main limitation for an increase resolution of the core field. We will present how these induced fields have been handle in a recent version of the model and future steps to progress further in the representation of the different sources of the geomagnetic field.

How to cite: Lesur, V. and Ropp, G.: Sequential modelling of the Earth magnetic field, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6860, https://doi.org/10.5194/egusphere-egu2020-6860, 2020.

EGU2020-11741 | Displays | EMRP2.4 | Highlight

Implications of magnetic secular variation for interpretation of crustal field anomalies

Rick Saltus, Aaron Canciani, Brian Meyer, and Arnaud Chulliat

We usually think of crustal magnetic anomalies as static (barring some major seismic or thermal disruption).  But a significant portion of the crustal magnetic field is caused by the interaction of magnetic minerals with the Earth’s magnetic field.  This induced magnetic effect is dependent on the direction and magnitude of the ambient field.  So, of course, as the Earth’s magnetic field changes over time, the form and magnitude of induced magnetic anomalies will vary as well.  These changes will often be negligible for interpretation when compared with measurement and other interpretational uncertainties.  However, with the reduction of various sources of measurement noise and increased fidelity of interpretation, these temporal anomaly changes may need to be considered.

In addition to considerations relating to interpretation uncertainty, these temporal anomaly changes, if they are measured in multiple magnetic epochs, can theoretically provide valuable information for use in source inversion.  For example, since crustal magnetic anomalies arise from a combination of induced (dependent the ambient field) and remanent (not dependent on ambient field) magnetic sources, measurements of secular magnetic variation can assist in separating these two sources during inversion.

We will report modeling of the expected form and magnitude of predicted induced anomaly variations, the possible implications of these variations for data compilation and interpretation, and on the availability of relevant data for measuring them.  Recent research into the use of high-resolution magnetic anomaly maps for airborne magnetic navigation has also brought the issue of changing magnetic fields into focus.  Initial work indicates that changes in induced anomalies could affect navigation accuracy in certain situations.

How to cite: Saltus, R., Canciani, A., Meyer, B., and Chulliat, A.: Implications of magnetic secular variation for interpretation of crustal field anomalies, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11741, https://doi.org/10.5194/egusphere-egu2020-11741, 2020.

EGU2020-2003 | Displays | EMRP2.4

Reprocessing of aeromagnetic data under consideration of satellite data for interpretation and modelling

Jörg Ebbing, Dilixiati Yixiati, Eldar Baykiev, Peter Haas, Fausto Ferraccioli, and Stephanie Scheiber-Enslin

How to cite: Ebbing, J., Yixiati, D., Baykiev, E., Haas, P., Ferraccioli, F., and Scheiber-Enslin, S.: Reprocessing of aeromagnetic data under consideration of satellite data for interpretation and modelling, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2003, https://doi.org/10.5194/egusphere-egu2020-2003, 2020.

EGU2020-11204 | Displays | EMRP2.4

Tantalising new magnetic views of Precambrian and Pan-African age crustal architecture in interior East Antarctica

Fausto Ferraccioli, Graeme Eagles, Alexander Golynsky, Jorg Ebbing, Wu Guochao, Chris Green, Bruce Eglington, and Egidio Armadillo

East Antarctica is the least understood continent on Earth due to its vast size, major ice sheet cover and remoteness. Coastal outcrops and glacial erratics have yielded cryptic but nevertheless fascinating clues into up to 3 billion years of East Antarctica’s geological and tectonic evolution. These geological constraints represent in turn the pillars to address global geodynamic linkages between East Antarctica, Australia, India, South Africa and Laurentia in the growth, assembly and dispersal of Gondwana, Rodinia and Nuna during the complex evolution of Earth's supercontinent cycles. However, due to the lack of drilling, our ability to project, test and augment such supercontinental linkages and several speculative geological interpretations in the interior of the continent beneath the East Antarctic Ice Sheet remains very limited.

While airborne and satellite gravity data and seismology are providing key new constraints on crustal and lithosphere thickness and help unveil large-scale heterogeneity in the East Antarctic lithosphere, detailed imaging of the architecture of individual crustal domains and their tectonic boundaries relies critically on magnetic anomaly data interpretation.

Here we exploit ongoing analyses of a recent continental-scale magnetic anomaly compilation (ADMAP 2.0) (Golynsky et al., 2018, GRL) augmented by major new datasets we recently collected, processed and compiled over the Recovery and South Pole frontiers and enhanced satellite magnetic imaging to:

1) reveal a more complex mosaic of distinct but in several places still cryptic Precambrian crustal provinces that represent the building blocks of interior East Antarctica;

2) provide new geophysical constraints that can be used to test different hypotheses of East-West Gondwana amalgamation along several candidate suture zones, including in particular the Shackleton suture zone, which provides a unique window on several distinct Precambrian terranes at the inferred leading edge of the composite Mawson Continent, as well as unique occurrences of Pan-African age rocks of ophiolitic affinity and

3) re-assess potential paths and the significance of the Kuunga suture zone between Greater India and East Antarctica and re-evaluate the tectonic origin of a major magnetic and gravity lineament previously thought to delineate the Indo-Australo-Antarctic suture and finally

4) propose new surveys in other frontier regions including in particular the under-explored interior of Princess Elizabeth Land and Recovery Subglacial Highlands that are critical in order to test the possible connectivity of the Kuunga, Gamburstev and potentially also Shackleton suture zones. 

Finally, we showcase examples of how we are combining aeromagnetic and gravity interpretations for East Antarctica with global magnetic and gravity datasets, geochronology, geochemistry, geology, tectonics and paleomagnetic data in an evolving plate kinematic framework (in GPlates) to re-assess supercontinent reconstructions with particular emphasis so far on Nuna and Gondwana.

How to cite: Ferraccioli, F., Eagles, G., Golynsky, A., Ebbing, J., Guochao, W., Green, C., Eglington, B., and Armadillo, E.: Tantalising new magnetic views of Precambrian and Pan-African age crustal architecture in interior East Antarctica , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11204, https://doi.org/10.5194/egusphere-egu2020-11204, 2020.

EGU2020-12387 | Displays | EMRP2.4

On Compensating for Magnetometer Swing in UAV Magnetic Surveys

Callum Walter, Alexander Braun, and Georgia Fotopoulos

Natural resource exploration has advanced in recent years through integrating unmanned aerial vehicles (UAVs) with high-resolution magnetometer payloads. One design consideration when integrating these systems for mineral exploration applications is ensuring that the magnetic measurement quality is comparable to the previously established methods of terrestrial magnetic and aeromagnetic surveying. High-resolution optically pumped magnetometers, employing a resolution of 0.1 - 0.01 nT, are the standard magnetic sensors used in both manned terrestrial magnetic and aeromagnetic surveys. Integrating a high-resolution optically pumped magnetometer in a multi-rotor UAV payload bay will compromise the integrity of the total magnetic intensity (TMI) measurements due to the electromagnetic interference generated by the brushless permanent magnet synchronous motors and other onboard electromagnetic components. One solution involves physically suspending the high-resolution magnetometer below the resolvability limit of the electromagnetic interference via a semi-rigid mount. However, the swinging motions of the high-resolution magnetometer through the geomagnetic field while in this configuration have the potential to introduce periodic variations in the collected TMI data, compromising quality. Within this study, a UAV-borne aeromagnetic survey was conducted over a mineral exploration target to assess the potential impact of magnetometer swing on collected UAV-borne TMI data. A DJI-S900 multi-rotor UAV and a GEM Systems Potassium Vapour Magnetometer (GSMP-35U) were used to fly a 500 m by 700 m grid, using a line spacing of 25 m and a flight elevation of 35 m above the ground.The optically pumped magnetometer was suspended outside the resolvability limit of the electromagnetic interference below the UAV via a semi-rigid mount. A nine degrees of freedom inertial measurement unit (IMU) was fixed to the semi-rigid mount and a Kalman filter was applied to post-process the measurements calculating the positional variations (pitch, yaw and roll) of the magnetometer. Spectral analysis was applied to the UAV-borne TMI measurements and the IMU positional data assessing contributions to the TMI signal from the swinging, semi-rigidly mounted magnetometer. Periodic signals were observed within the recorded TMI data directly relating to the swinging frequency of the magnetometer in pitch and roll throughout flight. The amplitude of the periodic TMI variations was variable (< 1 nT – 5 nT) throughout the survey and depended on the horizontal gradient of the ambient magnetic field and the arc length of the magnetometer swing. The magnetometer swinging frequency (~0.35 Hz) was determined to be primarily dependant on the magnetometer suspension length. Overall, the wavelength of the periodic TMI variations due to the swinging motions was characterized with the IMU measurements and determined to be spectrally unique from the longer wavelength geological signals targeted within the survey area. Due to the wavelengths of the targeted and untargeted signals not spectrally overlapping, the TMI variations related to magnetometer swing noise were filtered out. The design factors controlling the wavelengths of the targeted geologic signals (flight speed) and untargeted magnetometer swing noise (suspension length) must be considered when integrating high-resolution magnetometers on multi-rotor UAVs, such that the wavelengths do not spectrally overlap and phase-based compensation algorithms are not required.

How to cite: Walter, C., Braun, A., and Fotopoulos, G.: On Compensating for Magnetometer Swing in UAV Magnetic Surveys, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12387, https://doi.org/10.5194/egusphere-egu2020-12387, 2020.

EGU2020-5320 | Displays | EMRP2.4

Assessing and ameliorating edge effects in magnetic data transformations

Peter Lelièvre, Dominique Fournier, Sean Walker, Nicholas Williams, and Colin Farquharson

Reduction to pole and other transformations of total field magnetic intensity data are often challenging to perform at low magnetic latitudes, when remanence exists, and when large topographic relief exists. Several studies have suggested use of inversion-based equivalent source methods for performing such transformations under those complicating factors. However, there has been little assessment of the importance of erroneous edge effects that occur when fundamental assumptions underlying the transformation procedures are broken. In this work we propose a transformation procedure that utilizes magnetization vector inversion, inversion-based regional field separation, and equivalent source inversion on unstructured meshes. We investigated whether edge effects in transformations could be reduced by performing a regional separation procedure prior to equivalent source inversion. We applied our proposed procedure to the transformation of total field magnetic intensity to magnetic amplitude data, using a complicated synthetic example based on a real geological scenario from mineral exploration. While the procedure performed acceptably on this test example, the results could be improved. We pose many questions regarding the various choices and control parameters used throughout the procedure, but we leave the investigation of those questions to future work.

How to cite: Lelièvre, P., Fournier, D., Walker, S., Williams, N., and Farquharson, C.: Assessing and ameliorating edge effects in magnetic data transformations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5320, https://doi.org/10.5194/egusphere-egu2020-5320, 2020.

EGU2020-18945 | Displays | EMRP2.4

Modelling of airborne Full Tensor Magnetic Gradiometry using data from the INFACT project

Jouni Nevalainen, Elena Kozlovskaya, Jukka-Pekka Ranta, Joan Marie Blanco, Moritz Kirsch, Richard Gloaguen, Michael Schneider, and Jens Kobow

The measurement of the magnetic field has been a “backbone” geophysical method in mineral exploration since the 17th century. The existing instrumentation that measures Total Magnetic field Intensity (TMI) are a routinely used in ground, borehole and airborne surveys. In the TMI intensity data it is possible to observe certain signatures of magnetised objects, but retrieval of both magnetisation intensity and shape of 3-D magnetised objects from TMI can be difficult due to the vector nature of magnetisation and fundamental non-uniqueness of potential fields interpretation. Moreover, the presence of magnetic material in the host rock and/or presence of remanent magnetisation are challenges for TMI data interpretation.

Full Tensor Magnetic Gradiometry (FTMG) measurements provide the complete description of the magnetic field and hence an opportunity to get more information on the size, shape and material property of the magnetic rock mass. This is because the signatures in magnetic field originating from a specific magnetic object is observed in all independent components of magnetic field gradient tensor and thus, joint analysis of these tensor components constrains the number of possible magnetic models that fit the same data. In addition, observing the full tensor of magnetic field makes it possible to estimate the remanent magnetization with respect to the induced magnetization field if no a-priori information of remanent magnetization is available.

Highly sensitive magnetometers based on SQUID (Superconducting QUantum Interference Devices) technology has been successfully adopted in FTMG airborne measurements during the past decade. This achievement has given magnetic methods a new opportunity in terms of purely magnetic data modelling. In our work the benefits of interpretation of tensor airborne FTMG data are demonstrated through forward modelling and inversion with the grid search multiobjective global optimisation. As a case study, we consider airborne FTMG data measured with Supracon® JESSY STAR system in Northern Finland during the INFACT project.

Acknowledgements: This study has been done in the framework of EU Horizon 2020 funded INFACT project (webpage: https://www.infactproject.eu).

How to cite: Nevalainen, J., Kozlovskaya, E., Ranta, J.-P., Blanco, J. M., Kirsch, M., Gloaguen, R., Schneider, M., and Kobow, J.: Modelling of airborne Full Tensor Magnetic Gradiometry using data from the INFACT project, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18945, https://doi.org/10.5194/egusphere-egu2020-18945, 2020.

EGU2020-13436 | Displays | EMRP2.4

New technique to diagnose the geomagnetic field based on the single circular current loop model

Zhaojin Rong, Yong Wei, Wenyao Xu, Dali Kong, Jun Cui, Chao Shen, Rixiang Zhu, Weixing Wan, Masatoshi Yamauchi, Jun Zhong, and Lihui Chai

A quick and effective technique is developed to diagnose the geomagnetic dipole field based on an unstrained single circular current loop model. In comparsion with previous studies, this technique is able to separate and solve the loop parameters successively. With this technique, one can search the optimum full loop parameters quickly, including the location of loop center, the loop orientation, the loop radius, and the electric current carried by the loop, which can roughly indicate the locations, sizes, orientations of the interior current sources. The technique tests and applications demonstrate that this technique is effective and applicable. This technique could be applied widely in the fields of geomagnetism, planetary magnetism and palaeomagnetism. The further applications and constrains are discussed and cautioned.

How to cite: Rong, Z., Wei, Y., Xu, W., Kong, D., Cui, J., Shen, C., Zhu, R., Wan, W., Yamauchi, M., Zhong, J., and Chai, L.: New technique to diagnose the geomagnetic field based on the single circular current loop model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13436, https://doi.org/10.5194/egusphere-egu2020-13436, 2020.

EGU2020-711 | Displays | EMRP2.4

Analysis of Geomagnetic Variability by Empirical Orthogonal Functions

Chi-Hua Chung and Benjamin Fong Chao

We examine the secular variations of global geomagnetic field on long temporal scales using the IGRF model given in Gauss coefficients for 1900 - 2020. We apply the Empirical Orthogonal Function (EOF) analysis to the geomagnetic field truncated at degree 6 and downward continue it to the core-mantle boundary (CMB) under the assumption of an insulating mantle. The first three EOF modes show the periods around 120, 75 and 60 years with corresponding spatial structures. These oscillational modes potentially support the manifestation of magnetic, Archimedes and Coriolis (MAC) waves in the stably stratified layer near CMB (Buffett, 2016). We also model and decompose the geomagnetic field to standing and drifting components according to trajectories of the Gauss coefficients similarly to Yukutake (2015). We then use the Complex EOF (CEOF) analysis on the drifting field. The results indicate the presence of the westward drift phenomenon but only weakly given the fact that the westward drift has only completed a fraction of a cycle during this time.

How to cite: Chung, C.-H. and Chao, B. F.: Analysis of Geomagnetic Variability by Empirical Orthogonal Functions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-711, https://doi.org/10.5194/egusphere-egu2020-711, 2020.

EGU2020-19012 | Displays | EMRP2.4

Simplified model for axial dipole moment of the geomagnetic field from Brownian fluctuations

Alberto Molina Cardín, Luis Dinis Vizcaíno, and María Luisa Osete López

The magnetic field of the Earth is generated in its core by the process called the geodynamo, which involves convection in the fluid and electrical conducting outer core. The evolution of this complex process is simulated by magnetohydrodynamic models, which provide the state of the core and the magnetic field at any point and any time of the simulation. Nevertheless, the complexity of these models implies a high computational cost. That is why conceptual simple models describing only the main mechanisms from a statistical perspective can also be useful.

In this work we present a conceptual model that reproduces the main features of the axial dipole moment (ADM) of the Earth magnetic field. It is based on the stochastic dynamics of two Brownian particles interacting with each other within a double-well potential. The obtained simulations are able to mimic the random temporal distribution of reversals and excursions and the asymmetric temporal evolution of ADM during reversals. The relation between the model features and the real mechanisms that lead to the observed behaviour is discussed.

How to cite: Molina Cardín, A., Dinis Vizcaíno, L., and Osete López, M. L.: Simplified model for axial dipole moment of the geomagnetic field from Brownian fluctuations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19012, https://doi.org/10.5194/egusphere-egu2020-19012, 2020.

EGU2020-16408 | Displays | EMRP2.4

Decomposing the geomagnetic field: oscillation modes and characteristics

Cristiana Stefan, Venera Dobrica, and Crisan Demetrescu

Using the COV-OBS.x1 (Gillet et al., 2015) main geomagnetic field model, covering the time span 1840–2020, respectively IGRF-13 (1900-2020), we decomposed the geomagnetic field at Earth’s surface in oscillation modes by means of empirical orthogonal functions (EOF) as well into a long term and a cyclic component using HP filtering (Hodrick and Prescott, 1997). Further, the long term component is filtered using a Butterworth filter (1930) with different cut-off periods in order to obtain oscillation at inter-centennial (> 100 years) and sub-centennial (60-90 years) timescales. The EOF analysis shows that the first three oscillation modes are characterized by periodicities of >100 years while modes 4 and 5 are characterized by dominant periodicities of 70-90 year. Although the variance of the modes 4 and 5 is rather small compared to that of the first three modes, these two modes are responsible for the detailed structure of the geomagnetic field. A comparison between the results of both methods is done as well.

How to cite: Stefan, C., Dobrica, V., and Demetrescu, C.: Decomposing the geomagnetic field: oscillation modes and characteristics, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16408, https://doi.org/10.5194/egusphere-egu2020-16408, 2020.

EGU2020-7054 | Displays | EMRP2.4

Applied magnetic cartography is a tool for understanding the self-organization of the planet Earth system and its energy supply

Tamara Litvinova, Дмитрий Кашик, and Сергей Тихомиров

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The energy supply of geodynamic processes is one of the most important factors in the evolution of the planet earth system.The continuity and relatively stable regime of the planetary dipole magnetic field of the Earth is due, first of all, to the constant level of rotation energy continuously generated by the Earth during its rotation around its axis. In the mantle, asthenosphere, and the earth's crust, the determining energy factor is the density inhomogeneity of matter.

The Earth’s magnetic field, which is 99% generated by its internal sources, responds to phase transitions, which are the basis of the processes of self-organization of the planet Earth system.

The report presents ideas about energy sources, mechanisms and patterns of formation, transformation and replenishment of its reserves will significantly increase the reliability of the interpretation of cartographic information about structural and geophysical anomalies and related mineralogenesis processes.

How to cite: Litvinova, T., Кашик, Д., and Тихомиров, С.: Applied magnetic cartography is a tool for understanding the self-organization of the planet Earth system and its energy supply , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7054, https://doi.org/10.5194/egusphere-egu2020-7054, 2020.

EGU2020-3915 | Displays | EMRP2.4

First Results of the 2019 Algerian Magnetic Repeat Station Network

Abdenacuer Lemgharbi, Abdeslam Abtout, Mohamed Hamoudi, Abdelhamid Bendekken, Fatma Annad, Abderrahman Hemmi, Abdallah Mansouri, Ener Aganou, Moussa Allili, and Anis Mazari

Abstract:

The second part of the history of the Algerian magnetic repeat station network goes back to 1989 when the new one was started with 37 stations. It was then followed by three other networks in 1993, 1997 and 2005. The first part of this history started at the beginning of the XXth and ended ca 1956.

After a 14-year break, we launched a new repeat stations network in February 2019. The number of carried out stations was increased to 51 to try to cover all the territory.

Each repeat station network consists of stations of periodically, say  5-6 years, measured of three components of the Earth's magnetic field. to try to derive the spatial distribution of the geomagnetic field of Algeria and it's secular variation. This periodicity is also very important for the need to update local as well as global geomagnetic field models such as the International Geomagnetic Reference Field (IGRF).

In this work we describe the new 2019 Algerian repeat station network. Then we will discuss the steps of the absolute measurements using two methods. The first one is called the ‘method of zero’ and the second one ‘method of residuals’. The accuracy and resolution of the instruments and data reduction used and their effect on the final results will as well be discussed. We derive the spatial distribution of the geomagnetic field, and its secular variation. Finally, we will show how local, for instance regional polynomial modeling, is the key issue.

Key words: geomagnetic repeat network, absolute measurements, zero method, residual method, magnetic maps of Algeria, secular variation.

How to cite: Lemgharbi, A., Abtout, A., Hamoudi, M., Bendekken, A., Annad, F., Hemmi, A., Mansouri, A., Aganou, E., Allili, M., and Mazari, A.: First Results of the 2019 Algerian Magnetic Repeat Station Network , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3915, https://doi.org/10.5194/egusphere-egu2020-3915, 2020.

EGU2020-22098 * | Displays | EMRP2.4 | Highlight

The World Digital Magnetic Anomaly Map: version 2.1

Jerome Dyment, Yujin Choi, Vincent Lesur, Andreina Garcia-Reyes, Manuel Catalan, Takemi Ishihara, Tamara Litvinova, and Mohamed Hamoudi

The World Digital Magnetic Anomaly Map (WDMAM) is an initiative of the IAGA (International Association of Geomagnetism and Aeronomy) supported by the CGMW (Commission for the Geological Map of the World) of UNESCO. The second version was released in 2015 (Dyment et al., 2015; Lesur et al., 2016), and mandate was given to the authors to update this version 2.0 using the same methodology as often as newly available data would make it necessary. Five better datasets justify the preparation and release of version 2.1: (1) the complete digital aeromagnetic map of Brasil made available to CGMW by Agência Nacional do Petróleo, Gás Natural e Biocombustíveis; (2) an improved version of the aeromagnetic map of Russia prepared at VSEGEI; (3) the second version of the Antarctic Digital Magnetic Anomaly maP (ADMAP; Golynsky et al., 2018) which construction results from a remarkable international effort during and after the Second International Polar Year; (4) a new map of the Caribbean plate and Gulf of Mexico resulting from the compilation and re-processing of existing marine and aeromagnetic data in the area (Garcia, 2018); and (5) a new compilation of marine magnetic data worldwide. The new map shows significant improvements over the previous versions and will be shortly available at wdmam.org.

How to cite: Dyment, J., Choi, Y., Lesur, V., Garcia-Reyes, A., Catalan, M., Ishihara, T., Litvinova, T., and Hamoudi, M.: The World Digital Magnetic Anomaly Map: version 2.1, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22098, https://doi.org/10.5194/egusphere-egu2020-22098, 2020.

EGU2020-3314 | Displays | EMRP2.4

Analysis of the Czech magnetic anomaly data obtained by ground-based and airborne magnetic surveys

Pavel Hejda, Dana Čápová, Eva Hudečková, and Vladimír Kolejka

The modern epoch of ground magnetic surveying activity on the Czech territory was started by the Institute of Geophysics by setting up a fundamental network of the 1st order in 1957-58. It consists of 199 points and was reoccupied in 1976-78 and 1994-96. The anomaly maps were constructed by subtraction of the IGRF model.

Extensive aeromagnetic measurements have been performed from 1959 to 1972 by permalloy probe of Soviet provenience. The accuracy of the instrumentation was about (and often above) 10 nT. The second period of airborne survey started in 1976. Thanks to the deployment of proton precession magnetometer, the accuracy improved to ~ 2 nT. Since 2004 the measurements were carried out by caesium magnetometer. The data were digitized, known anthropogenic anomalies were cleared away and data were transformed to the regular grid with step 250 m. The final data file of magnetic anomalies ΔT, administered by the Czech Geological Survey, represents a substantial contribution to the exploration of ore deposits and to the structure geology in general.

In view of the fact that data file of magnetic anomalies was compiled from data acquired by heterogeneous methods in the course of more than 50 years, our recent study is aimed at looking into the homogeneity of the data by comparison them with ground-based magnetic survey. A simple comparison of the contour maps showed good similarity of the large regional anomalies. For more detailed analysis, the variation of ΔT in the neighbourhood of all points of the fundamental network was inspected and the basic statistic characteristics were computed. Summary results as well as several examples will be presented accordingly as the INSPIRE compliant services and eventually as the user-friendly web map application and made available on the CGS Portal http://mapy.geology.cz/ and on the updated web of the CzechGeo/EPOS consortium www.czechgeo.cz. Incorporating the map into the World Digital Magnetic Anomaly Map (WDMAM – IAGA) is also under consideration. This data will also be interesting for the EPOS.

How to cite: Hejda, P., Čápová, D., Hudečková, E., and Kolejka, V.: Analysis of the Czech magnetic anomaly data obtained by ground-based and airborne magnetic surveys, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3314, https://doi.org/10.5194/egusphere-egu2020-3314, 2020.

EGU2020-11192 | Displays | EMRP2.4

Tectonic structure and metallogeny of the Western Chukotka: insights from comprehensive geophysical dataset interpretation.

Kseniia Antashchuk, Alexey Atakov, Kirill Mazurkevich, and Oleg Petrov

Geological structure of western part of the Chukotka fold belt has been studied basing on the results of joint interpretation of geophysical data. The potential-field data, seismic and magnetotelluric data along two regional profile crossed the area and the off-shore seismic data obtained on the East Siberian sea were used in this study. The NE and NW oriented fault systems which control the mineragenous zones location were first detected and delineated. Joint interpretation of seismic and MT data along regional profiles allowed us to study: the deep structure of NW directed thrusts; the intrusion bodies morphology; the structural features of the Paleozoic and Mesozoic formations and the structure of volcanic deposits. The models of geological structure along regional profiles were used as a reference for potential field interpretation. Architecture of crystalline basement of the area was studied and several “steps” were detected. The depth of crystalline basement increases from north to south and reaches the largest depth under the volcanic deposits of Ochotsk-Chukotsk Volcanic Belt (OCVB).

How to cite: Antashchuk, K., Atakov, A., Mazurkevich, K., and Petrov, O.: Tectonic structure and metallogeny of the Western Chukotka: insights from comprehensive geophysical dataset interpretation., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11192, https://doi.org/10.5194/egusphere-egu2020-11192, 2020.

EGU2020-11275 | Displays | EMRP2.4

The structure of the Southeastern part of the Yano-Kolyma Fold Belt by the results of interpretation of various scale magnetic data

Denis Zubov, Kseniia Antashchuk, Alexey Atakov, Kirill Mazurkevich, and Marina Petrova

The wide range of anomalies caused by different geological structures from local to regional are studied by the heterogeneous datasets. They usually include the surveys of highly variable scales, resolution and quality. These parameters determine the methodology and technique used in further interpretation. The absence of detail and high quality surveys of geomagnetic field for large areas does not allow the implementation of the system analysis approach to full spectra of anomalies of magnetic field. The possibilities of system analysis using for various scale magnetic surveys to clarify of the tectonic settings and geological structure of the southeastern part of the Yano-Kolyma fold belt are considered. The geological structure of this area was studied earlier by the seismic and magnetotelluric investigations along 2DV regional profile. The tectonic settings are represented by several folded areas and cratons which are covered and knit together by Late Mesozoic bends and volcanic belt. The system interpretation of various scale magnetic surveys allowed us to obtain the geological and tectonic models of this area that include the following principal components: the deep structure of joint zones of different tectonic blocks; the structure and thickness of Paleozoic – Mesozoic deposits of sedimentary cover, crystalline basement and bends; the structure of volcanic belt deposits.

How to cite: Zubov, D., Antashchuk, K., Atakov, A., Mazurkevich, K., and Petrova, M.: The structure of the Southeastern part of the Yano-Kolyma Fold Belt by the results of interpretation of various scale magnetic data, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11275, https://doi.org/10.5194/egusphere-egu2020-11275, 2020.

EGU2020-11576 | Displays | EMRP2.4

New insights into the evolution of the Mt. Melbourne volcanic field (Northern Victoria Land, Antarctica) from high–resolution aeromagnetic data

Alessandro Ghirotto, Egidio Armadillo, Laura Crispini, Andrea Zunino, and Fausto Ferraccioli

The Mt. Melbourne field is interpreted as a quiescent volcanic complex, located in Northern Victoria Land, Antarctica, at the boundary between the Transantarctic Mountains (TAM) and the West Antarctic Rift System (WARS). It is one of the handful Antarctic volcanoes with the potential for large–scale explosive eruptions [1], with resulting key effects on the local environment and potentially on climate.

The geological and geophysical structure of this volcanic field remains poorly known, despite its key relevance to better comprehend the Cenozoic tectonic and geodynamic processes responsible for the opening of the WARS and the uplift of the TAM rift flank.

Here we present results derived from a novel high–resolution aeromagnetic dataset, collected in the austral summer 2002/2003 during the XVIII Italian Expedition, with the aim of investigating the geophysical structure of the main volcanic centres of the field.

Aeromagnetic data were processed and Digital Enhancement and Depth to Magnetic Source analysis performed to reveal the distribution of the main fault systems affecting the Mt. Melbourne volcanic field, particularly beneath the ice–covered areas. The results highlight NNE–SSW, NW–SE and E–W trending structural systems, in agreement with the available tectonic information for the study area [2, 3]. Furthermore, similar NNW–SSE trending pervasive negative anomalies are detected beneath both the Mt. Melbourne edifice and Cape Washington, superimposed by positive ones forming radial patterns.

With the aid of laboratory magnetic susceptibility data from rock samples collected in the field [4], we carried out forward and inverse modeling across the volcanic centres in order to image their subglacial internal structure.

Based on our results, considering the ambiguity and narrowness of the available geochronological data [1, 5, 6], we propose two (non–mutually exclusive) interpretative models to explain the evolution steps of the Mt. Melbourne volcanic complex. In the former, a major volcanic phase responsible for building of the inner part of the main volcanic centres likely occurred prior to the last magnetic polarity reversal (i.e. before 0.78 Ma, Matuyama Chron), explaining the negative anomalies detected as due to remnant magnetisation. During the Pleistocene–Holocene period, a following second volcanic phase put in place at shallower levels, primarily with present–day magnetization. In the alternative model, magma pulses originated at the lithospheric step between the thick East Antarctic craton and the thinner Ross Sea crust [7] caused i) widespread volcanism at the surface of the volcanic complex, particularly with the building up of the Mt. Melbourne edifice, and ii) a regional upward of the Curie isotherm at depth, causing partial de–magnetisation of the underlying volcanic rocks.

References:

[1] Giordano et al. (2012). Bull. Volcanol., 74, 1985-2005.

[2] Storti et al. (2006). J. Struct. Geol., 28, 50-63.

[3] Vignaroli et al. (2015). Tectonophysics, 656, 74-90.

[4] Pasquale et al. (2009). Ann. Geophys., 52(2), 197-207.

[5] Armstrong (1978). New Zeal. J. Geol. Geophys., 21(6), 685-698.

[6] Armienti et al. (1991). Mem. Soc. Geol. Ital., 46, 427-452.

[7] Park et al. (2015). Earth Planet. Sci. Lett., 432, 293-299.

How to cite: Ghirotto, A., Armadillo, E., Crispini, L., Zunino, A., and Ferraccioli, F.: New insights into the evolution of the Mt. Melbourne volcanic field (Northern Victoria Land, Antarctica) from high–resolution aeromagnetic data, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11576, https://doi.org/10.5194/egusphere-egu2020-11576, 2020.

The Rogaland Igneous Complex (RIC), in southwest Norway, is well known for its iron-titanium ore deposits (i.e. Storgangen and Tellnes), and potential apatite and vanadium-rich magnetite deposits. A better understanding of the subsurface structure of the complex and surrounding anorthosites will help to locate new mineral deposits, and in estimating the extent of the known mineralized zones. The RIC consists of anorthosites, leuconorites, mangerites, and the Bjerkreim-Sokndal (BK) layered intrusion. These igneous rocks were intruded into granulite facies rocks at 0.93-0.92 Ga, during the late-stage of the Sveconorwegian orogeny.

There is a strong correlation between the geology of the RIC and the magnetic and gravity anomaly patterns, with contrasting signatures between the three large anorthosite bodies (Egersund-Ogna, Haland-Helleren, and Ana-Sira) and the extensive BK layered intrusion.

Particularly, the Bouguer gravity map shows gravity lows over the anorthosites and the granulites, while a positive gravity anomaly ranging from 10 to 30 mGal correlates with the norite and mangerite rocks. In the aeromagnetic anomaly map, the anorthosites correlate with moderate to strong negative magnetic anomalies (below background) while mangerites and granulites have positive anomalies. More complex is the magnetic pattern over the BK layered intrusion. The latter is made up by 6 mega-cyclic units subdivided into a sequence of zones, defined by the presence or absence of certain index minerals which control the magnetic properties of the rocks and the magnetic pattern. This is clearly visible in the striking negative anomaly observed on the east limb of the Bjerkrem Lobe at Heskestad, with amplitude of -13000 nT in a high-resolution helicopter survey, and below -30000 nT in ground magnetic survey.

This area has long been explored, and a large set of geophysical data have been collected during multiple campaigns including gravity, seismic, airborne magnetic and radiometric data. Recently acquired ground magnetic data over the BK layered intrusion complement these data. Here, we used the geophysical data, and an extensive petrophysical dataset of over 1000 samples to investigate the shallow and deep structure of the RIC. A 3D gravity and magnetic model of the study area, built across multiple cross-sections, is presented. The BK layered intrusion is modeled in a doubly-plunging syncline structure and has a preliminary depth extent of approximately 4 km which agrees with previous seismic interpretations indicating the base at 4 to 5 km.

How to cite: Pastore, Z. and McEnroe, S.: 3D gravity and magnetic model of the Rogaland Igneous Complex in southwest Norway: a tank for ilmenite, apatite and magnetite resources , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18825, https://doi.org/10.5194/egusphere-egu2020-18825, 2020.

EGU2020-20023 | Displays | EMRP2.4 | Highlight

Crustal architecture of the largest pull-apart basin in East Antarctica unveiled

Laura Crispini, Fausto Ferraccioli, Egidio Armadillo, Andreas Läufer, and Antonia Ruppel

The West Antarctic Rift System (WARS) is known to have experienced distributed/wide mode extension in the Cretaceous, followed by narrow mode and variably oblique extension/transtension in the Cenozoic, the latter potentially linked to the onset of oceanic seafloor spreading within the Adare Basin (Davey et al., 2016, GRL). However, onshore the extent and impact of Cenozoic extension and transtension within the Transantarctic Mountains sector of East Antarctica is currently much less well-constrained from a geophysical perspective.

Here we combine aeromagnetic, aerogravity, land-gravity and bedrock topography imaging to help constrain the extent, architecture and kinematics of the largest Cenozoic pull-apart basin recognised so far in East Antarctica, the Rennick Graben (RG).

Enhanced potential field imaging reveals the extent of a Jurassic tholeiitic Large Igneous Province preserved within the RG and the inherited structural architecture of its basement, including remnants of uplifted ca 530-500 Ma arc basement in the northern Wilson Terrane and a ca 490-460 Ma subglacial thrust fault belt separating the Cenozoic western flank of the RG from the eastern margin of Wilkes Subglacial Basin (WSB).

The architecture of the RG is best explained in terms of a major composite right-lateral pull-part basin that extends from the Oates Coast to the Southern Cross Mountains block. We propose that Cenozoic strike-slip deformation kinematically connected the RG with both the western edge of the WARS and the eastern margin of the WSB. An earlier phase of left-lateral strike slip deformation is also emerging from recent geological field work in the study region but only relatively subtle offsets in aeromagnetic anomaly patterns are visible in currently available regional datasets.

We conclude that the RG is part of a wider distributed region of the continental lithosphere in East Antarctica that was deformed in response to an evolving Cenozoic transtensional tectonic setting that may have also affected enigmatic sub-basins such as the Cook Basins in the adjacent WSB region.

How to cite: Crispini, L., Ferraccioli, F., Armadillo, E., Läufer, A., and Ruppel, A.: Crustal architecture of the largest pull-apart basin in East Antarctica unveiled, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20023, https://doi.org/10.5194/egusphere-egu2020-20023, 2020.

EGU2020-1647 | Displays | EMRP2.4

Detection and Removal of Noisy Land Magnetic Data Spikes

Saad AlHumidan

Detection of land magnetic data spikes is an important issue in magnetic data processing. In addition, the presence of noisy contributions such as spikes, stripes and zigzag effects in magnetic data visualization represents the most common flaw that may degrade the image. Rendering the correct detection and identification of features very uncertain. In this study, a script called "Window_Despike" was written to mark the spike data points and give the index of each spike point. Spikes were replaced by linearly interpolating the adjacent "good" amplitudes, or replaced by Not a Number (NaN). Different windows size starting from window size of 3 till 9 compared and found that the best window size is 5.

How to cite: AlHumidan, S.: Detection and Removal of Noisy Land Magnetic Data Spikes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1647, https://doi.org/10.5194/egusphere-egu2020-1647, 2020.

Spectral analysis is among the most old and common techniques for the processing and interpretation of potential field data. This is related to the decay properties of the field power spectra which allows an easy estimation of the depths to the top and to the bottom of the sources of magnetic and gravity field anomalies. Such analysis can be accomplished however in different theoretical frameworks, assuming either a statistical ensemble of homogeneous sources or random fractal source distribution. Here, we present the many existing spectral analysis techniques to compare them with respect to estimating the depth to the source top and bottom. We evidence practical constraints on the depth estimation and inherent assumptions/limitations of the different approaches. Depth estimation using spectral methods requires a critical evaluation of window size, window location, and wavenumber range. Careful consideration of the merits and of the limitations of these different spectral techniques for different source distribution models may lead to robust and geologically meaningful outcomes. In fact, despite the several different approaches all the methods give quite consistent and often similar estimates of the source depths. However, due to ambiguities on the correction spectral factor, the best estimates are obtained if this factor is constrained by a priori information. Finally, we estimate the depth to the magnetic sources beneath Sicily, which may provide additional constraints to better understand the deep crustal geometry and thermal gradients of the region.

How to cite: Kelemework, Y. and Fedi, M.: Determination of depth to the magnetic sources using spectral analysis with application to Sicily, Southern Italy, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8863, https://doi.org/10.5194/egusphere-egu2020-8863, 2020.

EGU2020-13181 | Displays | EMRP2.4

Selecting Optimal Frequency Range for Estimating Depth to Magnetic Sources

Stefan Westerlund, Richard Chopping, Quanxi Shao, and Juerg Hauser

The depth to the top of the deepest magnetic source is a proxy for the depth to basement under the assumption that basement is magnetic rock covered by non-magnetic sediment. Spectral domain methods allow for rapid estimation of the depth of magnetic sources from a magnetic anomaly map by analysing the power spectra calculated from a window of magnetic field data. The choice of an appropriate window size is critical when employing these methods, due to the trade-off between robustness and locality. Larger windows are desirable as they include more data to average out randomness and noise, and larger windows are needed to observe the low-frequency components which relate to the deepest sources. But they may also include multiple objects which can confuse analysis and spatially smear results, so smaller windows are desirable for improved spatial resolution.

The three properties typically estimated are the depth to the bottom of the layer zb, the depth to the top of the magnetic layer zt and the magnetic fractal parameter β, which describes how the magnetic source changes with scale. For our purposes we are not interested in the depth to the underside of the layer and consequentially can use smaller windows as we do not require the low frequencies to constrain the depth to the bottom of the layer. Hence the minimum window size is now limited by the expected depth to the top of the layer. A wide frequency range is critical to best separate the effects of zt and β and obtain robust depth estimates. Above a certain frequency the spectrum is dominated by shallow sources and different types of noise; it no longer contains information about the deepest magnetic source. For a given window size we can obtain more robust estimates be carefully identifying this upper limit for the frequency range. The frequency at which the spectrum changes from being dominated by the magnetic layer to dominated by other sources can vary from location to location.

Here we therefore introduce a methodology that selects a locally optimal upper limit for the frequency range by analysing the goodness of fit as a function of this frequency. For each location we identify candidate frequencies based on the R2 value for a linear model for the power of the signal as a function of the frequency. From these candidate frequencies we chose the one resulting in the lowest root mean square error for the fitted spectral model. We use synthetic tests to derive an empirical relationship between the recoverable depth to the top of the magnetic layer and the window size; and illustrate the degree of undesirable spatial smoothing caused by an unnecessary large window for a given recoverable depth. Recovered trends for the depth to basement for Australia are comparable to solutions obtained from different sources of information. The true value of our improved spectral method though lies in its suitability for application in a real-time environment due to its efficiency.

How to cite: Westerlund, S., Chopping, R., Shao, Q., and Hauser, J.: Selecting Optimal Frequency Range for Estimating Depth to Magnetic Sources, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13181, https://doi.org/10.5194/egusphere-egu2020-13181, 2020.

EMRP2.5 – Electromagnetic induction in geophysics

EGU2020-7051 | Displays | EMRP2.5

Lateral crustal flow along discrete flat-lying lower crustal shear zones during Archean continent construction

Graham Hill, Eric Roots, Ben Frieman, Jim Craven, Richard Smith, Ras Haugaard, Saeid Cheraghi, and Ademola Adetunji

The nature of lithospheric evolution and style of the driving ‘tectonic’ processes occurring during Archean continent construction remain enigmatic. A significantly hotter thermal regime characterised the early Earth and was pervasive for much of the Archean. This resulted in construction of continents that were significantly weaker and unable to support the thick crustal sequences and topographies common to modern orogens. Gravitational collapse of these early continents may have occurred when deeper material became less dense by heating or partial melting and created a density contrast beyond the crustal competence and/or due to post-orogenic relaxation. Such a collapse could result in large scale horizontal spreading within the middle to lower crust and the development of lateral crustal flow along flat-lying shear zones producing fluid-deposited graphitic and metallic sulphide films at these depths, which, if preserved would produce broad scale quasi-horizontal mid-lower crustal low resistivity anomalies. Here we show 3D magnetotelluric resistivity models of the Archean Superior Province of Canada that reveal these types of anomalies that could represent lateral crustal flow in the middle to lower crust. Further, the magnetotelluric model shows narrow sub-vertical zones of low resistivity extending from the mid crust to the near surface, interpreted to represent remnant fluid pathways that potentially formed prior to gravitational collapse. These sub-vertical low resistivity features correlate spatially with crustal-scale deformation zones that potentially are host to hydrothermal ore deposits and abundant metasomatic mineral assemblages. The well preserved record of primary crustal amalgamation within the Superior Province of Canada with both features expected of autochthonous vertical ‘drip’ tectonics (sub-vertical fluid pathways) and allochthonous horizontal plate tectonics (flat-lying lower crustal shear zones) regimes, suggests a potential transitional period of tectonic evolution might have characterised the region during the late Archean.

How to cite: Hill, G., Roots, E., Frieman, B., Craven, J., Smith, R., Haugaard, R., Cheraghi, S., and Adetunji, A.: Lateral crustal flow along discrete flat-lying lower crustal shear zones during Archean continent construction, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7051, https://doi.org/10.5194/egusphere-egu2020-7051, 2020.

EGU2020-6386 | Displays | EMRP2.5

Probabilistic Cover-Basement Interface Characterization in Cloncurry, Australia, using Magnetotelluric Soundings

Hoël Seillé, Gerhard Visser, Jelena Markov, and Janelle Simpson

Cloncurry is located in the Mount Isa province in Queensland, NE Australia. The Mount Isa Province is a well-known metallogenic province in Australia which hosts many IOCG deposits. One of them is the Ernest Henry IOCG deposit, which was found below cover in the 90’s. The cover in this area comprises of regolith and the Jurassic-Cretaceous sediments of Eromanga and Carpentaria Basins. This deposit appears to belong to a complex mineral system which extend over the entire Cloncurry District.

A magnetotelluric (MT) survey was conducted in 2016 by Geoscience Australia and The Geological Survey of Queensland in the vicinity of the Ernest Henry IOCG deposit, in order to characterize the electrical properties of the mineral system beneath it. The derived 3D electrical conductivity model highlights the variable cover thickness over the area, and a correlation between conductors located in the upper crust and known mineral occurrences such as the Ernest Henry mine.

The use of 3D deterministic inversions of MT data is very powerful to image the electrical structure of the mineral system at the crustal scale but lacks resolution to image a realistic sharp cover-basement interface and precludes quantitative assessment of uncertainty around the results.

In this work we propose a workflow to image a geologically realistic cover-basement interface and bring insights on the reliability and robustness of different parts of the model using a probabilistic inversion approach.

We selected a subset of the MT survey and for each site we ran a probabilistic 1D trans-dimensional Markov chain Monte Carlo sampler for estimating subsurface conductivity and its associated uncertainty. These inversions are designed to be robust to non-1D effects present in the data. Next, we performed a petrophysical analysis using available down hole measurements to derive constraints on the electrical conductivities of the different lithologies found in the area. Then these petrophysical constraints, coupled to spatial lateral constraints, are used to fuse the 1D probabilistic ensembles into a 3D posterior ensemble.

The pseudo 3D model obtained is compared to a 3D model derived from a conventional 3D deterministic inversion using the same data to assess the value and validity of the workflow. Preliminary interpretation of the results is performed using petrophysical data and established local geology knowledge. Conclusions around the benefits of this workflow to give a different perspective on the characterization of a mineral system located under cover and to provide basis for future survey planning are presented.

How to cite: Seillé, H., Visser, G., Markov, J., and Simpson, J.: Probabilistic Cover-Basement Interface Characterization in Cloncurry, Australia, using Magnetotelluric Soundings, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6386, https://doi.org/10.5194/egusphere-egu2020-6386, 2020.

Space weather as a geohazard to modern technological infrastructure has come to the forefront of electromagnetic research in the past years. Geomagnetically induced currents (GICs) are generated by the rapidly changing magnetic fields during geomagnetic storms and sub-storms and the resulting induced electric fields into the ground. GICs can pose great risk to e.g. transformers in HV power grids and their monitoring and modelling is an ongoing effort in many higher and mid-latitude countries. Modelling of GICs in HV power grids requires knowledge about the magnetic field variations, the induced electric field via a conductivity model or through the magnetotelluric (MT) impedance tensor, and a detailed representation of the grid topology.

In the UK we have traditionally used a thin-sheet model for the calculation of electric fields during storm times due to very limited availability of MT data, but also as a fast and computational cost-effective approach. Using the Differential Magnetometer Method (DMM) in several locations of the grid has enabled us to indirectly measure GICs and validate them against the model. Here we present a case study from a location in Scotland, where we incorporate the different approaches and data sets that combine to a comprehensive analysis of GICs in this subset of the UK power grid.

How to cite: Huebert, J., Beggan, C. D., Richardson, G. S., and Thomson, A. W. P.: Utilizing magnetotelluric and differential magnetometer measurements for the validation of geomagnetically induced current models in a complex power network, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8374, https://doi.org/10.5194/egusphere-egu2020-8374, 2020.

There is a significant interest in constraining the mantle conductivity beneath oceans. One of the main sources of data that can be used to reveal the conductivity distribution in the oceanic mantle are time-varying magnetic fields measured at island geomagnetic observatories. From these data local electromagnetic (EM) responses are estimated and then inverted in terms of conductivity. The challenge here is that island responses are strongly distorted by the ocean induction effect (OIE) originating from the lateral conductivity contrasts between the conductive ocean and resistive land. OIE is generally modeled by global simulations using relatively coarse grids (down to 0.25 degree resolution) to represent the bathymetry. Insufficiently accurate accounting for the OIE may lead to the wrong interpretation of the observed responses. We study whether the small-scale bathymetry features influence the island responses. To address this question we developed a global-to-Cartesian 3-D EM modeling framework based on a nested integral equation approach, which allows to efficiently account for the effects of high-resolution bathymetry. Two geomagnetic observatories, located in Indian (Cocos Island) and Pacific (Oahu Island) Oceans, are chosen to study the OIE in long-period responses. Numerical tests demonstrate that accounting of the very local bathymetry (down to 1 km resolution) dramatically change modeling results. Remarkably, the anomalous behavior of the imaginary parts of the responses at Cocos Island, namely, the change of sign at short periods, is reproduced by using highly detailed bathymetry.

How to cite: Chen, C., Kruglyakov, M., and Kuvshinov, A.: Global-to-Cartesian 3-D EM modeling using a nested IE approach with application to long-period responses from island geomagnetic observatories, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8993, https://doi.org/10.5194/egusphere-egu2020-8993, 2020.

EGU2020-135 | Displays | EMRP2.5

3D imaging of the subsurface electrical conductivity structure in West Bohemia covering mofettes and Quaternary volcanic structures by using magnetotellurics

Anna Platz, Ute Weckmann, Josef Pek, Svetlana Kováčiková, Radek Klanica, Johannes Mair, and Basel Aleid

The West Bohemian Massif represents the easternmost part of the geo-dynamically active European Cenozoic Rift System. This region hosts different tectonic units, the NE-SW trending Eger Rift, the Cheb Basin and a multitude of different faults systems. Furthermore, the entire region is characterised by ongoing magmatic processes in the intra-continental lithospheric mantle. These processes take place in absence of active volcanism at surface, but are expressed by a series of phenomena, including e.g. the occurrence of repeated earthquake swarms and massive degassing of CO2 in the form of mineral springs and mofettes. Active tectonics is mainly manifested by Cenozoic volcanism represented by different Quaternary volcanic structures e.g. the Eisenbühl, the Kammerbühl and different maars. All these phenomena make the Eger Rift a unique target area for European intra-continental geo-scientific research. Therefore, an interdisciplinary drilling programme advancing the field of earthquake-fluid-rock-biosphere interaction was funded within the scope of the ICDP. Magnetotelluric (MT) measurements are applied to image the subsurface distribution of the electrical conductivity from shallow surface down to depths of several tens of kilometres. The electrical conductivity is a physical parameter that is particularly sensitive to the presence of high-conductive phases such as aqueous fluids, partial melts or metallic compounds. First MT measurements within this ICDP project were carried out in winter 2015/2016 along two 50 km long perpendicular profiles with 30 stations each and a denser grid of 97 stations close to the mofettes with an extension of 10 x 5 km2. Muñoz et al. (2018) presented 2D images along the NS profile of one regional profile. They reveal a conductive channel at the earthquake swarm region that extends from the lower crust to the surface forming a pathway for fluids into the region of the mofettes. A second conductive channel is present in the south of the model. Due to the given station setup, the resulting 2D inversion allows ambiguous interpretations of this feature. 3D MT data and inversions are required to distinguish between different scenarios and to fully describe the 3D structure of the subsurface. Therefore, we conducted a large MT field experiment in autumn 2018 by extending the study area towards the south. Broad-band MT data were measured at 83 stations along three 50-75 km long profiles and some additional stations across the region of the maars, the Tachov fault and the suture zone allowing for 2D as well as 3D inversion on a crustal scale. To improve the data quality, advanced data processing techniques were applied leading to good quality transfer functions. Furthermore, the previously collected MT data were reprocessed using the new approaches. This entire MT data set across the Eger Rift environment together with old MT data collected within the framework of the site characterisation in the surrounding of the KTB drilling are used to compute 3D resistivity models of the subsurface, with combining different transfer functions. These 3D inversion results will be introduced and discussed with regard to existing geological hypotheses.

 

How to cite: Platz, A., Weckmann, U., Pek, J., Kováčiková, S., Klanica, R., Mair, J., and Aleid, B.: 3D imaging of the subsurface electrical conductivity structure in West Bohemia covering mofettes and Quaternary volcanic structures by using magnetotellurics, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-135, https://doi.org/10.5194/egusphere-egu2020-135, 2020.

EGU2020-460 | Displays | EMRP2.5

Near-surface conductivity structures of quaternary volcanic maars in the Western Bohemian Massif: 3D imaging using the Radio-Magnetotelluric method

Gregor Willkommen, Radek Klanica, Světlana Kováčiková, Jan Mrlina, Anna Platz, and Ute Weckmann

As part of the Bohemian Massif, the Cheb Basin is one of the most active areas of the European Cenozoic Rift System. Separated from the ENE-WSW striking Eger Rift to the west by the morphological prominent Mariánské Lázne Fault Zone (MLF), the basin shows presently no active volcanism at the surface. Nonetheless it is characterized by degassing of mantle derived CO2 in mofettes and mineral springs and by repeated occurrences of swarm earthquakes along the Pocátky-Plesná Zone (PPZ) and MLF near Nový Kostel. All these activities are vivid signs of ongoing magmatic processes in the lithospheric mantle. Over the last 15 years four potential maar diatreme structures were discovered and join the two known scoria cones Komorní hurka and Zelezná hurka in the western part of the Cheb Basin. Unlike scoria cones there are no prominent morphological indications for maar diatreme structures, why only modern approaches in remote sensing and systematic gravimetrical surveys led to the discovery of the Mýtina Maar in 2007 (Mrlina et. al., 2007), the Neualbenreuth Maar in 2017 (Rohrmüller et. al., 2017) and recently the two potentials Ztracený rybník maars close to Libá (Hosek et. al., 2019; Mrlina et. al. 2019). All these quaternary volcanic structures are located very close along the Tachov Fault Zone (TFZ), one of the major NNW-SSE striking fault zones of the Bohemian Massif. Maar volcanoes were formed when rising magma interacts explosively with groundwater. Advancing explosions left a cone-shaped diatreme that has been filled with post-eruptive sediments which could conduce as a climate archive for the last 300.000 years in central Europe. An interdisciplinary Project "Drilling the Eger Rift" within the International Continental Scientific Drilling Program (ICDP) targets the interactions between fluids, deep biosphere, CO2 degassing and earthquake activity to shed light on the tectonic structure and related geodynamic processes. As a part of this project, Radio-Magnetotelluric (RMT) measurements were applied to image the near-surface electrical conductivity structure of these maar volcanoes. From May 2018 on, we conducted field experiments encompassing six 500 m RMT profiles across the Neualbenreuth maar, three 700 m profiles across Mýtina Maar and finally eight 400 - 1200 m long profiles over both Ztracený rybník maars. Compared with geo-electric resistivity tomography (ERT), our RMT measurements are more sensitive to conductors such as fluids or metallic compounds and were done with an areal coverage for 3D inversion and interpretation. With advanced and statistically robust data processing techniques typically applied to MT data resulted in impedance tensors in a period range of 10 kHz to 250 kHz. This RMT data sets are then modelled using inversion. The resulting 3D electrical conductivity models across the maar diatreme structures show distinct contrasts between the resistive rocks of the diatreme and the rather conductive post-eruptive sediments. The inversion results will be compared and discussed, in particular regarding a position for a potential core drilling in one of the maar structures. 

How to cite: Willkommen, G., Klanica, R., Kováčiková, S., Mrlina, J., Platz, A., and Weckmann, U.: Near-surface conductivity structures of quaternary volcanic maars in the Western Bohemian Massif: 3D imaging using the Radio-Magnetotelluric method, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-460, https://doi.org/10.5194/egusphere-egu2020-460, 2020.

EGU2020-18401 | Displays | EMRP2.5

Fluid network framing the hydrothermal system beneath Mt. Erciyes, Central Turkey

Mustafa Karaş, Serkan Üner, and Sabri Bülent Tank

Fluid contribution in a tectonic process is a crucial parameter for characterization of its products. In the geophysical point of view, illustrating the electrical resistivity structure of any tectonic system can be used to determine effects of fluid contribution. Magnetotellurics is globally used to decipher characteristics of fault zones, volcanoes and hydrothermal systems which are related to driving tectonic regime in collision and transition zones. Mt. Erciyes, which is the largest composite volcano of the Central Anatolian Volcanic Province in Turkey, developed in two particular stages during the Quaternary. Igneous activities in Koçdağ and New Erciyes stages created a plausible environment to observe dominant calc-alkaline products while alkaline and tholeiitic components are also present in the region. Geochemical evidences offer that fractional crystallization combined with low degree crustal assimilation were experienced during the formation of the volcano in addition to potential magma mixing processes occurred in the magma chambers. As part of NSF-funded Continental Dynamics/Central Anatolian Tectonics Project (CD/CAT), this study aims to investigate electrical resistivity structure beneath Mt. Erciyes by means of three-dimensional modeling of the wide-band magnetotelluric data collected at 48 sounding locations. Current model depicts a high conductivity anomaly beneath Mt. Erciyes, which corresponds to its hydrothermal system and related clay cap. Within Erciyes pull-apart basin, local branches of Ecemiş Fault Zone that possibly reinforced the convenient setting for the upwelling of volcanic materials, bound the interconnected highly conductive zones in shallow depths. Under these circumstances, a complex resistivity distribution arises as a consequence of various electrical transfer mechanisms contemplated for the study region.

How to cite: Karaş, M., Üner, S., and Tank, S. B.: Fluid network framing the hydrothermal system beneath Mt. Erciyes, Central Turkey, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18401, https://doi.org/10.5194/egusphere-egu2020-18401, 2020.

EGU2020-20994 | Displays | EMRP2.5

Improved interpolation scheme at receiver positions for 2.5D frequency-domain marine CSEM forward modelling

Gang Li, Shuangmin Duan, Hongzhu Cai, Bo Han, and Yixin Ye

We present an improved interpolation scheme for 2.5D marine controlled-source electromagnetic (CSEM) forward modeling problem. As the resistivity contrast between the seawater and seafloor sediment layers is significant, it is usually difficult to compute the EM fields accurately at receivers which are usually located at the seafloor. In this study, a new interpolation scheme at receivers is proposed, in which the interpolation of EM fields at the cell nodes for the whole computational domain to arbitrary receiver locations is discussed in detail. Numerical tests indicate that, our improved interpolation is more accurate for simulating the EM responses at receivers located on the seafloor, compared with the linear or rigorous interpolation.

How to cite: Li, G., Duan, S., Cai, H., Han, B., and Ye, Y.: Improved interpolation scheme at receiver positions for 2.5D frequency-domain marine CSEM forward modelling, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20994, https://doi.org/10.5194/egusphere-egu2020-20994, 2020.

While the Ultra Wide-Band Magnetotelluric technology (10,000Hz - 50,000 seconds) covers AMT, MT and LP bends in simultaneously measured time series, many research and exploration projects use dedicated MT (300Hz to 10,000 seconds) or LP (10 seconds - 50,000 seconds) systems. This is usually dictated by available equipment or a traditional approach to deep target studies. Surface anomalies (up to 2-5km) formed by conductive mineral bodies or fault systems considered to be less important and are often completely ignored during deep lithospheric studies. The upper layers conductivity is being estimated and averaged over the whole survey area. As it is very well known, the Magnetotelluric sounding signal measured on the surface represents an apparent resistivity at a depth dependent on frequency and conductivity of averaged ground thickness above. This assumption works generally well in smoothly layered geology, but might integrate an error in estimations and inversions in more complicated situations. In our previous studies we observed an effect of an upward shift of anomalies obtained after inversion of smaller grid size data for long period measurements. This seems to happen when the localized 3D conductive bodies are becoming dominant over the average layer conductivity and it cannot be assumed as a homogeneous thickness. In this study, our intention is analysing the efficient grid size that would be effective for MT band and LP band surveys. To achieve our estimated results, we designed a geoelectrical model that would be typical for Canadian shields with different grid sizes for MT band signal and LP band signals. We did run inversions using ModEM and observed vertical fit of inversion results to an original model.

How to cite: Vetrov, A. and Erdogan, E.: Synthetic Study for Optimizing an Efficient Grid Size for MT and Long Period MT Measurements, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22341, https://doi.org/10.5194/egusphere-egu2020-22341, 2020.

EGU2020-7908 | Displays | EMRP2.5

Introducing new global electromagnetic modeling solver

Mikhail Kruglyakov and Alexey Kuvshinov

In this contribution, we present novel global 3-D electromagnetic forward solver based on a numerical solution of integral equation (IE) with contracting kernel. Compared to widely used x3dg code which is also based on IE approach, new solver exploits alternative (more efficient and accurate) numerical algorithms to calculate Green’s tensors, as well as an alternative (Galerkin) method to construct the system of linear equations (SLE). The latter provides guaranteed convergence of the iterative solution of SLE. The solver outperforms x3dg in terms of accuracy, and, in contrast to (sequential) x3dg, it allows for efficient parallel computations, meaning that the code has practically linear scalability up to the hundreds of processors.

How to cite: Kruglyakov, M. and Kuvshinov, A.: Introducing new global electromagnetic modeling solver , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7908, https://doi.org/10.5194/egusphere-egu2020-7908, 2020.

EGU2020-1395 | Displays | EMRP2.5

3D CSEM Forward Modelling: Testing Adaptive Mesh Refinement Approaches on an Ore Body Model

Paula Rulff, Thomas Kalscheuer, Laura Maria Schmidt, and Mehrdad Bastani

We develop a forward modelling code to simulate 3D land-based controlled-source electromagnetic (CSEM) problems in frequency domain with unstructured tetrahedral meshes. The algorithm accounts for isotropic electrical resistivity and magnetic permeability variations in the subsurface. The latest addition to the software is a goal-oriented adaptive mesh refinement strategy driven by error estimators based on “face-jumps” of current density and magnetic flux density. In this study, we demonstrate that the goal-oriented adaptive refinement approaches are suitable to design a problem-specific mesh, which helps to solve 3D CSEM forward problems efficiently and accurately.
In mineral exploration, ore bodies often exhibit a strong resistivity contrast and sometimes a non-negligible contrast in magnetic permeability to their host rock. Accurate 3D modelling of electromagnetic measurement setups is therefore needed for feasibility studies and incorporation of the forward modelling in inversion approaches. To obtain sufficiently accurate solutions in time- and memory efficient computations, one option is to employ guided mesh refinement strategies. 
The so called goal-oriented adaptive mesh refinement method aims at designing a mesh, which is fine where necessary and coarse where discretisation errors do not influence the accuracy of the solution at the points of interest, typically the receiver sites. We apply the total electric field approach and first order Nédélec basis functions as interpolation functions defined on the edges of the finite elements to solve the electromagnetic diffusion equations. Thus, we achieve continuity of the electric and magnetic fields inside the elements and tangential to the edges and faces. However, the continuity of the normal components of current density and magnetic flux density across element interfaces cannot be ensured, resulting in small errors in the solution. We calculate these so called “face-jumps” and use them in combination with the elemental residuals and the dual solution of the problem to obtain error estimators that guide our adaptive refinement approach. The dual problem simulates influence sources at the receiver sites to weight the elemental error estimators with their influence to the solution accuracy at the receivers. 
We utilise a model of an iron ore body in central Sweden with a known magnetic permeability contrast and unknown electrical resistivity to study the behaviour of our implemented adaptive mesh refinement approaches. This is combined with a feasibility study to investigate the detectability of the ore body with CSEM. 
From literature examples on magnetotelluric forward modelling we know, that the error estimator based on the continuity of the normal current density shows robust performance, when modelling for electrical resistivity. We observe the same behaviour after adapting it to the controlled-source problem. The error estimator using the continuity of the magnetic flux density seems mathematically most promising to improve the mesh, when variations in magnetic permeability are significant. Numerical experiments with the ore body model indicate, that best results can be achieved, when mesh refinement guided by both error estimators is applied.


Acknowledgements: This work was supported by Smart Exploration project, which has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No.775971.

How to cite: Rulff, P., Kalscheuer, T., Schmidt, L. M., and Bastani, M.: 3D CSEM Forward Modelling: Testing Adaptive Mesh Refinement Approaches on an Ore Body Model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1395, https://doi.org/10.5194/egusphere-egu2020-1395, 2020.

EGU2020-21066 | Displays | EMRP2.5

Porosity estimates from marine controlled source electromagnetic dipole-dipole data at the Scanner Pockmark in the North Sea

Romina Gehrmann, Giuseppe Provenzano, Christoph Böttner, Naima Yilo, Gaye Bayrakci, Hector Marin-Moreno, Jonathan Bull, Tim Minshull, and Christian Berndt

As part of the EU Horizon2020 ‘STEMM-CCS’ project, controlled source electromagnetic (CSEM) and seismic data were acquired in 2017 at the Scanner Pockmark in the UK sector 15/25 of the North Sea, which is actively venting methane gas, to contribute to the evaluation of risk from potential fluid pathways to the sequestration of carbon dioxide in geological formations. We will present some preliminary results and relate electrical resistivities to sediment properties such as porosity and gas saturation.

The CSEM data presented were acquired with a University of Southampton deep-towed electric dipole source and two towed three axis dipole receivers (Vulcan, Scripps) along 12 profiles across an active pockmark. The data were processed in the frequency domain and the electrical resistivity structure was inferred with a 2D regularized inversion algorithm (MARE2DEM, K. Key).

To estimate porosities and their uncertainties to about 200 m below the seafloor, we use the empirical Archie’s law and calibrate Archie’s coefficient using physical properties measured with the multi-sensor core logger on gravity cores and sediment cores from the British Geological Survey Rock Drill 2 rig. Geological horizons identified on reflection seismic data are used as constraints in the resistivity model. The resulting porosity profile decreases with depth due to compaction and can be related to marine and glacial deposition environments.

How to cite: Gehrmann, R., Provenzano, G., Böttner, C., Yilo, N., Bayrakci, G., Marin-Moreno, H., Bull, J., Minshull, T., and Berndt, C.: Porosity estimates from marine controlled source electromagnetic dipole-dipole data at the Scanner Pockmark in the North Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21066, https://doi.org/10.5194/egusphere-egu2020-21066, 2020.

Injection of fluids (e.g. brines, CO2, steam) is commonly used in enhanced oil recovery (EOR) techniques to push crude oil in place towards the production wells. To optimize EOR procedures, it is essential to know the spatial propagation of injected fluids in the subsurface. Electromagnetic monitoring methods are particularly useful to decipher the spatio-temporal distribution of typically resistive oil versus typically conductive fluids.

We present an overview of soft- and hardware developments, modelling results, and time-lapse field data obtained over five years in an oilfield in NW Germany. CSEM modelling studies showed that conventional surface-based measurements alone do not provide sufficient resolution to changes within a thin (<15 m) reservoir structure located at ~1200 m depth. Combination with sources and/or receivers with vertical components increase sensitivity to such reservoirs very significantly. Based on these findings, a novel horizontal-vertical dipole source using the steel casing of a 1.3 km deep abandoned oil-well was successfully used for current injection in three time-lapse CSEM surveys (2014-2016) across the oilfield. We developed a novel numerical framework to compute the effect of metal casings on CSEM data and included it into our existing modelling and inversion (imaging) software. We also developed a receiver chain to measure the vertical electric field in a shallow observation borehole. Repeatability of the measured data – an essential prerequisite for any monitoring application – was excellent between the repeat surveys despite of high noise levels in an active oil field.

We also show results of a new numerical framework for 4D (time-lapse) CSEM inversion which allows direct imaging of changes within the 3D electrical conductivity structure of a reservoir. A cascaded inversion scheme in combination with a-priori information (conductivity constraints) and weighting of subdomains of the modelling space shows promising results in solving this mathematically ill-posed problem.

 

How to cite: Ritter, O., Patzer, C., and Tietze, K.: Reservoir monitoring with controlled source electromagnetics - a case study from a producing oil field in NW Germany, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16448, https://doi.org/10.5194/egusphere-egu2020-16448, 2020.

EGU2020-20232 | Displays | EMRP2.5

Conductive anomalies related to the Mérida Andes derived from the 3D inversion of a magnetotelluric profile

José Cruces, Oliver Ritter, Ute Weckmann, Kristina Tietze, and Michael Schmitz
Abstract
The geodynamic settings of north-western Venezuela are conditioned by the expulsion of the North Andean Block (NAB), and the deformational structures related to the Caribbean – South America plate interactions. The most prominent feature is the 100 km wide Mérida Andes (MA) that reach from the Colombian border to the Caribbean coast over more than 500 km. We present the analysis of a magnetotelluric (MT) profile acquired at 72 broadband sites along a 240 km long profile with station spacing between 3 and 5 km across the central part of the MA, including the sedimentary Maracaibo (MB) and Barinas-Apure (BAB) basins. Phase Tensors (PT) are consistent with 1D/2D dimensionality above the MB and the BAB, however, stations above the MA show a clear 3D dimensionality, with the induction vectors indicating the presence of off-profile structures. Following the dimensionality analysis, 3D inversions of the entire dataset were performed, employing the finite differences code ModEM. The results are in agreement with prominent geological structures, and are particularly effective in modelling the depth extensions of major fault systems. A conductive structure east from the profile at mid crustal levels seems to correlate with the tectonic escape of the Trujillo block, which is part of the NAB.

How to cite: Cruces, J., Ritter, O., Weckmann, U., Tietze, K., and Schmitz, M.: Conductive anomalies related to the Mérida Andes derived from the 3D inversion of a magnetotelluric profile , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20232, https://doi.org/10.5194/egusphere-egu2020-20232, 2020.

EGU2020-10695 | Displays | EMRP2.5

3-D Geoelectrical Characterisation of the Central Volcanoes of São Miguel Island (Azores Archipelago, Portugal) using Broad-Band Magnetotelluric Data

Duygu Kiyan, Colin Hogg, Volker Rath, Andreas Junge, Rita Carmo, Rita Silva, and Fatima Viveiros

The Azores islands are located at the triple junction between the North American, Eurasian and African plates. The Mid-Atlantic Ridge separates the North America from Eurasia and African plates, while Azores-Gibraltar Fracture Zone is the boundary between Eurasia and African plates. São Miguel Island, situated at the southeastern part of the western segment of the Azores-Gibraltar Fracture Zone, has three active strato-volcanoes, Sete Cidades, Fogo (Água de Pau), and Furnas. At Furnas and Fogo volcanoes, intense circulation of volcanic fluids at depth leads to high CO2 outgassing and flank destabilisation, whereas its neighbour Congro Fissural volcanic system, located between Fogo and Furnas volcanoes, experiences significant seismic swarm activity and poses considerable threat to the local population. Enhanced electrical conductivity values are typically associated with volcanic-hydrothermal systems and the modelled conductivity structures can provide constraints on these volcanic and hydrothermal processes.

Our previous work on Furnas volcano, which yielded a revised conceptual model developed from 39 high-frequency magnetotelluric soundings that imaged the hydrothermal system of the volcano to a depth of 1 km directly beneath the caldera, has now been expanded to include 35 additional broad-band magnetotelluric soundings from a recent field campaign conducted in late 2018, to image deeper and broader to gain new insights into the regional context of the Furnas volcanic system. The resistivity model of Furnas shallow hydrothermal system constructed from high-frequency dataset delineated two enhanced conductive zones, one at 100 m and another at 500 m depth, separated by a resistive layer. The shallow conductor has conductivity less than 1 S/m, which can be explained by clay mineral surface conduction with a mass fraction of at least 20% smectite. The deeper conductor extends across the majority of the survey area and is located at depths where smectite is generally not formed. We interpret this as the result of saline aqueous fluids near the boiling point, inferring temperatures of at least 240 oC. The less conductive layer found between these conductors is interpreted to be steam-dominated and coincides within the mixed-clay zone found in many volcanic hydrothermal systems. 3-D inversions using the deep-probing data indicate continuation of a strong conductive zone towards the south, beneath the 1630 Dome, which represents the most recent phase of eruptive activity in the multi-caldera complex. During the 2018 field campaign, we have enlarged our study to include 50 broad-band soundings on the adjacent Fogo (Água de Pau) volcano and Congro Fissural volcanic system. The Fogo-Congro region is subjected to seismic swarm activity and its relationship with the geoelectrical structure is being investigated.

How to cite: Kiyan, D., Hogg, C., Rath, V., Junge, A., Carmo, R., Silva, R., and Viveiros, F.: 3-D Geoelectrical Characterisation of the Central Volcanoes of São Miguel Island (Azores Archipelago, Portugal) using Broad-Band Magnetotelluric Data, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10695, https://doi.org/10.5194/egusphere-egu2020-10695, 2020.

EGU2020-13385 | Displays | EMRP2.5

Detecting lineaments in Northern Bavaria from magnetotelluric soundings

Eugenio D'Ascoli and Max Moorkamp

Northern Bavaria shows an elevated surface heatflow in combination with prominent lineaments
and potentially hydraulically active tectonic faults. Sharp resistivity contrasts, as they might appear
for geothermal fluids migrating along faults and altering the host rock resistivity environment, are
an ideal target for electromagnetic measurements. Magnetotelluric (MT) measurements have been
conducted in northern Bavaria in October 2019 for the investigation of the subsurface resistivity
structure of the lineaments and faults for possibly future geothermal explorations. Magnetotelluric
data sampled in highly populated areas are often contaminated with anthropogenic electromagnetic
noise and result in strong outliers in the impedance tensor estimates. A robust remote reference
method and several pre-stack data selection criteria have been applied in order to retrieve meaningful
estimates of the impedance tensor. To derive an image of the subsurface resistivity distribution a
three dimensional inverse modelling of the impedance tensor estimates has been applied.

How to cite: D'Ascoli, E. and Moorkamp, M.: Detecting lineaments in Northern Bavaria from magnetotelluric soundings, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13385, https://doi.org/10.5194/egusphere-egu2020-13385, 2020.

EGU2020-20745 | Displays | EMRP2.5

Integrated modelling based on shallow cross-gradient joint inversion and deep petrological approach on 2D/3D data in the Western Carpathians

Ján Vozár, Vladimír Bezák, Miroslav Bielik, Javier Fullea, and Max Moorkamp

We present the integrated geophysical modelling based on magnetotelluric (MT) method included in the crustal joint inversion with gravity data performed by JIF3D code and geophysical-petrological LitMod3D thermaly-selfconsistent mantle modelling. Performed geophysical modeling is primarily based on MT and regional gravity data with supporting information from seismic methods and geothermal data like Moho and lithospheric-asthenospheric boundary (LAB) depth used for building of the starting models. The integration among geophysical models is provided by the cross-gradient coupling method for the crustal structures and in the mantle, the coupling is provided petrological relationship based on compositional, temperature and pressure distribution information. The case study is focused on 3D modelling of the seismic 2T profile in central Slovakia crossing the major Carpathian tectonic units and the contact zone between European platform and Inner Carpathian block, which coincide with Carpathian Conductivity Anomaly (CCA).

The geoelectrical models from the 3D integrated modeling image the CCA in depths of about 10 - 20km and shows great improvement in comparison with 2D MT models. The CCA exhibits 3D features represented by the offset, along the fault, in the northsouth direction in the northern part of the modelled area. The four basic segments were identified in the crust structure of the central Slovakia part of the Western Carpathians. The southernmost physically distinctive segment with high full crust conductivity caused by young volcanic activity shows the presence of the partial melt, with high geoelectrical conductivity, in the middle and lower crust caused by higher heat flow. These structures are situated to the southwest from the profile and finger type conductors indicate its penetration in northeast direction. These volcanic processes in the south are not connected with CCA presence and its origin, which is supposed to be the presence of graphite or mineralized water in mylonitized rocks on the sheer contact zones of European platform and Inner Western Carpathians.

For mantle part of the integrated models, we studied different mantle compositions and fluid content within the lithospheric mantle to explain differences in electrical and seismic LAB. The calculated petrological conductivity model shows sensitivity of MT data on the LAB depth change, the correct input of composition parameters of lithospheric mantle and thermal field. The thermal steady state approximation was used to calculate surface heat flow in the area is lower than measured and estimated values from previous thermal studies. The differences between calculated and measured heat flow is primarily caused by high radiogenic production within the crust and not by the contribution from mantle.

How to cite: Vozár, J., Bezák, V., Bielik, M., Fullea, J., and Moorkamp, M.: Integrated modelling based on shallow cross-gradient joint inversion and deep petrological approach on 2D/3D data in the Western Carpathians, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20745, https://doi.org/10.5194/egusphere-egu2020-20745, 2020.

The results of the magnetoteluric investigations carried out along the profiles are presented in the form of sections in which the variations of the different parameters, the 2-D modeling, as well as the inversions.

The results of the geophysical researches (magnetotellurics, gravity, geomagnetics) obtained were aimed at obtaining a unitary image on the deep geological structure in the investigated area. A number of information was obtained regarding such as:

  • Determination of the thickness of the package of formations belonging to the post-tectogenetic sedimentary cover of the Transylvanian Depression; sedimentary sedimentary cover, conductive, with a maximum thickness of approximately 4000 m in the Pannonian Depression;
  • Contouring of the Tethysian Major Suture (near the town of Alba Iulia in the Transylvanian Basin), represented by the Transylvanian nappes system (ofiolitic complex and sedimentary formations), with resistivities of about 500 Ohm*m, which separates two blocks with continental crust of different thicknesses (22- 27 km for Internal Dacids and 32-36 km for Median Dacids);
  • Highlighting the change of nappes systems belonging to the Transylvanians, with a wide development both to the north (Căpâlnas-Techereu nappes and the nappes of Groşi and Criş), as well as to the east (the ophiolite complex and sedimentary cover), over the Biharia nappes system, respectively Central-Eastern Carpathian nappes; extension of the Codru and Biharia - Arieşeni nappes, the last with higher resistivities (200 Ohm*m);
  • Highlighting the transcrustal fault that marks the contact between the Inner Dacides and the Median ones;
  • Individualization at the level of the lower crust of a transition zone; significant decrease of resistivity, as a consequence of the presence of the fluids in the transition zone, from the pressure in the pores from lithostatic type to the hydrostatic type (occurs at depths of 22 - 30 km and at temperatures of 350º - 400º C).

The electromagnetic data reflect the anomalies of electrical conductivity in a sensitive way, but due to the many causes that can generate them, a careful analysis of the particularities existing for each case, especially the superficial ones, was necessary.

The correlation of the all the information provided in sections (resistivity, phases, density, magnetic susceptibility), inversions, modeling, lead to the validation of the model.

How to cite: Asimopolos, N.-S. and Asimopolos, L.: Coroboration of magnetotelluric investigations with other geophysical anomalies for a case study located in North-West to central part of Romania, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10242, https://doi.org/10.5194/egusphere-egu2020-10242, 2020.

EMRP3.2 – Advances in Rock, Bio-, and Environmental Magnetism

EGU2020-915 | Displays | EMRP3.2 | Highlight

Under pressure: How pressure affects magnetic remanence

Michael Volk, Roger Fu, and Josh Feinberg

Rocks have complicated histories and form under various conditions. However, all rocks, terrestrial and extraterrestrial, have been subjected to some form of pressure during their genesis. The effect of pressure (strain) on the magnetic remanence is a largely unexplored problem, with most of the work being focused on the study of meteorites. 

In the absence of a magnetic field, subjecting a rock to pressure can demagnetize the natural remanent magnetization (NRM). This loss of magnetic remanence can lead to an underestimation of paleointensities. On the other hand, in the presence of a magnetic field, magnetic minerals can record a pressure remanent magnetization (PRM). The superposition of the remaining NRM and a newly acquired PRM can influence the remanence direction as well as the paleointensity. Since the reconstruction of the temporal changes of Earths’ magnetic relies on robust estimations of direction and intensity, the effects of pressure on the remanence should be taken into account.

Here we present a series of experiments that aim to explore the acquisition process of PRMs and their net contribution with respect to the rock’s original magnetization. Stoichiometric magnetites of four different grain sizes (65 nm, 440 nm, 16.9 µm, and 18.3 µm) and magnetic domain states were subjected to crustal pressures (226, 301, and 376 MPa) in the presence of a magnetic field. Surprisingly, the PRM intensity showed no detectable dependence on grain size. However, because the acquisition of a thermal remanence (TRM) is strongly dependent on particle size,  populations of large multidomain particles can acquire a PRM, which may represent up to 30% of a TRM acquired in the same field.

Finally, we will show how the influence of pressure on the magnetic remanence can be visualized by modern magnetic imaging techniques like the quantum diamond microscope (QDM). The QDM has a  ~1 µm maximum spatial resolution that is able to resolve the magnetic fields of individual mineral assemblages with ~10 µm diameter. The high spatial resolution and sensitivity enables us to visualize the changes in magnetic remanence due to pressure cycling and can help to better understand the possible implications for paleomagnetism.

How to cite: Volk, M., Fu, R., and Feinberg, J.: Under pressure: How pressure affects magnetic remanence, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-915, https://doi.org/10.5194/egusphere-egu2020-915, 2020.

EGU2020-20552 | Displays | EMRP3.2 | Highlight

Micromagnetic modeling of silicate-hosted magnetic inclusions using SEM-FIB slice-and-view nanotomography

Even. S. Nikolaisen, Richard. J. Harrison, Suzanne. A. McEnroe, and Karl Fabian

Slice-and-view nanotomography uses a dual beam SEM-FIB to reconstruct the 3D volume of a mineralogical sample using a sequential series of nanoscale slices created with a focussed beam of Ga ions. This method reveals the true shapes and forms of naturally occurring magnetic inclusions hosted by the silicate minerals feldspar and pyroxene. High-resolution 3D morphological data for the magnetic minerals is extracted, converted to tetrahedral meshes, and micromagnetically modelled using the MERRILL software.

This study optimises the step-by-step process of extracting and processing micromagnetic data from polished thin-sections to generate a full rock magnetic classification of the remanence carriers in silicates. Slice-and-view nanotomography follows known preparation methods with a protective platinum layer, carbon rod guides and trenches, but also introduces a carbon slab along the Z-direction for e-beam alignment. This method reduces the need for auto focus, as the e-beam alignment will have a constant imaging distance and generates a good reference point for stack alignment. Image processing is limited to 3D a gaussian blur and 3D mean filters. Paraview is used to set the correct voxel dimensions and to generate the surface mesh. Freeware software Meshmixer and Meshlab are used for their powerful smoothing, mesh interaction tools and geometric calculations. The tetrahedral volume mesh is produced with iso2mesh in Matlab.

Micromagnetic hysteresis and back-field simulations of >400 inclusions with a broad range of morphologies have been performed using MERRILL using 20 different field directions, enabling average magnetic properties to be calculated for a random ensemble. The results give a detailed and direct description of the micromagnetic structure of naturally formed magnetic minerals that compliments macroscopic approaches, such as FORC analysis.

How to cite: Nikolaisen, E. S., Harrison, R. J., McEnroe, S. A., and Fabian, K.: Micromagnetic modeling of silicate-hosted magnetic inclusions using SEM-FIB slice-and-view nanotomography, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20552, https://doi.org/10.5194/egusphere-egu2020-20552, 2020.

EGU2020-18114 | Displays | EMRP3.2 | Highlight

First steps towards deriving rock magnetic and paleomagnetic data from subsets of magnetic grains in lavas using Micromagnetic Tomography

Lennart de Groot, Karl Fabian, Annemarieke Béguin, Martha Kosters, Roger Fu, Richard Harrison, Auke Barnhoorn, and Tristan van Leeuwen

Our understanding of the behavior of the geomagnetic field arises from magnetic signals stored in geological materials, e.g. lavas. Almost all experiments to determine the past state of the Earth’s magnetic field use bulk samples (typically 1 - 10 cc) and measure their magnetic moment after series of laboratory treatments. Lavas, however, consist of mixtures of different iron-oxide grains that vary in size, shape, and chemistry. Some of these grains are good recorders of the Earth’s magnetic field; others are not. Only a small amount of adverse behaved magnetic grains in a sample already hampers all classical experiments to obtain paleointensities; success rates as low as 10-20% are common, i.e. for 80-90% of all lavas vital information on paleointensities is lost before it can be uncovered.

Recently, we showed that it is possible to determine the magnetization of individual grains inside a synthetic sample using a new technique: Micromagnetic Tomography. The individual magnetizations of grains are determined by inverting scanning magnetometry data from the surface on the sample onto the known locations, sizes and shapes of the magnetic grains that are obtained from a microCT scan of the sample. The synthetic sample used for our proof-of-concept, however, was optimized for success: the dispersion of magnetic markers was low, and the magnetite grains had a well-defined grain size range. Furthermore, the scanning SQUID microscope used requires the sample to be at 4 K, below the Verweij transition of the magnetite grains.

Here we present the first Micromagnetic Tomography results from natural samples. We used two magnetic scanning techniques that operate at room temperature, a Magnetic Tunneling Junction set-up and a Quantum Diamond Magnetometer, to acquire the magnetic surface scans from a Hawaiian lava and calculated magnetic moments of individual grains present. We show that it is possible to acquire rock magnetic information as function of grain size from these natural samples and reveal the first results of interpreting a paleomagnetic direction from selected subsets of grains in our samples. These are the first steps towards deriving rock magnetic and paleomagnetic information from subsets of known good recorders inside lava samples, a technique that will revolutionize our field of research.

How to cite: de Groot, L., Fabian, K., Béguin, A., Kosters, M., Fu, R., Harrison, R., Barnhoorn, A., and van Leeuwen, T.: First steps towards deriving rock magnetic and paleomagnetic data from subsets of magnetic grains in lavas using Micromagnetic Tomography, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18114, https://doi.org/10.5194/egusphere-egu2020-18114, 2020.

EGU2020-19911 | Displays | EMRP3.2

Mineral sources of magnetic anomalies: Insights from scanning magnetic microscopy

Suzanne McEnroe, Zeudia Pastore, Nathan Church, Falko Langenhorst, and Hirokuni Oda

Development in instrumentation and technology now allows for mapping of magnetic anomalies, caused by spatial variations in magnetization in the source materials, over a wide range of scales, from the millimeter mineral scale to the km crustal scale.

Traditional rock magnetic methods, used to investigate the magnetization in natural rock samples, are bulk measurements, which cannot be directly correlated to the individual mineral phases, or particles.  Scanning magnetic microscopy is a high-resolution mapping technique that allows for detailed investigation of the magnetization in natural rock samples. The technique generates a map of the magnetic field distribution over a planar surface of a rock sample with sub-millimeter resolution that can be used to correlate specific magnetic signals to the underlying mineralogy. This information is vital for an understanding of the origin of rock bulk behavior measured in both the laboratory, and in magnetic surveys.

Here we use 3D magnetic modeling to investigate the sources of the magnetic anomalies mapped over a sample thin section. The oxide grains in the thin sections are modeled using information from optical and electron microscopy (SEM and TEM) to constrain the source geometry, and with magnetic property data. The internal geometry of the oxide mineral phases (exsolution lamellae, intergrowths, symplectites) and compositions are constrained by EMP and TEM. 

Magnetic scans aid in locating the magnetic sources, and resolving the different magnetic components contributing to the bulk rock properties.  By modeling the small-scale variations in the oxides the direction and intensity of the magnetic grains are determined.  Aeromagnetic and ground magnetic data from the sample locations are used in conjunction with thin section magnetic mapping. Thin section results can be up-scaled to compare with ground and aeromagnetic data.

How to cite: McEnroe, S., Pastore, Z., Church, N., Langenhorst, F., and Oda, H.: Mineral sources of magnetic anomalies: Insights from scanning magnetic microscopy , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19911, https://doi.org/10.5194/egusphere-egu2020-19911, 2020.

The NRM of the ocean floor is carried by titanomagnetite grains that undergo low-temperature oxidation after initial cooling. Progressing oxidation is known to generate shrinkage cracks in   grains larger than approximately 5 mkm, and is suspected to   control the long wavelength variation of NRM-intensity across the ocean floor. Here we develop a quantitative theory of single-phase oxidation and crack formation by solving the vacancy-diffusion equation that describes the oxidation process for spherical titanomagnetite particles, where the diffusion coefficient strongly decreases with vacancy concentration. The latter dependence has been experimentally demonstrated and is essential to explain the peculiarities of the observed variations of oxidation-degree with ocean-floor age. The calculated diffusion profiles provide the exact stress distributions inside oxidized titanomagnetite spheres, and  predict a size limit for shrinkage-crack formation that agrees   with   microscopic observations of crack appearance in ocean-floor basalt samples. The new diffusion model provides a unified explanation of long-known experimental facts that 1) temperatures for the onset of low-temperature oxidation during laboratory heating are theoretically estimated as 200-400 °C, depending on grain size, and 2) that heating to 400-500 °C is required to obtain a sufficiently high degree of oxidation z about 0.8  for the development of  high-temperature exsolution lamellae. Calculations for ocean-floor conditions    quantitatively suggest that a rapid decrease of NRM intensity during the first 40 ka  results from  a deflection of magnetization by strong stresses that emerge in titanomagnetite grains of sub-critical sizes, and randomization of domain-state by crack formation in larger grains. This work was supported by Russian Science Foundation grant 19-47-04110401 (VS)

How to cite: Shcherbakov, V. and Fabian, K.: Theory of low-temperature deuteric oxidation with application to time evolution of ocean magnetic anomalies, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3733, https://doi.org/10.5194/egusphere-egu2020-3733, 2020.

EGU2020-9355 | Displays | EMRP3.2 | Highlight

Beyond single model and single domain : Using big data to answer fundamental questions in rock magnetism

Lesleis Nagy, Wyn Williams, Lisa Tauxe, Adrian Muxworthy, and Karl Fabian

Interpretation of palaeointensity measurements is difficult since natural samples are made up of magnetic grains of different size, shape and chemical composition. Néels single-domain (SD) model is the main theoretical tool used to understand paleomagnetic measurements, but it is limited since it only applies to uniformly magnetized grain assemblies.

Recent work has shown that the single-vortex (SV) state is not only significantly more common than SD as it occupies a much larger range of grain sizes but is also surprisingly thermally stable [1]. As these grains likely account for the majority of the signal measured in the laboratory, they must also be responsible for the range of observations that lead to large inaccuracies in measurement, for example, pTRM tails. Additionally, if we have a clear understanding of the SV state, can we account for the true cooling rate dependence of grain assemblies, and also find a physical link between magnetic coercivity and blocking temperature.

We present a framework of tools for paleomagnetists to interpret measurements in the light of advances in numerical modelling and the increase in speed of modern computers. We hope that these tools will advance our understanding of the basic mechanisms by which samples record and preserve the Earth’s magnetic field and so will allow for the quantification of errors observed in real samples.

[1] L.  Nagy,  W.  Williams,  A.  R.  Muxworthy,  K.  Fabian,  T.  P.  Almeida, P. Ó Conbhuí, and V. P. Shcherbakov. Stability of equidimensional pseudo–single-domain magnetite over billion-year timescales. PNAS 114(39):10356–10360, 2017

How to cite: Nagy, L., Williams, W., Tauxe, L., Muxworthy, A., and Fabian, K.: Beyond single model and single domain : Using big data to answer fundamental questions in rock magnetism, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9355, https://doi.org/10.5194/egusphere-egu2020-9355, 2020.

EGU2020-11247 | Displays | EMRP3.2

A pseudo-single domain theory of paleomagnetic recording

Karl Fabian and Lennart V. de Groot

The common theory of paleomagnetic recording is based on the acquisition of thermoremanent magnetization in  single-domain (SD) particles (Neel, 1955). The physical consequences of this theory  agree  well enough  with observations as to be of utmost use in understanding and assessing the quality of paleomagnetic data in almost all practical applications in paleomagnetism. This is to a large extent due to the statistical nature of the interpretations based on SD theory, which apparently is sufficiently robust to make up for minor descrepancies between the real remanence carriers and their physical description on which the statistical interpretation is founded. Exceptions to this rule are becoming more important as increasingly sophisticated technical methods for paleomagnetic measurements are developed and used, that involve fewer and fewer individual magnetic particles to contribute to the measured signal. Examples are the determination of paleointensity from individual mineral grains, and the development of scanning magnetometers that average over relatively small numbers of magnetic grains in a sample. The statistical uncertainties of paleomagnetic quantities resulting from using small sample sizes have been studied for SD particle ensembles for example by Berndt et al. (2016).  First experimental data indicate that in case of PSD particles the statistical sample size required to  reconstruct paleomagnetic field direction may be smaller than for SD particles. Here, a theoretical study is presented that describes the micromagnetic background of this hypothesis and allows to test  it for a simplified mathematical model of TRM acquisition in PSD particles.

How to cite: Fabian, K. and de Groot, L. V.: A pseudo-single domain theory of paleomagnetic recording, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11247, https://doi.org/10.5194/egusphere-egu2020-11247, 2020.

EGU2020-12040 | Displays | EMRP3.2

Determination on iron species in plagioclase crystals toward the understanding on the magnetite exsolution

Ryoichi Nakada, Masahiko Sato, Masashi Ushioda, Yujiro Tamura, and Shinji Yamamoto

Speciation analysis of Fe in single plagioclase crystals separated from two different gabbros was performed to understand the crystallization mechanisms of magnetite exsolution. Iron species in single crystals were measured using Fe K- and LIII-edge X-ray absorption fine structure (XAFS) analysis. The K-edge pre-edge analysis showed variation in the averaged valence state of Fe in plagioclase crystals even if they had been separated from the same gabbro that was further confirmed by the LIII-edge analysis. The K-edge pre-edge analysis also suggests the various degrees of contribution from tetrahedral Fe. The mixing of tetrahedral and octahedral Fe leads to an underestimation of the averaged valence state of Fe for the K-edge pre-edge analysis; thus, we adopted the LIII-edge result for the valence state of Fe in plagioclase crystals. Iron K-edge extended XAFS (EXAFS) analysis of two plagioclase crystals separated from the same gabbro clearly showed different coordination environments. A weakening of EXAFS oscillation was recognized in one sample, because two Fe‒O bonds (Fe3+‒O1 and Fe2+‒O2) cancelled out the oscillations of each other. The EXAFS spectrum of the other plagioclase crystal suggested a homogeneous distribution of Fe. The content of exsolved magnetite in these crystals is nearly identical, indicating that the exsolution of magnetite in plagioclase crystal had been completed before the temperature decrease that stopped the ordering of Fe ions in tetrahedral sites.

Reference: Nakada et al. (2019) G-Cubed, vol. 20 (11), 5319-5333.

How to cite: Nakada, R., Sato, M., Ushioda, M., Tamura, Y., and Yamamoto, S.: Determination on iron species in plagioclase crystals toward the understanding on the magnetite exsolution, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12040, https://doi.org/10.5194/egusphere-egu2020-12040, 2020.

EGU2020-12922 | Displays | EMRP3.2

Magnetic properties of sedimentary smythite (Fe9S11)

Andrew Roberts and Chorng-Shern Horng

Smythite (Fe9S11) is an occasionally reported magnetic iron sulphide mineral that occurs in varied geological settings and co-occurs commonly with other magnetic iron sulphide minerals. Determining the magnetic properties of smythite is important to understand its geological distribution and paleomagnetic and environmental magnetic significance. We have identified sedimentary smythite from three locations in Taiwan (one terrestrial and two marine), which suggest that smythite forms in methanic diagenetic environments into which sulfide has been reintroduced. We report the magnetic properties of our purest smythite sample and compare them with those of other magnetic iron sulfide minerals. The magnetization of smythite is controlled by multi-axial anisotropy, with magnetic easy axes that lie within the crystallographic basal plane. Smythite has stable magnetic properties with no low-temperature magnetic transition. The magnetic properties of smythite at elevated temperatures are dominated by thermal alteration, which precludes Curie temperature determination. Hysteresis and coercivity properties of stable single domain smythite are similar to those of greigite at, and below, room temperature. In contrast to greigite, and similar to pyrrhotite polytypes, smythite crystals occur as hexagonal plates. This morphological contrast facilitates discrimination of smythite from greigite in electron microscope observations, but it does not assist discrimination from pyrrhotite. Similar magnetic and morphological properties between smythite and other magnetic iron sulfides means that diagnostic mineralogical analyses (e.g., X-ray diffraction) are needed to identify these minerals. Further work is needed to obtain pure samples to develop a comprehensive domain state dependent magnetic property framework for smythite.

How to cite: Roberts, A. and Horng, C.-S.: Magnetic properties of sedimentary smythite (Fe9S11), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12922, https://doi.org/10.5194/egusphere-egu2020-12922, 2020.

How to cite: Nowaczyk, N.: Re-sedimentation experiments with mud from the Southeastern Black Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4399, https://doi.org/10.5194/egusphere-egu2020-4399, 2020.

EGU2020-12027 | Displays | EMRP3.2

Origin of the Scotia Sea Magnetic Susceptibility Signal Across the MIS6-MIS5 Transition

Stefanie Brachfeld, Brendan Reilly, Lisa Tauxe, Bridget Lee, Michael Weber, Maureen Raymo, Trevor Williams, Ian Bailey, Marga Garcia, Michelle Guitard, Sidney Hemming, Yasmina Martos, Suzanne OConnell, Lara F. Perez, Thomas Ronge, Xufeng Zheng, and Kathy Licht and the Expedition 382 Scientists

Patterns of variability in Pleistocene magnetic susceptibility (k) from deep-sea sediment cores from the Scotia Sea show a striking similarity to patterns of dust flux recorded in the EPICA Dronning Maud Land (EDML) ice core.  Antarctic marine k records broadly reflect the interplay of lithogenic sediment provenance, biological productivity, sediment transport processes, and post-depositional diagenesis. Here we explore the origin of the Scotia Sea k record via a detailed rock magnetic study across the transition from MIS 6 to MIS 5. We analyzed bulk sediment and grain size separates in order to construct magnetic signatures of iceberg rafted debris (IBRD), sortable silt, and eolian input. The MIS 6-MIS 5 transition consists of three lithologies, a high k silty-clay-rich diatomaceous mud deposited during the glacial interval, an IBRD-rich but low k silty clay that marks the onset of deglaciation, and a low k diatomaceous ooze in which IBRD decreases forward through time. The high k glacial sediment is characterized by multi-domain hysteresis parameters, low χARM/χ values, S ratios near 1, and thermomagnetic curves indicative of low-Ti titanomagnetite. The absence of k peaks in the IBRD-rich silty-clay and IBRD rich diatomaceous ooze likely reflects the weakly magnetic lithogenic detritus supplied by Weddell Sea Embayment (WSE) ice streams, such as sandstone, quartzite, metasedimentary lithologies, phyllite and schist observed in lateral moraines adjacent to ice streams of the eastern WSE. The deglacial interval is characterized by elevated MR/MS, χARM/χ, and HIRM values, and decreased S-ratios in the bulk sediment, suggesting a greater proportion of high coercivity minerals such as hematite or goethite in the iron oxide assemblage. Preliminary data from grain size separates indicates that the clay mass fraction is > 0.5 in all three lithologies. Clay is also the dominant size fraction in the EDML ice core dust, with particle sizes generally < 5 μm. The Scotia Sea clay fraction k values are a factor 1.5 to 5 weaker than the silt fraction k values, and therefore are not the main carrier of the bulk k signal. The rock magnetic signatures of Scotia Sea sediment will be compared to those of terrestrial till and bedrock from the WSE, and to those of potential dust sources in South America to identify the sediment sources and environmental processes responsible for the k signal.

How to cite: Brachfeld, S., Reilly, B., Tauxe, L., Lee, B., Weber, M., Raymo, M., Williams, T., Bailey, I., Garcia, M., Guitard, M., Hemming, S., Martos, Y., OConnell, S., Perez, L. F., Ronge, T., Zheng, X., and Licht, K. and the Expedition 382 Scientists: Origin of the Scotia Sea Magnetic Susceptibility Signal Across the MIS6-MIS5 Transition, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12027, https://doi.org/10.5194/egusphere-egu2020-12027, 2020.

Obtaining reliable global and regional records of the past climatic changes during the glacial Pleistocene is of prime importance for building up consistent climate models of the near and far future. Magnetic signature along sequences of alternating loess and (paleo)soil units from the terrestrial environments is considered as semi-continuous record of climate change in the geological past. However, soil formation in aeolian landscapes may occur under different and changing conditions of dust sedimentation. Viewing from this standpoint the depth variations of several rock magnetic characteristics along profiles of Holocene soils from low Danube area allowed us to establish a set of criteria for identification of the past regimes of aeolian sedimentation persisted during the soil forming periods. A conceptual model for the time evolution of the grain size of the pedogenic magnetic fraction  with soil depth is proposed,  which is build upon  the mechanism of soil formation – accretional or  stable land surfaces,  or a combination of the two. According to the proposed conceptual model, discrimination between accretional soils and soils developed without dust additions during soil forming period can be done. Accretional soils are characterized by parallel changes in grain size sensitive magnetic proxies. Soils, developed at stable landscape conditions show gradation of the depths at which maximum enhancement of various proxies occurs with deepest occurrence of the maximum in frequency dependent magnetic susceptibility, followed by depth of maximum anhysteretic susceptibility and the normalized anhysteretic to isothermal remanence acquired at 100mT field. It is shown that the mean coercivity of the pedogenic component of accretional soils is higher than that of soils developed without eolian input at equal temperature conditions because of the soils’ thermal gradient and different depths, at which pedogenic minerals form in the two settings.

How to cite: Jordanova, D. and Jordanova, N.: Deducing the role of eolian dust sedimentation during soil forming periods on mineral magnetic records and its implications for paleoclimate reconstructions , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4523, https://doi.org/10.5194/egusphere-egu2020-4523, 2020.

EGU2020-21076 | Displays | EMRP3.2

Zinc and Nickel signature for abiogenic and biogenic magnetite: implications for the origin of magnetite in banded iron formations

Xiaohua Han, Elizabeth Tomaszewski, Ronny Schönberg, Yongxin Pan, James Byrne, and Andreas Kappler

There are longstanding and ongoing controversies about the abiogenic or biogenic origin of magnetite in banded iron formations (BIFs). The trace element composition of magnetite was proposed as a promising tracer for distinguishing biogenic from abiogenic magnetite, which, however, remains to be explored quantitatively. Here, we compared the partitioning of trace elements Zinc (Zn) and Nickel (Ni) in both abiogenic and biogenic magnetite produced either by an abiotic reaction of ferrihydrite with by Fe2+aq or by Fe(III)-reducing bacteria Shewanella oneidensis MR-1. We compared the transformation of three different ferrihydrite (Fh) starting materials: 1) Control Fh without added trace elements, 2) ferrihydrite with co-precipitated Zn (ZnFh) and 3) ferrihydrite with co-precipitated Ni (NiFh) – both in either NaHCO3 or HEPES buffer. We monitored Fe concentration and speciation in both aqueous and solid phases over time using the spectrophotometric ferrozine assay, analyzed Fh transformation products by Mössbauer spectroscopy as well as X-ray diffraction and quantified Zn and Ni in solution and in the minerals by iCAP-Qc quadrupole mass spectrometer after acidic dissolution of the minerals. In summary our results revealed that both Zn and Ni are much more depleted in abiogenic magnetite than those in biogenic magnetite, independent of whether magnetite was precipitated in NaHCO3 or HEPES buffer. Although further analyses are needed, this suggests that the trace element distribution could be a chemical signature to distinguish biogenic from abiogenic magnetite in BIFs.

How to cite: Han, X., Tomaszewski, E., Schönberg, R., Pan, Y., Byrne, J., and Kappler, A.: Zinc and Nickel signature for abiogenic and biogenic magnetite: implications for the origin of magnetite in banded iron formations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21076, https://doi.org/10.5194/egusphere-egu2020-21076, 2020.

EGU2020-2510 | Displays | EMRP3.2 | Highlight

Safety of hypomagnetic field and its effects on the skeletal system

Peng Shang and Yanru Xue

    All organisms survive and multiply under the geomagnetic magnetic field (GMF) [1]. With the launch of the Moon and Mars space program, during long-distance space mission, astronauts will inevitably be exposed to an environment with a hypomagnetic field (HyMF), which several thousand times weaker than GMF[2]. Spatial hypomagnetic field exists on the surface of the moon or in the deep space of the solar system, and its magnetic intensity is less than 5 μT[3]. At present, the research on the effects of HyMF on the health of astronauts is mainly focused on the conditions of ground simulation experiments, including the central nervous system, blood system and brain cognition [4,5]. However, relevant safety of the skeletal system studies about HyMF are deficient. Our recent research indicated that the effects of HyMF on bone cannot be overlooked. In vivo, our study found that HyMF aggravated bone loss induced by hindlimb unloading (HLU) in rats and mice, which related to the changes in iron metabolism[6,7]. In addition, HyMF also inhibited the recovery of simulated microgravity-induced osteoporosis of mice, probably by restraining elevated iron return to normal levels[8]. Meanwhile, we found that HyMF can inhibit osteoblast differentiation and mineralization[9], promote osteoclast formation and bone resorption in vitro[10]. The research results have significant academic values in the field of magneto-biology and the potential application values in space activities for the manned moon landing exploration.

Key words: 

Hypomagnetic field, geomagnetic field, safety management, iron storage.

References

[1]Dubrov A P. The Geomagnetic Field and Life: Geomagnetobiology. Bioscience. 1978. 978-1-4757-1610-8.

[2]Belyavskaya N. Biological effects due to weak magnetic field on plants. Advances in space Research, 2004, 34(7): 1566-1574.

[3]Mo W C, Ying L & He R Q. Hypomagnetic field, an ignorable environmental factor in space? Science China -Life Sciences, 2014. 57(7): 726-728.

[4]Mo WC, Liu Y & He RQ. A biological perspective of the hypomagnetic field: from definition towards mechanism. Prog Biochem Biophys, 2012, 39: 835–842 

[5]Jia B, Zhang WJ, Xie L, Zheng  Q, Tian ZC & Shang P. Effects of hypomagnetic field environment on hematopoietic system in mice. Space Medicine &Medical Engineering, 2011.24(5): 318-322.

[6] Jia B, Xie L, Zheng Q, Yang P F, Zhang W J & Shang P. A hypomagnetic field aggravates bone loss induced by hindlimb unloading in rat femurs. PloS one, 2014, 9(8): e105604.

[7] Yang J, Meng X, Dong D, Xue Y, Chen X & Shang P. Iron overload involved in the enhancement of unloading-induced bone loss by hypomagnetic field. Bone, 2018 Sep;114:235-245.

[8]Xue YR, Yang JC, Luo J, Ren L, Shen Y & Shang P, Disorder of iron metabolism inhibits the recovery of unloading-induced bone loss in hypomagnetic field. Journal of bone and mineral research.2020. DOI: 10.1111/JBMR.3949.

[9]Yang J, Zhang J, Ding C, Dong D & Shang P. Regulation of Osteoblast Differentiation and Iron Content in MC3T3-E1 Cells by Static Magnetic Field with Different Intensities. Biological trace element research, 2017, 184(7): 1-12.

[10]Zhang J, Meng X, Ding C, Xie L, Yang P & Shang P. Regulation of osteoclast differentiation by static magnetic fields. Electromagnetic biology and medicine, 2017, 36(1): 8-19.

 

How to cite: Shang, P. and Xue, Y.: Safety of hypomagnetic field and its effects on the skeletal system, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2510, https://doi.org/10.5194/egusphere-egu2020-2510, 2020.

EGU2020-8865 | Displays | EMRP3.2

Curie temperature variations in synthetic titanomagnetite single crystals

Sophie-Charlotte Lappe, Georg Winkens, Joerg Persson, Shibabrata Nandi, and Oleg Petracic

Paleomagnetic measurements provide very important methods to study the evolution of and variations in the Earth’s magnetic field throughout time. A vital tool used in paleomagnetism are natural magnetic minerals, such as the titanomagnetite (TM) solid solution series (Fe3-xTixO4, 0 ≤ x ≤ 1). The main source of magnetic information in TMs is the thermal remanent magnetisation (TRM) they retain whilst being cooled below their Curie temperature (TC) during their formation.

The key factor determining the TC  is the composition. However, recent studies on natural and synthetic TM powders [1,2,3] have shown that their TC  is also heavily influenced by their thermal history. Annealing various natural and synthetic TM powders at temperatures between 300°C and 425°C for timescales of hours to months resulted in changes in their TC  of up to 150°C.

The accuracy of many paleomagnetic measuring techniques, such as geomagnetic paleointensity estimates and paleomagnetic paleothermometry, depends on the exact knowledge of the Curie temperature. Changes in TC  of such a considerable extend could deeply impact those techniques or even render them doubtable. So far, vacancy-mediated chemical clustering at the octahedral site of the TM structure has been postulated as the mechanism causing this phenomenon [2,3]. To further investigate the underlying processes, we synthesised a large (~6.5 mm diameter;  ~27 mm length) TM single crystal using an optical floating zone furnace. Via SEM-EDX techniques it was established that the crystal was homogenous over its whole length with a composition of  Fe2.64Ti0.36O4. Using a Physical Properties Measurement System (PPMS) the Curie temperatures of several pieces of the crystal were determined after different annealing treatments. For the first time it has been possible to detect systematic changes in TC  with annealing in a TM single crystal.

Additionally within the scope of this project it was possible to determine the relationship between the extend of change in TC  and the microstructure for polycrystalline samples.

 

[1] Bowles, J. A., Jackson, M. J., Berquó, T. S., Solheid, P. A. and Gee, J. S. (2013), Nature Communications, 4, 1916. https://doi:10.1038/ncomms2938

[2] Jackson, M. J., and Bowles, J. A. (2018), J. Geophys. Res., 123, 1-20. https://doi:10.1002/2017JB015193

[3] Bowles, J. A., Lappe, S.‐C. L. L., Jackson, M. J., Arenholz, E., & van der Laan, G. (2019). Geochem. Geophy. Geosy. 20. https://doi.org/10.1029/2019GC008217

How to cite: Lappe, S.-C., Winkens, G., Persson, J., Nandi, S., and Petracic, O.: Curie temperature variations in synthetic titanomagnetite single crystals, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8865, https://doi.org/10.5194/egusphere-egu2020-8865, 2020.

EGU2020-13170 | Displays | EMRP3.2

Magnetic properties of nanotextured greigite.

Barbara Lesniak, Michalis Charilaou, and Andreas Gehring

Greigite (Fe3S4) is a ferrimagnetic mineral widespread in sedimentary environments, commonly found in lacustrine and marine sediments that records ancient geomagnetic field variations and environmental processes. However, its magnetic properties are not yet well understood due to the lack of a single crystal greigite suitable for magnetic measurements. In particular, the dependency of its magnetic properties with respect to structural and morphological properties remains uncertain.

In the present study, we analyzed the structural and magnetic properties of synthetic, polycrystalline greigite formed by controlled colloidal synthesis [Rhodes et al. 2017]. X-ray diffractometry and transition electron microscopy reveal that greigite forms flakes of about 100 nm that consist of epitaxial intergrown nanoparticles with a mean coherence length of 19 nm. Therefore, our synthetic greigite can be considered as polycrystalline flakes with a nanotexture.

The saturation magnetization (Ms) of the nanotextured greigite is 32.7 Am2kg-1 and the coercivity is Bc = 41 mT. The Ms is about 45% below the value for relatively large, synthetic crystal and this in turn is probably caused by the nanotexture, e.g., interfaces between nanocrystallites. The ratios Mr./MS = 0.54 and Bar/BSc = 1.33 indicate single-domain (SD) particles with pre-dominant uniaxial anisotropy [Roberts 1995]. The FORC diagram at room temperature shows an oval contour plot supporting that the flakes are nanotextured with interacting SD particles. The hysteresis parameters Bc and MS continuously increase upon cooling to 10 K.

Low-temperature cycling of the magnetization between 300 and 10 K in fields between 10 mT and 1000 mT shows the expected behavior for ferrimagnets with the superposition of the cooling and warming curves at fields B ³ 500 mT. At weaker fields a slight magnetic induction upon warming is found and the relative increase in magnetization is field dependent. This irreversibility most likely stems from the magnetization of the nanoparticle interfaces and their interactions in the flakes.

Ferromagnetic resonance spectroscopy (FMR) at room temperature shows a resonance field Bres= 213 mT and linewidth DB = 160 mT. Upon cooling the Bres decreases continuously down to 50 K followed by a pronounced shift to lower values down to 10 K. The shift goes along with markedly linewidth broadening. The discontinuity of the spectral parameters at T < 50 K points to a change in the effective anisotropy of the flakes most likely due to changes of the magnetocrystalline and the interaction anisotropies in the nanotexture, because the shape anisotropy of the polycrystalline flakes undergoes no significant change. 

In summary, the magnetic properties of greigite can be critically affected by the nanotexture. The response of the nanotexture to the magnetization and anisotropy properties can be taken to identify and characterize greigite nanoparticles in natural environments and to critically evaluate their use for paleomagnetic studies.

Rhodes, Jordan M., et al. "Phase-controlled colloidal syntheses of iron sulfide nanocrystals via sulfur precursor reactivity and direct pyrite precipitation." Chemistry of Materials 29.19 (2017): 8521-8530.

Roberts, Andrew P. "Magnetic properties of sedimentary greigite (Fe3S4)." Earth and Planetary Science Letters 134.3-4 (1995): 227-236.

How to cite: Lesniak, B., Charilaou, M., and Gehring, A.: Magnetic properties of nanotextured greigite., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13170, https://doi.org/10.5194/egusphere-egu2020-13170, 2020.

EGU2020-19223 | Displays | EMRP3.2

Magnetization of lower crustal rocks - potential sources of long wavelength anomalies

Geertje ter Maat, Suzanne McEnroe, Nathan Church, and Hirokuni Oda

The occurrence and nature of primary magnetic phases in ultramafic rocks is a subject of debate. Studies of ultramafic rocks originating in the deep crust commonly report secondary magnetic phases due to later metamorphism, serpentinization, or alteration as sources for long-wavelength anomalies. To assess the potential magnetic contribution from primary magnetic minerals occurring ‘in situ’ in deep-seated ultramafic rocks, the stability of these phases at lower crustal pressure and temperature conditions must be addressed. However, to study the magnetization of deep-crustal rocks, we are limited to exposures of unaltered uplifted rocks. Studying the petrophysical and rock magnetic properties of these ultramafic rocks can aid in predicting magnetic behavior deeper in the crust. 

Here, we present the results of a petrophysical and rock magnetic study on the ultramafic rocks of the Reinfjord Ultramafic Complex (RUC). These rocks are part of the Seiland Igneous Province, a magmatic plumbing system that formed in the deep crust (25-35 km depth). The dunites and wehrlites are minimally serpentinized, which indicates that the magnetic oxides in these rocks may be representative of those at depth. The primary magnetic carriers in these rocks were characterized using optical and electron microscopy, hysteresis and FORC measurements, backfield unmixing curves, and scanning magnetic microscopy. The primary magnetic carriers in the RUC are Cr-magnetite blebs exsolved from Al-chromite, and exsolved magnetite lamellae within clinopyroxene. The magnetic carriers have a range of domain states from SD to MD. 

The ultramafic rocks from the RUC are remarkably pristine and therefore provide insight into the magnetization of the lower crust. Due to the presence of SD magnetic carriers, these rocks may hold a stable remanence at lower crustal conditions and therefore be a potential source for long-wavelength anomalies.

How to cite: ter Maat, G., McEnroe, S., Church, N., and Oda, H.: Magnetization of lower crustal rocks - potential sources of long wavelength anomalies, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19223, https://doi.org/10.5194/egusphere-egu2020-19223, 2020.

EGU2020-8095 | Displays | EMRP3.2

Rock magnetic signature as a result of gas hydrate dissociation off southwestern Taiwan

Yin-Sheng Huang, Chorng-Shern Horng, Chih-Chieh Su, Shu-Kun Hsu, and Jing-Yi Lin

Marginal areas off southwestern Taiwan have been widely considered a high potential reservoir of gas hydrates based on several geophysical, geological, and geochemical investigations since the past decades. First gas hydrate sample has been collected on 21 June 2018 during the cruise MD214 at the core site MD18-3542 on the South Yung-An East Ridge. In the study, we focus on magnetic properties of this MD core. The most attractive feature in the magnetic susceptibility is an abrupt drop recorded at about 4 meters core depth. To clarify and identify the dominant magnetic mineral in the core, hysteresis loop parameters were first measured and then presented on the Day Plot, and further the X-ray diffraction analysis was applied to the selected core samples. Based on the magnetic results, the clear drop in the magnetic susceptibility is related to the change of dominant magnetic minerals in core sediments. Before about 4 meters core depth, the dominant magnetic mineral remains detrital magnetite. Below the depth, however, core sediments should have been infected by methane released by gas hydrate dissociation. Authigenic greigite and pyrite have become dominant, and therefore low magnetic susceptibility appears below 4 meters core depth.

How to cite: Huang, Y.-S., Horng, C.-S., Su, C.-C., Hsu, S.-K., and Lin, J.-Y.: Rock magnetic signature as a result of gas hydrate dissociation off southwestern Taiwan, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8095, https://doi.org/10.5194/egusphere-egu2020-8095, 2020.

EGU2020-12655 | Displays | EMRP3.2

Paleomagnetic and rock magnetic investigation in marine sediments, Nankai Trough, offshore Cape Muroto

Myriam Kars, Tetsuya Fukuta, and Carina Becker

The Nankai Trough is an accretionary complex which extends over several thousands of kilometers along the Japanese Pacific coast. Many ocean scientific drilling expeditions have taken place in this zone to better understand the mechanisms of big earthquakes and generation of devastating tsunamis. Offshore Cape Muroto, Shikoku Island, is one of investigated zones. A recent International Ocean Discovery Program (IODP) expedition (IODP Expedition 370) in the area has focused on the temperature limit of life in deep subseafloor sediments. Here we present paleomagnetic and rock magnetic preliminary results on two neighboring sites in this zone drilled during two former Ocean Drilling Program (ODP) legs: Site 808 of ODP Leg 131 and Site 1174 of ODP Leg 190. At all sites, shipboard magnetostratigraphy was challenging because of a strong diagenetic alteration of the magnetic mineral assemblages. Four main downcore magnetic zones, characterized by specific magnetic properties and mineralogy, are identified. At Site 808, catagenesis of the organic matter has been proposed to explain the downcore rock magnetic properties. This explanation however could not stand for Site 1174. We present here a first attempt of a comparative paleomagnetic and rock magnetic study in high temperature marine sediments, off Cape Muroto.

How to cite: Kars, M., Fukuta, T., and Becker, C.: Paleomagnetic and rock magnetic investigation in marine sediments, Nankai Trough, offshore Cape Muroto , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12655, https://doi.org/10.5194/egusphere-egu2020-12655, 2020.

EGU2020-18890 | Displays | EMRP3.2

Environmental and rock magnetic investigations into provenance and processes of west Iberian margin sediments

Matthew Nichols, Chuang Xuan, David A. Hodell, Carl Richter, Gary D. Acton, and Paul A. Wilson

Ocean sediment records from the West Iberian margin can be correlated to both Antarctic and Greenland ice cores as well as to European terrestrial pollen data. Previous studies have focussed on comparatively short sediment cores collected from relatively deep-water sites (i.e. >~2500mbsl). Here we present magnetic mineralogy and grain size from Integrated Ocean Drilling Programme Sites U1385 (2585mbsl) and U1391 (1085mbsl) to further understand magnetic sediment provenance and palaeocurrent evolution on the west Iberian margin dating back to ~416 ka. The gradient of IRM acquisition curves, shape of hysteresis loops, and marked decrease in magnetic susceptibility at ~580°C indicate that magnetite is the dominant magnetic phase at Site U1391. At depth, increased contributions of a higher coercivity component are seen at intervals where the concentration of magnetic material is low. FORC diagrams indicate the presence of a narrow ridge elongated along the Bc axis consistent with a higher coercivity component observed in IRM acquisition data. Magnetic grain size proxy (kARM/k) records from Site U1391 also show a significant difference in pattern of variability at depth. After ~130 ka kARM/k closely follows relative sea level, however prior to ~130 ka there is higher frequency variability with apparent coarser magnetic grain size, suggesting the higher coercivity component could have resulted from diagenetic processes. This is particularly apparent during warm intervals where magnetic material concentration is low (MIS 7, 9 and 11). This behaviour differs from that observed at either Site U1385, or in the younger portion of the record at Site U1391. We infer that the intervals of diagenetically effected sediments at U1391 could have resulted from increased productivity, vertical migration of the Mediterranean outflow water and associated changes in bottom water ventilation. Further understanding of sediment composition, redox conditions, transport and provenance through the last few glacial cycles underpins much of the other palaeoclimatic investigation at these sites. Results from our analysis of rock magnetism will be used to guide the reconstruction of reliable relative palaeointensity records from the Iberian Margin sediments to assess past geomagnetic changes in the region.

How to cite: Nichols, M., Xuan, C., Hodell, D. A., Richter, C., Acton, G. D., and Wilson, P. A.: Environmental and rock magnetic investigations into provenance and processes of west Iberian margin sediments, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18890, https://doi.org/10.5194/egusphere-egu2020-18890, 2020.

EGU2020-6234 | Displays | EMRP3.2

Characterization of a magnetotactic bacteria-grazing ciliate in sediment from the intertidal zone of Huiquan Bay, China

Chen Si, Pan Hongmiao, Cui Kaixuan, Zhang Wenyan, Zhao Yicong, and Xiao Tian

Magnetotactic bacteria (MTB) represent a group of microorganisms with the ability to orient and swim along geomagnetic field lines. They can synthesize magnetosomes through the biomineralization processes. Previously studies have reported that some species of protozoa can graze MTB and accumulate magnetosomes in the cells. Here, we characterize a slightly magnetically responsive MTB-grazing ciliate from the intertidal sediment of Huiquan Bay. According to molecular biological information, the ciliate is tentatively identified as Uronemella parafilificum. Using transmission electron microscopy, we observed that two to four different shapes of magnetosomes were randomly distributed within this ciliate. Energy-dispersive X-ray spectroscopy and high-resolution transmission electron microscopy images of them were consistent with magnetite. Although the same shapes and components of magnetosomes were also detected in MTB occurred in the same environment, the size of them was larger than that in ciliates. The results suggest that this ciliate species is capable of grazing and ingesting different types of MTB. These data reveal broad diversity and wide distribution of magnetically responsive protozoa and provide us more possibilities for researching the origin of magnetoreception in eukaryotes.

Keywords: ciliate, magnetotactic bacteria, magnetosome, graze, ingest.

How to cite: Si, C., Hongmiao, P., Kaixuan, C., Wenyan, Z., Yicong, Z., and Tian, X.: Characterization of a magnetotactic bacteria-grazing ciliate in sediment from the intertidal zone of Huiquan Bay, China, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6234, https://doi.org/10.5194/egusphere-egu2020-6234, 2020.

Paleoenvironmental reconstructions from three mountaineous lakes located in northern Iberia are compared and completed with classical magnetic analyses in order to detect the influence of different processes on the record and preservation of magnetic properties. The lakes are located in the Cantabrian Mountains, Enol Lake, and in the Pyrenees, the Marboré Lake and Basa de la Mora Lake and share a similar composition of their catchment areas, dominated by limestones. They present other different characteristics, such as in the organic matter content, being Enol the one with the highest organic carbon values. Redox indicator (Mn/Fe) is higher and more variable in Basa de la Mora Lake, whereas in Enol and Marboré Lakes steadily increases towards the top of the sequences. New and revisited results from sedimentary cores unravel the significance of the magnetic changes respect to the geochemical and sedimentological variations found in the geological record.

The magnetic mineralogy present after analyses done in discrete samples (less than 500 mg) is magnetite in all samples, due to a sharp decrease at 120 K (Verwey crystallographic transition) and 580ºC (Curie temperature of magnetite) in the thermomagnetic curves performed in the MPMS and the Curie balance respectively. No indication of neither pyrrhotite (phase transition at 35 K) nor siderite is observed. The high temperature thermomagnetic analyses show the presence and creation of magnetite during heating, see an increasing of induced magnetization forming a broad peak above 450ºC in the heating curve. In addition, a subtle change in the induced magnetization is observed at around 300ºC. All analyses related with coercivity indicate the predominance of low coercitive minerals (“soft”) as magnetite is.

The combination of geochemical, sedimentological and magnetic proxies suggest that in Enol Lake the magnetic signal may be dominated by the formation of new minerals in relation to redox processes favored by the higher presence of organic matter (6%organic content), whereas in Marboré Lake, the increase of the magnetic signal toward the top of the sequence seems related to the oxic environment and the preservation of magnetite, since this lake is ultra-oligotrophic. In Basa de la Mora Lake, the source rock seems to play a role in the magnetic signal of the sequence.

These results indicate that diagenesis and changes in the redox conditions alter the concentration of magnetic minerals during the Late Pleistocene and Holocene and underlines their value as environmental and paleoclimate archives.

Acknowledgements

Funding for this research was provided by the Spanish Inter-Ministry Commission of Science and Technology through MEDLANT (CGL2016-76215-R) and DINAMO 3 (Ref CGL2015-69160-R) projects and by the European Commission (EFA056/15 REPLIM). The Institute for Rock Magnetism (IRM), the Instrumentation and Facilities program of the National Science Foundation of the Earth Science Division and the University of Minnesota are acknowledged for supporting visits and the free use of the facilities at the IRM, together with the both easy-going and expert guidance from the IRM staff.

How to cite: Oliva-Urcia, B., Moreno, A., and Valero-Garcés, B.: Standard magnetic properties in three mountain lakes of northern Iberia, ¿what is the influence of the major environmental processes?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8417, https://doi.org/10.5194/egusphere-egu2020-8417, 2020.

EGU2020-804 | Displays | EMRP3.2

Anisotropy of out-of-phase magnetic susceptibility as a tool for tracking heavy metals pollution: a new approach to environmental magnetism study

Katarzyna Dudzisz, Szymon Oryński, Beata Górka-Kostrubiec, and Wojciech Klityński

Soil contamination by heavy metals has become a severe problem in many parts of the world, affecting people and other living organisms. The anisotropy of magnetic susceptibility (AMS) was successfully used to track deformation and flow directions in rocks and unconsolidated sediment, however, it has been very rarely applied to soils. In this study, magnetic susceptibility, electromagnetic (EM) methods and AMS of soils around three historical mining areas at the Sudetes Mountains (Poland) were studied. These sites are diversified in terms of exploitation time and type of ore (Zloty Stok – gold and arsenic, Janowa Gora – iron and Szklary - nickel). They were selected in order to examine the spatial spread of contamination from mine tailings, their potential sources and to test the potential use of the AMS to study migration pathways.

Magnetic susceptibility (к), GCM (ground Conductivity Electromagnetic Method) and magnetometric measurements were carried out in situ to get a spatial resolution of the magnetic data. Bartington MS2 magnetic susceptibility meter was used for mapping of к, whereas GCM measurements were made to obtain conductivity distribution from 6 different depth ranges. Magnetometric measurements were conducted with GEM GSM-19T Overhauser Magnetometer integrated with GPS, allowing for measurement of the total magnetic field and its vertical gradient. Moreover, soils samples were taken for further analyses in the laboratory. For AMS measurements, all samples were oriented northward and carefully placed into 8 ccm plastic, non-magnetic cubic boxes to prevent artificial modification of in situ magnetic fabrics. Then, these samples were measured in three mutually perpendicular positions using KLY-5 Kappabridge (Agico).

The highest values of magnetic susceptibility (1-5x10-3 SI) are observed around nickel tailings, whereas the lowest values (60-120x10-6 SI) characterise iron mining area. Preliminary results of GCM and magnetometry indicate the occurrence of overlapping anomalies in the studied area. Mapping of in situ magnetic susceptibility shows variability within particular sites. For Szklary, all three methods indicate the presence of the elongated anomaly roughly NE-SW oriented. Although AMS axes of in-phase susceptibility are randomly distributed for all sites, the magnetic fabric created by ferromagnetic minerals (out-of-phase, opAMS) indicate well grouped maximum susceptibility axes mainly oriented NE-SW. There is a clear correlation between mapped anomaly around nickel tailings (Szklary) and opAMS lineation. Outside the anomaly, opAMS directions are oriented SE-NW. For other sites, opAMS is also in line with the results of EM methods. Taking into account these results, as well as landforms and hydrological conditions, it could be concluded that magnetic minerals accompanied with heavy metals, most likely, migrate with subsurface runoff and opAMS is capable of detecting changes in the direction of the pollution spread. However, more study is necessary to fully explain this mechanism.

How to cite: Dudzisz, K., Oryński, S., Górka-Kostrubiec, B., and Klityński, W.: Anisotropy of out-of-phase magnetic susceptibility as a tool for tracking heavy metals pollution: a new approach to environmental magnetism study, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-804, https://doi.org/10.5194/egusphere-egu2020-804, 2020.

We studied the magnetic properties and trace element concentrations (Cr, Cu, Fe, Mn, Pb, Ti, V, Zn) of urban topsoils from 111 urban sites in a large REE-Nb-Fe mining and smelting city, Baotou, Inner Mongolia, China. The results show that pseudo-single domain and multi-domain magnetite dominates the magnetic properties of the soil samples, and the magnetic concentration parameters show a large positive anomaly near the Baotou iron and steel works. The average contents of all trace metals exceeded their background level in soils in Inner Mongolia, except for Pb. The spatial distribution and correlation analysis show that magnetic parameters related to the magnetite concentration and Cr, Fe, Mn, Ti, V and Zn show similar trends of variation. In addition, the results of PCA show that Fe, Ti, and V are highly correlated with the magnetic particles derived from the Baotou iron and steel works, tailing dam, chromium plant, and cement plant. In contrast, Cr, Mn, Pb and Zn are derived from both the steel plant and traffic pollution. Using a PMF model, three potential pollution sources are identified: industrial pollution, including the steel works, tailing dam, cement plant and chromium plant, are reflected by χlf, χARM, SIRM and SOFT, and they account for 71.2%; traffic pollution is reflected by Pb and Zn and accounts for 9.0%; and natural sources, reflected by χfd%, χARM/χ, χARM/SIRM, HARD%, S-300, S-100 and Ti, contribute 19.8%. The results are potentially useful for developing control measures for reducing trace metal contamination in soils in Baotou city, and in addition we conclude that a combined magnetic approach and geochemical approach is an effective means for qualitative and quantitative sources apportionment of urban surface soil pollution.

How to cite: Wang, B., Xia, D., and Jia, J.: Source apportionment of soil-contamination in Baotou City (Northwestern China) based on a combined magnetic and geochemical approach, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3879, https://doi.org/10.5194/egusphere-egu2020-3879, 2020.

EGU2020-3602 | Displays | EMRP3.2

Investigation of correlations between magnetic susceptibility and elemental content in the Kupa River sediments (Croatia)

Stanislav Frančišković-Bilinski, Sanja Sakan, Dragana Đorđević, Aleksandar Popović, Sandra Škrivanj, and Halka Bilinski

The Kupa River basin occupies the west-central part of Croatia and is shared by two neighboring countries (Slovenia, Bosnia and Herzegovina). It is the tributary of the Sava River and meets the latter at Sisak after traversing a distance of 294 km. The Sava River belongs to the Danube River watershed and enters the Danube River at Belgrade (Serbia).      

An extreme barium anomaly in sediments of Kupica and Kupa rivers was discovered during 2003 (Frančišković-Bilinski, 2006). It is result of un-careful mine waste disposal. Therefore this river has a big potential to be used as a “natural laboratory” in the future to study sediment transport processes in rivers, so we repeated sampling on most important locations in 2018, to see which processes happened during that time frame. In the current study, we aim to investigate correlations between magnetic susceptibility (MS) and elemental content of 26 studied elements in the fine sediment fraction (<63 µm) of samples from 2018. MS method is a fast and cheap method, which can give indication of contamination with some metals, so we aimed to test its suitability on Kupa River sediments. Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) was used to determine the concentration of the studied elements, after sequential extraction procedure (Sakan et al., 2016). Total concentrations of each element were determined as the sum of concentrations determined in each fraction. The same method was applied recently for determination of Ba concentrations in Kupa River sediments by Frančišković-Bilinski et al. (2019).

Correlation analysis was performed to reveal statistical correlations between MS and 26 elements analyzed by ICP-OES. Ten of them showed negative correlation (As, B, Ba, Fe, K, Li, Mg, Na, P, S), while other elements showed positive. Chromium showed excellent correlation with MS (0.91) and is element with the highest correlation to MS. All other elements show much weaker correlation with MS. Element with strongest correlation to MS after Cr is vanadium (0.62), followed by Mn (0.52), Al (0.52) and Cd (0.50). All other elements have rather weak correlation with MS, among which highest are those of Sr (0.45), Zn (0.35), Be (0.28), Co (0.27), Pb (0.27) and Ti (0.26). Rest of elements has very low correlation.

Our research confirmed that MS is not suitable to study barium contamination in sediments, as they have low negative correlation (-0.18). Low negative correlation of MS with Fe (-0.12) indicates that MS is not caused by iron minerals in Kupa River sediments. MS values show extreme value at the same location as does Cr, which is bound to residual fraction of yet not known mineral composition.

Earlier data of Frančišković-Bilinski (2007) show that SiO2 group of minerals predominate in Kupa sediment at Pokupsko, where MS and Cr have highest values.

Rererences:

Frančišković-Bilinski, S. (2006). J.Geochem.Explor. 88, 1-3, 106-109.

Frančišković-Bilinski, S. (2007). Fresenius Env.Bull. 16, 5, 561-575.

Frančišković-Bilinski S., Bilinski, H., Sakan, S., Đorđević, D., Popović, A. (2019). SGEM Conference proceedings, 19, 3.1., 73-80.

Sakan, S., Popović, A., Anđelković, I., Ðorđević, D. (2016). Env.Geochem.Health 38, 855–867.

How to cite: Frančišković-Bilinski, S., Sakan, S., Đorđević, D., Popović, A., Škrivanj, S., and Bilinski, H.: Investigation of correlations between magnetic susceptibility and elemental content in the Kupa River sediments (Croatia), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3602, https://doi.org/10.5194/egusphere-egu2020-3602, 2020.

EGU2020-7344 | Displays | EMRP3.2

Magnetic characterization of ghost rocks from the Sterkfontein cave (South Africa): are iron oxides linked to biological activity?

Aude Isambert, Maud Watkinson, Céline Pisapia, Emmanuelle Gérard, Bénédicte Menez, Richard Maire, and Laurent Bruxelles

The Sterkfontein caves system in the Cradle of Humankind (South Africa) is a karstic environment resulting from a ghost-rock karstification process that developed in the Malmani dolomite formation presenting interlayered more resistant chert layers (Bruxelles, 2017). This process of karstification occurs under low hydrodynamic conditions leaving in place a residual highly porous altered rock, which preserves the structure of the initial bedrock, and which is called “ghost rock”. Due to its high porosity and in the presence of water, ghost-rocks can represent a potential habitat for microorganisms, ubiquitous on and in Earth, with metabolisms mainly relying on dissolution or precipitation processes of minerals. Thus some secondary mineralizations of manganese and iron oxides, found associated to microorganisms in cave systems, could have a biological origin (Banerjee and Joshi, 2012). To better characterize the alteration phases and understand the process of karstification and the potential role of microorganisms and biofilms, samples including dolomitic bedrock, cherts and ghost-rocks were collected at the Sterkfontein cave system. We report here magnetic properties of powdered samples (low-field susceptibility, hysteresis parameters, saturation magnetization and MPMS measurements). In parallel to these magnetic measurements, XRD analyses, FTIR spectroscopic analyses and microscopic observations (SEM) have been realized in order to better characterize the mineralogy of bedrock and secondary phases and to better constrain the alteration processes. We observe that the ghost-rock is mostly composed of quartz and oxides. The magnetic phases detected are mainly hematite and goethite, precipitated on the quartz grain boundaries. These first observations could be explained by a total dissolution of the main bedrock (dolomite) and a partial chemical alteration and mechanical erosion of cherts. To go further, an additional microbial ecology study in the cave system is needed to better constrain the role of microorganisms in the precipitation of oxides detected.

 

Banerjee, S., Joshi, S.R., 2013. Insights into Cave Architecture and the Role of Bacterial Biofilm. PNAS, India Section B: Biological Sciences 83, 277–290.

Bruxelles L., 2017. Des fantômes et des hommes. Le rôle de la fantômisation dans la formation des karsts à homininés d’Afrique du Sud. Karstologia 69, 1–8.

How to cite: Isambert, A., Watkinson, M., Pisapia, C., Gérard, E., Menez, B., Maire, R., and Bruxelles, L.: Magnetic characterization of ghost rocks from the Sterkfontein cave (South Africa): are iron oxides linked to biological activity? , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7344, https://doi.org/10.5194/egusphere-egu2020-7344, 2020.

EGU2020-6208 | Displays | EMRP3.2

Diversity of multicellular magnetotactic prokaryotes in the intertidal zone of Huiquan Bay, Qingdao

Zhao Yicong, Zhang Wenyan, Pan Hongmiao, Cui Kaixuan, Chen Si, and Xiao Tian

Multicellular magnetotactic prokaryotes (MMPs) are a group of aggregates composed of 10-100 gram-negative cells synthesizing intracellular magnetic crystals. Two morphotypes of MMPs have been identified, including several species of globally distributed spherical mulberry-like MMPs (sMMPs), and ellipsoidal pineapple-like MMPs (eMMPs). We recently collected MMPs from the intertidal zone of Huiquan Bay, Qingdao. Optical microscopy showed that there were two types of MMPs in the area, including sMMPs and eMMPs. We observed the size of eMMPs was 9.25 ± 0.79 × 7.48 ± 0.79 μm (n = 24), and the average diameter of sMMPs was 5 ± 0.66 μm (n = 24). Transmission electron microscopy showed that these MMPs contained three sizes of bullet-shaped crystals in parallel chains or clusters. The length and width ratios of the sizes of these magnetosomes were 4.16±0.64, 3.07±0.29 and 2.51±0.36 (n=44). The 16S rRNA gene of micromanipulation-purified sMMPs and eMMPs were cloned and sequenced. Phylogenetic analysis based on the 16S rRNA gene sequence revealed that 565 sequences of MMPs belonged to 16 OTUs, affiliated with Deltaproteobacteria. Four OTUs displayed >3.48% sequence divergence and two OTUs displayed >7.26% sequence divergence with respect to previously reported MMPs. This result suggested that they represented six new species and two novel genera. These results indicated that the intertidal zone of Huiquan Bay has a high diversity of MMPs that bio-mineralize iron crystals and play an important role in iron cycling in such a complex environment. These observations provide a new perspective of the diversity of MMPs in general and expand knowledge of the occurrence of MMPs in the Huiquan Bay.

Keywords: Intertidal zone, Diversity, Multicellular magnetotactic prokaryotes (MMPs),  16S rRNA gene, Magnetosomes

How to cite: Yicong, Z., Wenyan, Z., Hongmiao, P., Kaixuan, C., Si, C., and Tian, X.: Diversity of multicellular magnetotactic prokaryotes in the intertidal zone of Huiquan Bay, Qingdao, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6208, https://doi.org/10.5194/egusphere-egu2020-6208, 2020.

EGU2020-12310 | Displays | EMRP3.2

Magnetotactic bacteria in Tengchong hot springs, China

Jia Liu, Wensi Zhang, Fang Yuan, Yongxin Pan, and Wei Lin

Magnetotactic bacteria (MTB) biomineralize intracellular magnetic nanocrystals and can use the geomagnetic field to navigate towards specific microenvironments in water columns and sediments. MTB are a model system to study the mechanisms of microbial magnetoreception and biomineralization. The majority of MTB identified so far are from environments with pH values near neutral and at the normal range of temperature. MTB from extreme environments, such as hot springs, has not been observed and described until recently. However, our knowledge on extremophilic MTB is still very limited. Here we report the identification and characterization of various MTB in Tengchong hot springs, China, with a temperature range of 41.3-69.5 °C and a pH range of 7.1-8.6. Although MTB are diverse in cell morphology, they all form bullet-shaped magnetite magnetosomes organized into either one chain or multiple bundles of chains. Through genome-resolved metagenomics, we have reconstructed five genome bins of hot spring MTB that are all affiliated within the Nitrspirae phylum. Genomic analyses and metabolic reconstructions are now in progress. These results will help to better understand the extremophilic MTB and may shed new lights on the origin and evolution of microbial magnetoreception and biomineralization.

How to cite: Liu, J., Zhang, W., Yuan, F., Pan, Y., and Lin, W.: Magnetotactic bacteria in Tengchong hot springs, China, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12310, https://doi.org/10.5194/egusphere-egu2020-12310, 2020.

EGU2020-8524 | Displays | EMRP3.2

Paleomagnetic investigation of the Tarhanian deposits of Cop-Takyl section (Kerch peninsula, Crimea)

Eugeniya Filina, Olga Pilipenko, and Yuliana Rostovtseva

With the goal to provide the new magnetostratigraphic investigations of the Miocene marine deposits of the Black Sea Basin the forty-four oriented hand blocks of the Cop-Takyl section (45°N, 36°E, Kerch peninsula, Crimea) were collected during summer 2019 field work. The section is composed mainly of clays, has a total thickness of ~ 53 m and covered the Tarhanian stratigraphic interval. Standard paleomagnetic measurements have been performed to establish a new magnetostratigraphic record for the Cop-Takyl section. The composition of the ferromagnetic fraction was examined using dependences of magnetic susceptibility on temperature and saturation magnetic moment on temperature. These thermo magnetic analyzes showed that the low concentration of magnetite is the main carrier of the natural remanent magnetization NRM. Coercivity of remanence Bcr values, determined from backfield demagnetization measurements, range between ~34 and 91 mT. The structure of the magnetite grains is mainly pseudo-single domain. In order to determine true NRM directions, we studied the anisotropy of magnetic susceptibility. The rock sample possesses a planar anisotropy, which is a characteristic of the normal sedimentary rocks. The alternating field demagnetization of the samples (three duplicates from each level) was used for obtaining NRM vector angle elements. Demagnetization results were analyzed using orthogonal plots and stereographic projections. Polarity components were isolated in most samples between 15-60 mT. The values of the declination D and inclination I of the NRM satisfactory agree for all three duplicates from each level. This allows to average angle elements and construct curves of I and D variations over the thickness of the section. New paleomagnetic data of the Cop-Takyl section will used for assessing the effect of astronomical cyclicity on sedimentation processes. This work was supported by Russian Science Foundation, project № 19-77-10075.

How to cite: Filina, E., Pilipenko, O., and Rostovtseva, Y.: Paleomagnetic investigation of the Tarhanian deposits of Cop-Takyl section (Kerch peninsula, Crimea), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8524, https://doi.org/10.5194/egusphere-egu2020-8524, 2020.

EGU2020-17434 | Displays | EMRP3.2

Paleomagnetism and magnetostratigraphy of the Permian-Triassic red beds, East European Platform, Russia

Anna Fetisova, Roman Veselovskiy, Valeriy Golubev, Alvina Chistyakova, Mikhail Arefiev, and Tatyana Bagdasaryan

We present the combining results of 6-year comprehensive studies, which have been done on fifteen key sections the Permian-Triassic red beds located within the Russian Basin (East European platform). In our presentation we discuss some aspects of paleomagnetism and rock magnetism of sediments, such as inclination shallowing, anisotropy of magnetic susceptibility and so on. The main achievement of our work is getting the new mean Permian-Triassic paleomagnetic pole for the East European platform as well as calculation of its Late Permian and Early Triassic poles. We also present new version of the magnetostratigraphic correlation of studied sections within the Russian Basin and with Global Geomagnetic Polarity Time Scale, taking into account obtained results of U-Pb LA-ICPMS dating of detrital zircons and paleontological constraints. One of the most intriguing conclusions of our work is a suggestion about the existing of quite long-lasting time interval of non-GAD (Geocentric Axial Dipole) configuration of the Earth's magnetic field close to the Permian-Triassic boundary, evidences of which we have found in some of studied P-Tr sections. This study is supported by the grant of the RFBR (18-05-00593).

How to cite: Fetisova, A., Veselovskiy, R., Golubev, V., Chistyakova, A., Arefiev, M., and Bagdasaryan, T.: Paleomagnetism and magnetostratigraphy of the Permian-Triassic red beds, East European Platform, Russia, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17434, https://doi.org/10.5194/egusphere-egu2020-17434, 2020.

EGU2020-17533 | Displays | EMRP3.2

Magnetostratigraphy and environmental magnetism of the Aptian-Albian boundary of sedimentary core from Sergipe-Alagoas Basin: preliminary results

Raquel Gewehr de Mello, Gerson Fauth, Karlos Guilherme Diemer Kochhann, Carolina Gonsalves Leandro, Mauro Daniel Rodrigues Bruno, Guilherme Krahl, Fernando Marcanth Lopes, and Jairo Francisco Savian

The Early Cretaceous was dominated by greenhouse conditions and increases in ocean crust production rate, with critical climate, geography and oceanography changes and abrupt shifts in redox conditions in the oceans. Prior to the Aptian, regarding the Earth’s magnetic field, a high rate of polarity reversals dominated. However, thereafter, a period of polarity stability, known as the Cretaceous Normal Polarity Superchron (CNPS), was established for 34 Myr. Although, there are debates on the causes and consequences of these extreme events. The exact behavior of the geomagnetic field in this period is still poorly understood, and data from volcanic and sedimentary rocks are conflicting. The biostratigraphy data from the sedimentary succession from the Aptian-Albian interval in the Sergipe-Alagoas Basin (Brazil) are rare and correlations are weak with Tethyan realm. Since some of the major reservoirs of the terrestrial portion of the Sergipe-Alagoas basin are from Aptian-Albian ages the lack of age models brings difficulties to the oil industry. Magnetic parameters such as magnetic susceptibility, ARM, IRM, and magnetostratigraphy data were obtained with a resolution of 25 cm in the Core SER-03 from Sergipe-Alagoas Basin. The entire section varies 4 Myr, including the Aptian-Albian boundary. Here, we present preliminary environmental magnetism and magnetostratigraphic interpretation for this core. Therefore, these data will aid to develop an age model framework in order to assist this uncovered region and for future comparisons with Tethyan realm.

How to cite: Gewehr de Mello, R., Fauth, G., Guilherme Diemer Kochhann, K., Gonsalves Leandro, C., Daniel Rodrigues Bruno, M., Krahl, G., Marcanth Lopes, F., and Francisco Savian, J.: Magnetostratigraphy and environmental magnetism of the Aptian-Albian boundary of sedimentary core from Sergipe-Alagoas Basin: preliminary results, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17533, https://doi.org/10.5194/egusphere-egu2020-17533, 2020.

EGU2020-1162 | Displays | EMRP3.2

Magnetostratigraphy and Carbon isotopes of Ediacaran Avellaneda Formation, Rio de La Plata Craton, Argentina

Jhon Afonso, Ricardo Trindade, Pablo Franceschinis, and Augusto Rapalini

The Ediacaran Period (635-542 Ma) witnessed a series of extraordinary events. It arises with the end of the Marinoan Glaciation and deposition of worldwide enigmatic cap carbonate deposits. This abrupt shift in paleoclimatic conditions coincides with major fluctuations in the  isotope ratios of carbon and sulfur, and with significant changes in the concentration of redox-sensitive elements in marine sediments. The Ediacaran is also a period marked by rapid changes in geomagnetic polarity. Magnetostratigraphy may therefore provide high-resolution correlation between Ediacaran successions worldwide. Here, we combine stratigraphy logs, carbon isotopes and magnetostratigraphy on the Avellaneda Formation (590-560 Ma) which at the Rio La Plata Craton, eastern Argentina. We investigated two drill cores (TSE-34 and TSE-7) with a 0.3-0.7 m resolution covering the entire Avellaneda Formation, corresponding to 98 standard specimens (25 mm in diameter). The basal contact of the Avellaneda Formation with the underlying mudstone rocks from Loma Negra Formation (~ 590 Ma) is present in both cores. The upper contact with the Alicia Formation, only observed in TSE-34 core, is transitional. The TSE-7 displays an erosional contact between Avellenda and Cerro Negro Formations (~ 560 Ma). After stepwise thermal demagnetization up to 600°C, almost all samples provided a characteristic magnetization between 350°C and 600°C, therefore Ti-poor magnetite or titanohematite is likely the main carrier of the stable remanence in these rocks. A high-temperature, dual-polarity component is persistent and coherent in the two drill cores. The base of the unit is marked by normal polarity, followed by a reverse interval, followed by persistent normal polarity across to the upper part of the Avellaneda Formation. This magnetostratigraphic framework, together with the carbon isotope signal, will be compared with results recently obtained for potentially coeval successions in China, Canada and United States.

How to cite: Afonso, J., Trindade, R., Franceschinis, P., and Rapalini, A.: Magnetostratigraphy and Carbon isotopes of Ediacaran Avellaneda Formation, Rio de La Plata Craton, Argentina, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1162, https://doi.org/10.5194/egusphere-egu2020-1162, 2020.

EGU2020-13109 | Displays | EMRP3.2

High-field slope correction of hysteresis loops: are we doing it correctly?

Xiang Zhao, Andrew Roberts, and David Heslop

Presentation of magnetic hysteresis data has long been a standard component of paleomagnetic, rock magnetic, and environmental magnetic publications. It has become standard practice to correct the high-field slope of hysteresis loops using a line fit through data points between 70 and 100% of the maximum applied field. Implicit to this approach is that the magnetization is considered saturated if the loop is closed at the point at which 70% of the maximum applied field is reached. This approach treats hysteresis overly simplistically because it assumes that the irreversible magnetization, which is what gives rise to hysteresis, is the only relevant part of the magnetization. The reversible component of magnetization is also important; this component approaches saturation non-linearly following the so-called law of approach to saturation, where the magnetization continues to increase due to rotation of magnetic moments parallel to the applied field, which is resisted by the anisotropy of the material. Various mathematical formulations exist for the law of approach to saturation. Use of this law is not straightforward for geological materials because terms in the respective equations depend on the material analysed and must be approximated, which becomes problematical for samples with mixed magnetic components. Alternatively, hysteresis loops can be fitted and extrapolated to high fields to estimate the approach to saturation using hyperbolic functions. We illustrate issues associated with linear slope correction at 70–100% of the maximum applied field by comparing hysteresis parameters estimated using approach to saturation fitting with various maximum applied fields. In all cases, for maximum fields used typically in mineral magnetic studies (e.g., 1 T), conventional slope correction underestimates the saturation magnetization Ms and overestimates the ratio of the saturation remanent magnetization Mrs to Ms. Hysteresis loop undersaturation is likely to be widespread in mineral magnetic studies with inadequate slope correction probably causing a large uncertainty in published hysteresis parameters. We recommend routine application of approach to saturation fitting of hysteresis loops, which can help to better estimate Ms and Mrs/Ms, as well as help to indicate whether a maximum applied field is sufficient to achieve magnetic saturation.

How to cite: Zhao, X., Roberts, A., and Heslop, D.: High-field slope correction of hysteresis loops: are we doing it correctly?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13109, https://doi.org/10.5194/egusphere-egu2020-13109, 2020.

EGU2020-6879 | Displays | EMRP3.2

A new furnace for improving thermal demagnetization in paleomagnetism
not presented

Huafeng Qin, Xiang Zhao, Shuangchi Liu, Greig Paterson, Zhaoxia Jiang, Shuhui Cai, Qingsong Liu, and Rixiang Zhu

Thermal demagnetization furnaces are routine facilities for paleomagnetic studies. The ideal thermal demagnetizer should maintain “zero” magnetic field during thermal treatments. However, magnetic field noises, including residual magnetic fields of material and induced fields caused by the heating current in the furnace are always present. The key to making high-performance demagnetization furnace is to reduce the magnetic field noises. By combining efficient demagnetization of shielding and a new structure of heating wire, we have developed a new demagnetization furnace with low magnetic field noises. Repeated progressive thermal demagnetization experiments using specimens that were previously completely thermal demagnetized above their Curie temperature were carry out to explore the effects of field within various types of furnace during demagnetization. These experiment confirm that magnetic field noises in the furnace can have an observable and detrimental impact on demagnetization behavior. Comparison between commercial furnaces and our new design show a notable reduction in the impacts of on thermal demagnetization behavior. The new heating element design and procedure for reducing magnetic field noises represent a significant improvement in the design of thermal demagnetizers and allows for extremely weak specimens to be successfully measured.

How to cite: Qin, H., Zhao, X., Liu, S., Paterson, G., Jiang, Z., Cai, S., Liu, Q., and Zhu, R.: A new furnace for improving thermal demagnetization in paleomagnetism, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6879, https://doi.org/10.5194/egusphere-egu2020-6879, 2020.

EGU2020-18667 | Displays | EMRP3.2 | Highlight

Elimination of the geomagnetic field impairs adult hippocampal neurogenesis and cognition

Bingfang Zhang, Lei Wang, Aisheng Zhan, Lanxiang Tian, Min Wang, Weixiang Guo, and Yongxin Pan

Elimination of the geomagnetic field impairs adult hippocampal neurogenesis and cognition

Bingfang Zhang1,2,4, Lei Wang3,4, Aisheng Zhan1,2,4, Min Wang3, Lanxiang Tian1,2,*, Weixiang Guo3,4,*, Yongxin Pan1,2,4,

1Biogeomagnetism group, Key Laboratory of Earth and Planetary Physics, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China

2Innovation Academy for Earth Science, Chinese Academy of Sciences, Beijing 100029, China

3State Key Laboratory for Molecular and Developmental Biology, Institute of Genetics and Developmental Biology, Chinese Academy of Sciences, Beijing 100101, China

4University of Chinese Academy of Sciences, Beijing 100049, China

The geomagnetic field (present-day intensity 25-65 μT, GMF) plays a fundamental role in the survival and evolution of organisms, but organisms including human beings could be exposed to hypomagnetic field (HMF, intensity < 5 μT), e.g., during geomagnetic polarity reversals, some artificial environments without GMF such as magnetic shielded room, and the prolonged periods in deep-space travelling. Previous studies have shown that HMF exposure could trigger central nervous system (CNS) dysfunction-like behavioral effects and influence the cognitive processes of various animals, from insects to human beings. However, the underlying mechanism is still an enigma. In general, adult hippocampus continuously generates new-born neurons throughout animals’ life which are functionally integrated into hippocampal circuits and contribute to memory and learning, and the process of adult neurogenesis has been shown to be strongly influenced by a variety of environment stimuli. Here, we show that long-term HMF exposure markedly attenuates cell proliferation, influences multiple stages of neurogenesis of adult hippocampus, resulting in the impairments of hippocampal neurogenesis and hippocampus-dependent cognition of mice. This study provides new insights into the potential risk of long-term HMF exposure on adult hippocampus in deep space missions.

How to cite: Zhang, B., Wang, L., Zhan, A., Tian, L., Wang, M., Guo, W., and Pan, Y.: Elimination of the geomagnetic field impairs adult hippocampal neurogenesis and cognition, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18667, https://doi.org/10.5194/egusphere-egu2020-18667, 2020.

EMRP3.5 – The strength and evolution of the Earth´s magnetic field: methods, data and models

Néel theory (doi: 10.1080/0001873550010120 ) predicts that natural remanent magnetizations (NRMs) of thermal origin will be nearly linearly related to the magnetic field in which they are acquired for field strenghts as low as the Earth's. This makes it in principle possible to estimate the strength of ancient magnetic fields. In practice, however, recovering the ancient field strength is complicated. The simple theory only pertains to uniformly magnetized (single domain, SD particles). While SD theory predicts that a magnetization acquired at a temperature T should be demagnetized by zero-field reheating to T, yet failure of this “reciprocity” requirement has long been observed and the causes and consequences for grains with no domain walls are unknown. Recent experiments (Shaar and Tauxe, doi: 10.1073/pnas.1507986112 and Santos and Tauxe, doi:10.1029/2018GC007946) have demonstrated that, in contrast to the stability of SD remanences over time, the remanence in many paleomagnetic samples typically used in paleointensity experiments are unstable, exhibiting an "aging" effect in which the unblocking temperature spectrum changes over only a few years.  This behavior is completely unexpected from theory. Solving these mysteries is key to cracking the problem of paleointensity estimation. In this presentation we will demonstrate that it is a shift in unblocking temperatures observed over even relatively short time intervals (two years) in certain samples that leads to the failure of reciprocity which in turn limits the ability to acquire accurate and precise estimates of the ancient magnetic field. From rock magnetic experiments (xFORCs) it seems likely that magnetic grains larger than the highly stable single vortex state are the source of the non-ideal behavior. This non-ideal behavior which leads to differences between known and estimated fields that can be rather large (up to 10 μT) for individual specimens, does appear to lead to a bias in field estimates.  It is unclear how this behavior can be compensated for using the most common paleointensity estimation methods.   

How to cite: Tauxe, L., Santos, C., Zhao, X., and Roberts, A.: Transforming understanding of paleomagnetic recording: Insights from experimental observations of laboratory aged thermal remanences, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1960, https://doi.org/10.5194/egusphere-egu2020-1960, 2020.

The arrival of the great Maori waka and the settlement of New Zealand some seven or eight hundred years ago are described in oral history, but details of exactly when and how colonisation occurred are undocumented. Radiocarbon dating of early archaeological sites is particularly problematic, due to the inbuilt age of datable materials, and non-linearity and ambiguity in the calibration of measurements to calendar dates. Hangi stones, used as heat retainers in traditional Maori earth ovens, hold thermoremanent records of Earth’s magnetic field at the time of their last cooling. Matching the directions of these magnetizations to established reference curves provides alternative, archaeomagnetic, estimates of age. Our results cover the past 700 years, with a cluster of dates between 1500 and 1600 AD, from both North and South Islands, but none earlier than 1300 AD, thus supporting a model of rapid coordinated migration around that time. Archaeointensity data have been obtained from sixteen distinct archaeological features, including twelve hangi from eight sites, and from them the first archaeointensity record for New Zealand has been constructed. To this has been added other archaeointensity and palaeointensity data from the SW Pacific region and virtual axial dipole moments (VADMs) have been plotted. This plot outlines steady VADM values of about 8 x1022 Am2 from 1000-1300 AD, and 9.5 x1022 Am2 from 1500 AD to the present, with an intervening sharp peak in the early 15th century when the VADM reached about 13 x1022 Am2. This peak bears many similarities to archaeomagnetic “jerks” and “spikes” in northern hemisphere records from the first millennia BC and AD. However, it is the first such feature to be found in the southern hemisphere at this date, suggesting, in accordance with recent modelling, that it may be a feature of the non-dipole field, associated with rapid growth and decay of an intense flux patch on the core-mantle boundary.

How to cite: Turner, G. M., Kinger, R., McFadgen, B., and Gevers, M.: Archaeomagnetic directions and intensities from New Zealand: evidence for a fifteenth century AD archaeomagnetic “spike” in the SW Pacific?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13317, https://doi.org/10.5194/egusphere-egu2020-13317, 2020.

EGU2020-11300 | Displays | EMRP3.5

Field Intensity changes during the past 40 ka

Jean-Pierre Valet, Franck Bassinot, Ramon Egli, and Anojh Thevarasan

The period encompassing the past 40 ka is crucial to constrain the characteristic time of the axial dipole, which is computed so far from the historical period and still fails to be tested against long-term field changes. The past 7 kyr of geomagnetic history are primarily documented from archeological artefacts, yet the last 4 kyr remain relatively poorly constrained. Beyond this period, we are dealing with long-term changes of the dipole field that are relatively poorly documented by sedimentary records or by volcanic lava flows. Many measurements of absolute paleointensity do not incorporate directional information, while it is crucial to document the entire field vector and consequently can only be analyzed in terms of virtual axial dipole moments (VADM). In summary, no high resolution dataset covers the field changes which followed the Laschamp event and therefore we have poor knowledge of the pattern of fluctuations and the rate of the changes that were associated with the field recovery after the Laschamp. We have selected a set of marine sedimentary cores based on the quality of their oxygen isotope records. Their deposition rates are comprised between 10 and 20 cm/ka and therefore offer a great potential to constrain the filed intensity changes with a resolution of the order of 100 ka. during this period. We will present the results obtained from 7 marine core records and investigate their common and their discrepant features in order to identify the true paleointensity signal.

 

How to cite: Valet, J.-P., Bassinot, F., Egli, R., and Thevarasan, A.: Field Intensity changes during the past 40 ka, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11300, https://doi.org/10.5194/egusphere-egu2020-11300, 2020.

EGU2020-1652 | Displays | EMRP3.5

Reconstruction of geomagnetic dipole moment variations for the last glacial period based on cosmogenic radionuclides from Greenland ice cores

Minjie Zheng, Anna Sturevik-Storm, Andreas Nilsson, Florian Adolphi, Ala Aldahan, Göran Possnert, and Raimund Muscheler

Geomagnetic dipole moment variations, for example associated with polarity reversals and excursions, are linked to changes in cosmogenic radionuclide production rates. Therefore, it is possible to reconstruct past changes in the dipole moment based on cosmogenic radionuclide records from natural archives such as ice cores. Here we present a geomagnetic dipole moment reconstruction based on 10Be and 36Cl data from two Greenland ice cores over the period from 11.7 ka to 108 ka BP (before present AD 1950). We find significant correlations between the cosmogenic radionuclides and climate proxies which may be due to the common transport and deposition processes of these species. In an attempt to minimize climate-related variations in our dipole moment reconstruction, we apply a multi-linear correction method by removing common variability between 10Be and 36Cl and climate parameters (accumulation, δ18O and aerosol data) from the radionuclide records. The comparison of the resulting cosmogenic radionuclide-based dipole reconstruction with independent geomagnetic field records shows good agreement. This validates the use of cosmogenic radionuclides in ice cores to reconstruct past geomagnetic dipole moment variations after correction for the climate effect.

How to cite: Zheng, M., Sturevik-Storm, A., Nilsson, A., Adolphi, F., Aldahan, A., Possnert, G., and Muscheler, R.: Reconstruction of geomagnetic dipole moment variations for the last glacial period based on cosmogenic radionuclides from Greenland ice cores, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1652, https://doi.org/10.5194/egusphere-egu2020-1652, 2020.

EGU2020-11157 | Displays | EMRP3.5

Testing continuity of the Hadean-Eoarchean geodynamo with zircon paleomagnetism

John Tarduno, Rory Cottrell, and Axel Hofmann

Understanding the pre-Paleoarchean geodynamo is arguably the greatest technical challenge for paleomagnetism: only silicate crystals bearing magnetic inclusions now found in younger sedimentary units may have escaped the metamorphism that otherwise excludes extant Paleoarchean to Hadean whole rocks from consideration. The recent optical and electron microscope documentation of primary magnetite inclusions in Jack Hills zircons (Tarduno et al., PNAS, 2020), previously predicted by paleomagnetic unblocking temperatures, together with microconglomerate test results, Pb-Pb radiometric age data and Li-diffusion constraints, support a geodynamo as old as 4.2 billion-years-old. While the available record is to first-order consistent with a continuous geodynamo since the Hadean, there are several 50-100 m.y. gaps in the record. Herein we examine these gaps and further test the paleointensity history derived from Jack Hills zircons through study of Paleoarchean and older detrital zircons of the Singhbum craton of eastern India. Preliminary paleomagnetic and paleointensity data suggest the presence of a primary magnetism, magnetite inclusion carriers and field strengths similar to those of the Jack Hills record.

How to cite: Tarduno, J., Cottrell, R., and Hofmann, A.: Testing continuity of the Hadean-Eoarchean geodynamo with zircon paleomagnetism, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11157, https://doi.org/10.5194/egusphere-egu2020-11157, 2020.

EGU2020-4406 | Displays | EMRP3.5

Fast geomagnetic field variations recorded in glacial Black Sea sediments

Norbert Nowaczyk, Jiabo Liu, and Helge Arz

How to cite: Nowaczyk, N., Liu, J., and Arz, H.: Fast geomagnetic field variations recorded in glacial Black Sea sediments, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4406, https://doi.org/10.5194/egusphere-egu2020-4406, 2020.

EGU2020-10488 | Displays | EMRP3.5

Modelling fast geomagnetic reversals

Stefano Maffei, Philip Livermore, Sam Greenwood, and Jonathan Mound

Field reversals are some of the most prominent and commonly known temporal variations of the geomagnetic field. Polarity changes have been observed in seafloor magnetisation patterns, volcanic records, sediment sequences, speleothem records, and have been reported in geodynamo simulations. However, many open questions remain concerning the phenomenology and underlying causes of this process and whether precursory signals can be detected prior to a reversal. In particular, there is currently no scientific consensus regarding the temporal scales over which geomagnetic reversals occur. Simple order-of-magnitude arguments suggest that the geomagnetic field might reverse over the magnetic diffusion timescale, which for the Earth’s outer core is on the order of tens of thousands of years; numerical simulations aimed at understanding Earth’s million-year evolution have predicted a time scale on the order of thousands of years. On the other hand, analysis of a lacustrine sequence in the central Italian Appennines suggests that the most recent geomagnetic reversal (the Matuyama-Brunhes transition) took place around 786,000 years ago in as short as 13 years [Sagnotti, L. et al. (2015). GJI, 204(2), 798-812.]. This extremely short decadal time scale challenges our current understanding of the geodynamo and present-day numerical models.  

Here we attempt to answer the question: how fast can the axial dipole component of the geomagnetic field to reduce to zero during a magnetic reversal?  To do so, we derive fluid flows at the top of Earth’s outer core that optimise the rate of dipole decay, subject to a minimal number of physical ingredients. Specifically, we neglect the internal dynamics and prescribe a total flow kinetic energy that is consistent with observational bounds. This technique, previously employed for the study of paleomagnetic intensity spikes, is extremely versatile and allows us to explore a wide range of hypotheses concerning the flow geometry, its complexity, and the configuration of the geomagnetic field prior to the onset of the reversal. Although the resulting flows may not be physically realisable, this technique provides justified bounds on the fastest plausible polarity reversal time scale. 

How to cite: Maffei, S., Livermore, P., Greenwood, S., and Mound, J.: Modelling fast geomagnetic reversals , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10488, https://doi.org/10.5194/egusphere-egu2020-10488, 2020.

EGU2020-10130 | Displays | EMRP3.5

High-coercivity magnetic minerals in archaeological ceramics: new insights from remanence acquisition and demagnetization measurements at elevated temperatures

Andrei Kosterov, Mary Kovacheva, Maria Kostadinova-Avramova, Pavel Minaev, Nataliya Sal'naya, Leonid Surovitckii, Svetlana Yanson, and Elena Sergienko

The thorough understanding of magnetic mineralogy is a prerequisite of any successful palaeomagnetic, and in particular, archaeomagnetic study. Magnetic minerals in archaeological ceramics and baked clay may be inherited from the parent material, or, more frequently, formed during the firing process. The resulting magnetic mineralogy may be complex, including ferrimagnetic phases not commonly encountered in rocks. Towards this end, we carried out a detailed rock magnetic study on a representative collection of archaeological ceramics (baked clay from combustion structures and bricks) from Bulgaria and Russia. Experiments included measurement of isothermal remanence acquisition and demagnetization as a function of temperature between 20°C and >600°C, and a variant of Lowrie 3-axis IRM test with measurements performed at elevated temperatures. For selected samples, low-temperature measurements of saturation remanence and initial magnetic susceptibility between 1.8 K and 300 K have been carried out.
All studied samples contain a magnetically soft mineral identified as maghemite probably substituted by Al and/or Ti. Stoichiometric magnetite has never been observed, as evidenced by the absence of the Verwey phase transition. In addition, one or two magnetically hard mineral phases have been detected, differing sharply in their respective unblocking temperatures. One of these unblocking between 540°C and 620°C is believed to be substituted hematite. Another phase unblocks at much lower temperatures, between 140°C and 240°C, and its magnetic properties correspond to an enigmatic high coercivity, stable?, low unblocking temperature (HCSLT) phase of McIntosh et al. [McIntosh, G., M. Kovacheva, G. Catanzariti, M. L. Osete, and L. Casas (2007), Geophys. Res. Lett., 34, L21302, doi: 10.1029/2007GL031168]. In a few samples high- and low-unblocking temperature magnetically hard phases appear to coexist, in the others the HCSLT phase is the only magnetically hard mineral present. We finally compare the samples performance in archaeointensity experiments with their respective magnetic mineralogy.
This study is supported by Russian Foundation of the Basic Research, grant 19-55-18006, and Bulgarian National Science Fund, grant KP-06-Russia-10.

How to cite: Kosterov, A., Kovacheva, M., Kostadinova-Avramova, M., Minaev, P., Sal'naya, N., Surovitckii, L., Yanson, S., and Sergienko, E.: High-coercivity magnetic minerals in archaeological ceramics: new insights from remanence acquisition and demagnetization measurements at elevated temperatures , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10130, https://doi.org/10.5194/egusphere-egu2020-10130, 2020.

EGU2020-7814 | Displays | EMRP3.5

Influence of firing conditions on the rock magnetic properties. Preliminary results from experimental heating experiments

Petar Dimitrov, Maria Kostadinova-Avramova, Andrei Kosterov, and Deyan Lesigyarski

Archaeomagnetism deals with baked clay materials carrying a thermoremanent magnetization acquired in the Earth`s magnetic field, which determines its importance for two scientific fields – geophysics and archaeology. It is well known that the success of an archaeomagnetic study is closely related to the magnetic properties of the materials used. In turn, the magnetic properties depend on the initial clay mineralogy, firing conditions and burial history. In order to get more information about the influence of the firing process, samples prepared of raw clays (taken from six different sources) were subjected to the successive experimental baking in three experimental combustion structures: open-hearth, single-chamber round furnace and double-chamber rectangular kiln. Heating and cooling temperatures in the various parts of the structures were constantly monitored. Rock magnetic measurements and analyses were carried out prior to, after the first and after the fourth experimental firing.

The heating/cooling cycle in the single-chamber furnace was the most prolonged. The temperatures achieved vary from 400 to 540°C displaying very uneven distribution after 400 °C. Maximum temperatures of about 850 – 900°C were reached in the hearth and in the double-chamber kiln but they were retained for a relatively short time (5 – 10 min) whether or not extra fuel was added. The heating and especially the cooling were the most homogeneous in the double-chamber kiln, where the cooling temperatures in its different parts varied within 50°C. In contrast, these temperatures differ by about 250°C in the single-chamber furnace and almost 400°C in the hearth.

X-ray diffraction analyses classify the chosen six clays as calcareous (all grayish clays) and non-calcareous (all brownish clays).  Magnetic susceptibility behaviour monitored during stepwise heating and the shape of alternative field demagnetization curves of laboratory induced isothermal magnetization divided clays into three groups. Remanence and magnetic susceptibility measured after the first experimental firing are quite variable according to the clay type, structure and samples position, but some trends are obvious. The lowest magnetic properties generally correspond to the samples heated in the single-chamber furnace where the lowest firing temperatures developed. However, in many cases the measurements for samples baked in the hearth and/or in the kiln are very close. The highest magnetic enhancement was always achieved in the double-chamber kiln but only in the parts farthest from the entrance. The reheating increases (except for one clay) and homogenizes the magnetic properties of the kiln samples but this pattern is not systematically observed for the hearth. Magnetically soft minerals dominate. Presence of a high-coercivity carrier (probably hematite) is supposed for three clays single-baked in the hearth and the single-chamber furnace (but only when the samples were placed in the parts with the most oxygen access). During the multiple experimental firing, some samples disintegrated in different extent.

This study is funded by the grant KP-06-N30/2 from the Bulgarian National Science Fund. The support by Russian Foundation of the Basic Research grant 19-55-18006 is also acknowledged.

How to cite: Dimitrov, P., Kostadinova-Avramova, M., Kosterov, A., and Lesigyarski, D.: Influence of firing conditions on the rock magnetic properties. Preliminary results from experimental heating experiments, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7814, https://doi.org/10.5194/egusphere-egu2020-7814, 2020.

EGU2020-7717 | Displays | EMRP3.5

Geomagnetic field variations and low success rate of archaeointensity determination experiments for Iron Age sites in Bulgaria

Maria Kostadinova-Avramova, Andrei Kosterov, Neli Jordanova, Petar Dimitrov, and Mary Kovacheva

Bulgarian archaeomagnetic database is the longest local geomagnetic field record covering almost completely the last 8000 years. However, very poorly constrained periods in need of elucidation still exist. Among the most problematic are the last 1200 years BC corresponding to the Iron Age in Bulgarian lands. In contrast to the relatively well studied Neolithic, Eneolithic and Bronze Age settlements, the Iron Age sites (especially in the Early Iron Age phase) are not sufficiently investigated from the archaeological point of view. The lack of precise stratigraphic frames significantly hampers the specification of sites chronology, and as a result, baked clay features that could serve as reliable reference points are rather scarce.

The most recent compilation of the Bulgarian archaeomagnetic database contains 18 intensity and 16 directional reference points belonging to the Iron Age, which have very uneven temporal distribution. To extend the coverage, 26 baked clay structures from nine different archaeological sites were sampled and archaeomagnetically studied producing ten new directional, but only five intensity data. It seems quite often materials from Iron Age combustion structures to possess magnetic properties unfavorable for archaeomagnetism, generally reflected in non-linear and concave Arai plots. Hereby, the lowest success rate of archaeointensity determination experiments is registered for this period within the whole database. Usually it is much easier to determine the past geomagnetic field direction compared to the intensity and the failure of the Thellier experiment is not surprising. Nevertheless, more than 90% of the investigated features belonging to all the other epochs except Iron Age normally display success rates over 50%, as only for 1-2% the archaeointensity determination experiments fail completely. In contrast, in the Iron Age the successful features comprise only 56% of the total number studied with 28% failure. A possible explanation for this observation was sought in some specific inappropriate conditions during firing/burial time that affect magnetic properties of baked clay materials during and/or after their thermoremanence acquisition.

This study is supported by the grant KP-06-Russia-10 from the Bulgarian National Science Fund and Russian Foundation of the Basic Research grant 19-55-18006.

How to cite: Kostadinova-Avramova, M., Kosterov, A., Jordanova, N., Dimitrov, P., and Kovacheva, M.: Geomagnetic field variations and low success rate of archaeointensity determination experiments for Iron Age sites in Bulgaria, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7717, https://doi.org/10.5194/egusphere-egu2020-7717, 2020.

EGU2020-4419 | Displays | EMRP3.5

Archaeointensity data from Japan: current status and future perspectives

Tadahiro Hatakeyama, Evdokia Tema, and Naoko Matsumoto

Japan is a country with very rich cultural heritage and with many archaeological sites that can offer precious information about the geomagnetic field secular variation in the past. However, even though archaeomagnetic research in Japan started more than 60 years ago, with numerous studies focused on archaeodirection determinations of in situ archaeological structures, the available up to now archaeointensity data are still scarce. Most of the absolute intensity records come from archaeomagnetic studies carried in 60’s, 70’s and 80’s, mainly obtained with the original Thellier-Thellier method and/or its modifications. In none of these data, cooling rate and anisotropy corrections were applied. During the last 20 years, only two more archaeointensity studies have been published, applying the Tsunakawa-Shaw palaeointensity method on baked clays from Japanese kilns. This current status of archaeointensity studies in Japan makes evident the need of new high-quality reference data in order to reconstruct the geomagnetic intensity secular variation path in Japan. In this perspective, in the frame of the “Be-Archaeo” MSCA-RISE project, we have collected a total of 56 fragments of archaeological artifacts from the archaeological sites of Sada Higashizuka, Sada Nishizuka, Tatezaka, Tenguyama, Tatetskuki and Nima Ohtsuka, situated at the Okayama prefecture. The baked clays studied belong to ancient coffins, haniwa artifacts and pottery and their ages range from 100 AD to 675 AD. Preliminary rock magnetic and archaeomagnetic analysis including magnetic susceptibility, Q-ratio, isothermal remanent magnetization (IRM) curves, thermal demagnetization of a composite IRM component as well as stepwise thermal and alternating field (AF) demagnetizations were performed to investigate the magnetic mineralogy of the samples and their suitability for archaeointensity experiments. The results show the presence of a magnetic mineral with Curie temperature ranging from 480 to 560 oC, most probably magnetite and/or Ti-magnetite. IRM curves and AF demagnetization suggest also the presence of a high coercivity component in some samples, as saturation is not reached at 1 T and samples are not completely demagnetized at 180 mT. Demagnetization diagrams reveal a stable single component of magnetization for most of the samples. However, some samples demonstrate disturbed Zijderveld diagrams and/or two components of magnetization; no correlation between the quality of the results and the material studied (pottery, haniwa or coffin fragments) was found. These preliminary results were used to select promising samples for archaeointensity experiments, aiming to obtain new high-quality archaeointensity records for the Late Yayoi and Kofun periods.

How to cite: Hatakeyama, T., Tema, E., and Matsumoto, N.: Archaeointensity data from Japan: current status and future perspectives, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4419, https://doi.org/10.5194/egusphere-egu2020-4419, 2020.

EGU2020-5529 | Displays | EMRP3.5

Geomagnetic paleointensity and paleoinclination between 1200 and 1700 AD derived from brick buildings in northern and south-eastern Poland.

Jerzy Nawrocki, Karol Standzikowski, Olga Rosowiecka, Krystian Wójcik, Tomasz Werner, Maria Łanczont, Jan Gancarski, and Marcin Wiewióra

The bricks are one of the best material for archeomagnetic studies. They usually contain a very stable and intense remanent magnetization and their backing technique (i.e. horizontal location in the furnace) allow to determine the value of inclination of geomagnetic field.  The technique were not changing since the middle ages up to the half of the XIX century, when a machine production have started. Preliminary archeomagnetic studies of the brick samples from Poland that providing a general information about paleoinclination changes in Gdańsk since 1080 AD indicated that this material is suitable for determination of ancient geomagnetic field parameters. However, in spite of the presence of many brick objects, as well as early and great tradition of brick building in Poland, this region of Europe is still “Tabula Rasa”  on the map of current archeomagnetic investigations. The archeomagnetic curves for this part of  Central Europe will be constructed almost from foundations. Well defined curves with secular variations of geomagnetic field during last 2500 year were constructed for example in neighboring Germany. The bricks for our archeomagnetic study were selected from churches, castles and palaces of well-known age. The age uncertainty in each case was less than 25 years. In order to check historical ages, a comparative TL dating of selected bricks was also conducted. The paleointensity of geomagnetic field  was determined using the IZZI-Thellier-Thellier protocol. About 300 cylindrical  specimens from the bricks located in more than 50 historical objects were examined. Studies of magnetic carriers and studies of anisotropy of magnetic susceptibility and anisotropy of isothermal remanent magnetization were also performed. The obtained paleosecular curves were compared with the coeval data from other regions of Europe.

How to cite: Nawrocki, J., Standzikowski, K., Rosowiecka, O., Wójcik, K., Werner, T., Łanczont, M., Gancarski, J., and Wiewióra, M.: Geomagnetic paleointensity and paleoinclination between 1200 and 1700 AD derived from brick buildings in northern and south-eastern Poland. , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5529, https://doi.org/10.5194/egusphere-egu2020-5529, 2020.

EGU2020-5627 | Displays | EMRP3.5

Archeomagnetic studies of fired ceramics from Sakhtysh-I and Sakhtysh -II settlements (Ivanovo Region, Russia)

Olga Pilipenko, Inga Nachasova, Yuri Tsetlin, and Eugeniya Filina

With the goal to obtain new data of geomagnetic field intensity in the Bronze Age in the Eastern Europe the arheomagnetic study of fired ceramic samples from the settlements Sakhtysh-I and Sakhtysh - II were done. The settlements Sakhtysh-I and Sakhtysh -II are placed in Teikovo district of the Ivanovo region of Russia (56о48′ N, 40о33′ E). Archeological excavations of ancient ceramics were carried out by the Upper Volga Archeological Expedition of the Institute of Archeology RAS. The studied collection of pottery fragments belongs to three cultures: the Fatyanovo, the Fatyanoid (or the Fatyanivo-like) and the Textile ceramics culture.  The composition of the ferromagnetic fraction presented in the studied archaeological samples have been performed by the complex of standards petromagnetic methods. The thermomagnetic analysis (TMA) in dependence of the saturation magnetic moment on temperature and determination of the Curie points were carry out. Thus based on TMA one can conclude that the main carrier of the magnetisation of the samples is relatively resistant to heat maghemite. The size of grains lies in a pseudo single domain area. The determination of the ancient magnetic field intensity was carried out by modified Thellier method.  Based on the carbon-isotope dating the age of pottery fragments corresponds to the ~ 2000-700 years BC, and we can construct a curve of paleointensity variations of the geomagnetic field from the age.  The data obtained for this period can provide new information about variations of the geomagnetic field intensity during the Bronze Age, which will make it possible to specify the character of changes in geomagnetic field. Earlier for the time interval II millennium BC a certain amount of the geomagnetic field intensity data were obtained in the Russian Plain region. Due to the uncertainty of the dating, these data allowed us to evaluate only the general features of geomagnetic  field intensity variations.  Rapid sharp changes in field intensity occurred with an increase in the average level of the field intensity compared with the level in the previous two millennia. This work was supported by the Russian Foundation for Basic Research, project no. 19-55-18006 and the State task of the Schmidt Institute of Physics of the Earth RAS.

How to cite: Pilipenko, O., Nachasova, I., Tsetlin, Y., and Filina, E.: Archeomagnetic studies of fired ceramics from Sakhtysh-I and Sakhtysh -II settlements (Ivanovo Region, Russia), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5627, https://doi.org/10.5194/egusphere-egu2020-5627, 2020.

EGU2020-19991 | Displays | EMRP3.5

End-Member Modeling Analyses (EMMA) of pseudo-Thellier style experiments to derive absolute paleointensities from lavas

Liz van Grinsven, Tristan van Leeuwen, and Lennart de Groot

Over the past years several groups have made efforts to calibrate the ‘pseudo-Thellier’ technique to obtain paleointensities from materials that acquired their natural remanent magnetizations thermally, while avoiding heating the samples during the experiments. These calibrations revolve around mapping laboratory induced Anhysteretic Remanent Magnetizations (ARMs) to thermally acquired Natural Remanent Magnetizations (NRMs).

One approach has been to plot pseudo-Thellier slopes against paleointensities that are either known (for very young lavas) or result from different paleointensity techniques. Although the obtained calibration relation is linear and closely follows the data, the relation worryingly misses the origin, i.e. a pseudo-Thellier slope of 0 leads to a paleointensity of up to 14.7 µT. Currently, there is no satisfying explanation for this non-zero axis intercept. Another approach has been to calibrate the mapping between the TRM and ARM by giving (thermally stable) samples a remanent magnetization and force the calibration through the origin. Although to the current state of our knowledge this is theoretically correct, the mismatch between the calibration relation and the data introduced by this approach is evident. So far, neither of these approaches yielded a generically applicable and theoretically acceptable mapping between ARMs and TRMs.

Naturally occurring basalts, however, are assemblages of magnetic minerals differing in grain size, shape, and chemistry. Here we take a new approach to the interpretation of pseudo-Thellier data by trying to find end-members for the ARMs, through nonnegative matrix factorization, that represent these different magnetic minerals in the samples. With the idea that the quantity of these end-members in the different ARMs are related to the original NRMs intensities. We use a set of 580 samples from different volcanic edifices (Hawaii, Mt. Etna, Tenerife, Gran Canaria, and Iceland) that recently cooled in the Earth’s magnetic field, so in known field strengths. The first results that we will present are encouraging and address the current challenges with obtaining absolute paleointensities from lavas with a pseudo-Thellier approach.

How to cite: van Grinsven, L., van Leeuwen, T., and de Groot, L.: End-Member Modeling Analyses (EMMA) of pseudo-Thellier style experiments to derive absolute paleointensities from lavas, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19991, https://doi.org/10.5194/egusphere-egu2020-19991, 2020.

EGU2020-3242 | Displays | EMRP3.5

Preliminary absolute paleointensity estimation from a single volcanic-glass grain extracted from an unwelded pyroclastic flow

Yuhji Yamamoto, Hiromi Takeda, Masahiko Sato, and Hiroshi Kawabata

Many pyroclastic flows are distributed around Japan. They usually involve volcanic-glass grains. These grains are considered to form at the timing of volcanic eruptions and are expected to have magnetic inclusions consisting of tiny single (titano)magnetites with recording the paleomagnetic field. We have extracted single volcanic-glass grains of pumice-type with a diameter of 0.60-0.84 mm from an unwelded part of the Ito pyroclastic flow deposits (A-Ito, 26-29 ka; Machida and Arai, 2003), Kyusyu, Japan. A series of rock- and paleomagnetic measurements have been made on the grains.

Sixty-seven out of 88 grains had detectable intensities of natural remanent magnetization. Some of such grains were further investigated. Results of low-temperature magnetometry exhibited inflection points at 105-120 K, suggesting magnetite as a main remenence carrier. Stepwise alternating field demagnetization revealed an existence of stable characteristic remanence (ChRM) which was interpreted to be a primary component. 

Tsunakawa-Shaw method (Tsunakawa and Shaw, 1994; Yamamoto et al., 2003), one of the latest absolute paleointensity (API) techniques to date, was applied to selected grains having stable ChRMs. On the application we newly included measurements related to an isothermal remanent magnetization (IRM). Four successful results were obtained by an adoption of IRM corrections, giving an average API value of about 25 μT. This corresponds to a virtual axial dipole moment (VADM) of about 50 ZAm2, which is consistent with the contemporaneous VADM of the sedimentary record (PISO-1500; Channell et al., 2009). 

How to cite: Yamamoto, Y., Takeda, H., Sato, M., and Kawabata, H.: Preliminary absolute paleointensity estimation from a single volcanic-glass grain extracted from an unwelded pyroclastic flow, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3242, https://doi.org/10.5194/egusphere-egu2020-3242, 2020.

EGU2020-13712 | Displays | EMRP3.5

Constraining the Eruption History of the Rangitoto Volcano using Palaeomagnetic Data

Megan Allington, Andreas Nilsson, Mimi Hill, Neil Suttie, Ingeborg Hjorth, Linda Aulin, and Paul Augustinus

Rangitoto is an island volcano situated outside the city of Auckland, New Zealand. The volcano is the youngest and largest volcano in the monogenetic Auckland Volcanic Field (AVF), with the last eruption occurring about 550-500 calibrated years BP, a date determined from studying historical records. The eruption history of Rangitoto is unknown, however all other volcanoes in the AVF have a brief eruption history. In February 2014 a core spanning 127 metres in length was recovered, consisting of 53 lava flows varying in thickness from 1 to 15 metres. Radiocarbon dates taken from marine sediments found at the bottom of the core, underneath the Rangitoto’s lava flows, suggest that there was early activity as far back as 6000BP, after which Rangitoto may have been dormant until the main shield building phase at around 600BP. Magnetic mineralogy analysis has also shown that much of the core is a reliable recorder of the past geomagnetic field. 156 samples have been analysed for palaeodirectional data and 21 acceptable palaeointensity estimates have been accrued from a range of depths throughout the core length. The collected palaeomagnetic data are used to reconstruct variations in the geomagnetic field, which in turn are used to constrain the eruption rate. Preliminary results suggest that the palaeomagnetic data are incompatible with a short eruption duration of the shield building phase implied by the radiocarbon data (under 100 years) and more compatible with a longer duration of shield building for Rangitoto Island. We discuss alternative explanations for this discrepancy and potential implications of our results in regard to improving hazard planning in Auckland.

How to cite: Allington, M., Nilsson, A., Hill, M., Suttie, N., Hjorth, I., Aulin, L., and Augustinus, P.: Constraining the Eruption History of the Rangitoto Volcano using Palaeomagnetic Data, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13712, https://doi.org/10.5194/egusphere-egu2020-13712, 2020.

This work is devoted to paleomagnetic studies of lava samples from three volcanoes of Kamchatka in order to define the age of lava flows and to obtain data of paleosecular variations of the geomagnetic field for the Kamchatka region. We studied 53 samples from 7 sites from lava flows of the Avachinsky, the Gorely and the Tolbachik volcanoes. The study of paleosecular variations recorded in the magnetization of the lava flows of volcanoes makes it possible to create a magnetochronological scale for epochs of the same polarity.

According to the data of electron microprobe and thermomagnetic analyzes, the magnetic properties of samples from the lava flows of the Avachinsky volcano are mainly determined by titanic magnetite with a Curie temperature Tc = (540-580) °С. The study of magnetic mineral grains using electron and magnetic force microscopy showed the presence of decay structures in grains, indicating the high-temperature oxidation of titanomagnetite. Ferrimagnetic grains of samples from the Gorely and Tolbachik volcanoes are represented by titanomagnetite with a Curie temperature Tc = (200–300) °C. According to the hysteresis characteristics, the magnetic structure of the grains corresponds to a single-domain and pseudo-single-domain state. Thermal and magnetic cleanings showed the predominance of one component in the NRM. The geomagnetic field intensity was determined by the Thellier method in the Coe modification.

It was found that the paleointensity value Hanc = 55±3 μT, determined from the NRM of samples of the 2012 eruption from the Tolbachik volcano, differs from the modern magnetic field in the area of this volcano by the IGRF-12 model by only 4% (НIGRF = 53 μT). This indicates the reliability of our methodology for determining paleointensity from the most stable part of the NRM of igneous rocks.

A comparison of the coordinates of the paleomagnetic pole (N 66º±4º, E 266º±5º) and the virtual dipole magnetic moment of the Earth (VDM = 8.3±0.9*1022 A*m2) with data on variations of the geomagnetic field over the past 10,000 years [Burlatskaya, 2007; McElhinny, 1982] allows us to conclude that the investigated lava flow belongs to the historical eruptions of 1827. The coordinates of the virtual geomagnetic pole (N 83º±3º, E 254º±21º) and the value of VDM = 8.0±0.3*1022 A*m2 determined from the samples belonging to the second lava flow of the Avachinsky volcano indicate that rocks are formed in the result of the eruption, which occurred 5-5.5 thousand years ago.

It was revealed that the magnitude (Hanc =65±5μT) and the direction of paleointensity determined by the NRM of the samples from Gorely volcano significantly differ from the characteristics of the modern magnetic field. The assumption is made that the studied samples belong to the outpouring of lava, which occurred about 2.7 thousand years ago, during the "Sterno-Etrussia" geomagnetic excursion.

 

This work was supported by the Russian Foundation for Basic Research, project 20-05-00573.

How to cite: Sleptsova, I. and Maksimochkin, V.: Paleointensity derived from igneous rocks of Kamchatka volcanoes of the Late Pleistocene-Holocene epoch , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7954, https://doi.org/10.5194/egusphere-egu2020-7954, 2020.

A new full-vector palaeosecular variation curve for Italy is presented based on a selection of high-quality data from sites within a 1000 km radius around Viterbo. The intensity and direction curves were calculated separately, using an updated compilation of Italian archaeomagnetic data from both archaeological material and volcanic rocks. The quality of the data was carefully evaluated, with particular attention on the reliability of the dating of the volcanic rocks and on the quality of the archaeointensity determinations. Only data from volcanic rocks of undisputable age have been considered. The new curves were calculated using Bayesian statistics and cover the last three millennia. The directional curve is very well constrained whilst the intensity curve is characterized by a larger error envelope, highlighting the need for new high-quality intensity data from Italy. Despite the limited number of reference data, the Italian intensity curve confirms periods with high intensity values of around 80 μT at 800-700 BC and 700-800 AD, in accordance with the geomagnetic intensity spikes previously identified in Middle East and Western Europe. Thanks to the privileged geographical position of the Italian peninsula, situated almost in the center of the Mediterranean, the Italian secular variation (SV) curves were used to analyze the evolution of the geomagnetic field in Europe by comparing them with other recently published SV curves for Western and Eastern Europe and with geomagnetic field models. The new curves can be used for archaeomagnetic dating not only in Italy but also in other countries of Europe such as Croatia, Slovakia and Serbia where no local SV curves are available so far.

How to cite: Tema, E. and Lanos, P.: The first full vector palaeosecular variation curve for Italy based on revised data from archaeological material and volcanic rocks, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8261, https://doi.org/10.5194/egusphere-egu2020-8261, 2020.

Recent advances in geomagnetic field modelling of palaeomagnetic data have led to significant improvements of our understanding of the geomagnetic field and how it varies on millennial timescales. Among other things, palaeomagnetic field reconstructions have shown that large-scale non-dipolar field anomalies, similar to or even larger than the present day South Atlantic Anomaly (SAA), have occurred several times in the past, implying that such structures are not necessarily associated with polarity reversals or excursions. A recent study even suggests that such large-scale field asymmetries could be periodically recurrent features of the field. Here we present results from a new Holocene geomagnetic field model, pfm9k.2, constructed using a novel Bayesian approach to account for chronologic uncertainties and address problems with potential smoothing induced by post-depositional remanent magnetisations. We focus our attention on a particular time period, around 700BC, where our new model shows a large-scale field asymmetry in the northern hemisphere with a weak field anomaly in the North Pacific accompanied with strong field intensities in Europe. The field evolution predicted by the model during this time period shares many similarities with the present day field, including a rapid decay of the dipole moment, suggesting that there may be common driving processes. We discuss the physical implications of these results.

How to cite: Nilsson, A. and Suttie, N.: A North Pacific Anomaly around 700BC: a potential analogy to the present day geomagnetic field, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21424, https://doi.org/10.5194/egusphere-egu2020-21424, 2020.

EGU2020-449 | Displays | EMRP3.5

Pulses and decay of the dipolar field during the Holocene

Alicia González-López, Saioa A. Campuzano, Alberto Molina-Cardín, Francisco Javier Pavón-Carrasco, Angelo De Santis, and María Luisa Osete

Temporal changes in the main geomagnetic field, the so-called secular variation, can range from decades to millennia without showing any clear periodicity. A better knowledge of the secular variation behaviour is important to determine the mechanisms that maintain the magnetic field and can help to establish constraints in dynamo theories. Considering that the magnetic dipole contributes to around 90 % of the total main field, we have searched for periodicities in this component over the last 10,000 years using four global paleomagnetic field reconstructions (SHA.DIF.14k, CALS10k2, BIGMUDI4 and SHAWQ2k). We have applied three techniques commonly used in signal analysis: a) the Fourier transform to identify the characteristic frequencies of the dipole field; b) the Empirical Mode Decomposition to separate the secular variation of the dipole into short and long wavelength signals; and c) the wavelet analysis to know how the characteristic periods are distributed over the time studied. Results show that for short-wavelength terms we find a recurrent periodicity of 700 – 800 years, present throughout most of the last 10,000 years with small variations. Focusing on long-wavelength terms for SHA.DIF.14k and CALS10k2, we observed a drop in the dipole field, controlled by the axial dipole, starting around 7000 BC. We have fitted it as an exponential decay obtaining a relaxation time of 8,000 – 10,000 years, which well agrees with the theoretical diffusion time of the geomagnetic field. The dipole field starts to increase around 4,500 BC for nearly 4,000 years. If we consider that this increase is comparable to the charge of a capacitor, it would give a characteristic time of 15,000 years. However, the theoretical maximum value obtained for the axial dipole field is never reached and the charge stops at 40 % around the year 100 AD. At that time, the dipole impulse ended and the present large trend dipole decrease seems to start, with a relaxation time of 13,000 years.

How to cite: González-López, A., Campuzano, S. A., Molina-Cardín, A., Pavón-Carrasco, F. J., De Santis, A., and Osete, M. L.: Pulses and decay of the dipolar field during the Holocene, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-449, https://doi.org/10.5194/egusphere-egu2020-449, 2020.

EGU2020-6808 | Displays | EMRP3.5

Correlation based snapshot models of the archeomagnetic field

Maximilian Arthus Schanner, Stefan Mauerberger, Monika Korte, and Matthias Holschneider

For the global time stationary geomagnetic core field, a new modeling concept for Holocene archeomagnetic data is presented. Major challenges consist of the uneven data distribution, missing vector field components and non-linear relations between observations and the geomagnetic potential. Instead of a truncated spherical harmonics approach, we propose a fully Bayesian, Gaussian process based model. Inherently, the Bayesian approach provides location dependent uncertainties.

The geomagnetic potential is assumed to be a Gaussian process whose covariance structure is given by an explicit kernel function, including several hyperparameters. For this kind of non-parametric models, the full Bayesian posterior is numerically intractable. Instead, we propose an approximate computation using a Bayesian update system. In a first step, the full vector records are used to obtain, within Laplace approximation, a rough field estimate. This estimate serves as a point of linearization for the non-linear observations. The approximate posterior is then given by a Gaussian mixture. Marginals for all relevant parameters and the field itself can be computed. We are able to quantify the impact of data coverage on uncertainty reduction.

How to cite: Schanner, M. A., Mauerberger, S., Korte, M., and Holschneider, M.: Correlation based snapshot models of the archeomagnetic field, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6808, https://doi.org/10.5194/egusphere-egu2020-6808, 2020.

EGU2020-11984 | Displays | EMRP3.5

Global and regional geomagnetic variabilities recorded in late Quaternary sediments from the west Iberian Margin

Chuang Xuan, Matthew Nichols, Joseph Stoner, Carl Richter, and Gary Acton

High-resolution palaeomagnetic records preserved in sediments (especially those that are well dated) provide valuable continuous information on past changes in Earth’s magnetic field. These data are essential for us to develop better understanding on the dynamics and causes of geomagnetic changes at various time scales. In this study, we conducted palaeomagnetic analyses on continuous u-channel samples collected from well-dated late Quaternary sediment sequences cored in the west Iberian Margin during Integrated Ocean Drilling Program (IODP) Expedition 339. Natural remanent magnetisations (NRM) as well as a suite of laboratory-induced magnetisations of the samples were measured at 1-cm interval resolution on a superconducting rock magnetometer before and after stepwise alternating field (AF) demagnetisation. NRM demagnetization data of the samples reveal a very stable and well-defined primary magnetisation component. Chronology of the studied cores is well constrained and tied to the polar ice cores as well as the absolutely dated Asian speleothem records. Average sedimentation rates of the studied cores range between ~10 cm/kyr to over 70 cm/kyr. Relative palaeointensity (RPI) records reconstructed from these sediments, when placed on the acquired age models, correlate well with other global and regional RPI records on time scales of ~10 kyr or longer. RPI features recorded at higher sedimentation rate sites appear slightly younger (a few hundreds to a couple of thousand years), possibly due to effects of the sediment magnetisation lock-in process. These Iberian Margin RPI records also show common millennial to multi-millennial scale variabilities, especially after deconvolution and correction of the lock-in induced age offset.

How to cite: Xuan, C., Nichols, M., Stoner, J., Richter, C., and Acton, G.: Global and regional geomagnetic variabilities recorded in late Quaternary sediments from the west Iberian Margin, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11984, https://doi.org/10.5194/egusphere-egu2020-11984, 2020.

EGU2020-20101 | Displays | EMRP3.5

High resolution paleosecular variations recorded in Lake Pleshcheevo sediments (Yaroslavl region, Russia)

Danis Nurgaliev, Kuzina Dilyara, Kosareva Lina, Nurgalieva Nuriia, Krylov Pavel, Borisov Anatoly, and Khassanov Damir

In this paper we present the results of paleomagnetic investigations of the Lake Pleshcheevo sediments (Yaroslavl region, Russia). Sediments of modern lakes are a unique record's archive changes of environment, climate, geomagnetic field over the past millennia. From lake were selected 4 cores up to 6.3 meters. From the core using a special sampler was selected undeformed samples for petromagnetic and paleomagnetic investigations. Magnetic susceptibility, NRM (modulus and direction), demagnetization by an alternating magnetic field were made for all samples. The absolute age of the sediments was determined using the radiocarbon dating. To establish the absolute age of magnetization in sediments, it is necessary to use information from observational, archaeomagnetic, and other data. After correlation of data from Lake Pleshcheevo sediments with archaeomagnetic and other corrected limnomagnetic records, we were able to construct an adequate time scale for recording geomagnetic variations. The obtained changes of direction of the characteristic NRM component of sediments were compared with archaeomagnetic data and records of geomagnetic field variations reconstructed from studies of lake sediments in Western and Eastern Europe. There is observed a very good agreement of all these data. It testifies the high quality of the magnetic record in the sediments of the Pleshcheevo Lake and the need for special studies and obtaining a high-quality master curve of the geomagnetic field variations over the last 12 thousands years for the European part of Russia.

This work was funded by the Russian Science Foundation under grant № 18-17-00251. Authors acknowledge measurements at the resource Center "GEOMODEL", Scientific Park of Saint-Petersburg State University.

How to cite: Nurgaliev, D., Dilyara, K., Lina, K., Nuriia, N., Pavel, K., Anatoly, B., and Damir, K.: High resolution paleosecular variations recorded in Lake Pleshcheevo sediments (Yaroslavl region, Russia), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20101, https://doi.org/10.5194/egusphere-egu2020-20101, 2020.

EGU2020-3292 | Displays | EMRP3.5

Preliminary paleomagnetic results from PS97 cores from the Drake Passage for the past 110 ka

Jiabo Liu, Norbert Nowaczyk, Xufeng Zheng, Qinsong Liu, and Helge Arz

Paleomagnetic records reconstructed from globally distributed marine sediments have greatly improved our understanding of long-term paleosecular variations and geomagnetic excursions. Nevertheless, questions regarding to the development of the geomagnetic field anomaly in the Southern Atlantic Ocean and the asymmetric geomagnetic field between Northern and Southern Hemispheres are not yet satisfactorily resolved. Paleomagnetic data, particularly from the Southern Hemisphere, is needed to better define the global geomagnetic field configurations spanning paleosecular variations and excursions. In this study, three sediment cores (PS97-085, PS97-84, PS97-079) recovered from the Drake Passage, Southern Ocean were subjected to detailed rock magnetic and paleomagnetic investigations. Preliminary age models were obtained by correlating their magnetic susceptibility to the ẟ18O master record from Dome C, Antarctica. In addition, rock magnetic records of the studied PS97 cores were further correlated to that of core PS67/197-1 with AMS 14C age constraints. The results from PS97 cores are thus continuously covering the past about 110 ka. Rock magnetic results indicate titanomagnetite is the dominant magnetic carrier in the studied PS97 cores. Relative paleointensities (RPI) derived from these PS97 cores are comparable with the regional relative paleointensity records and the South Atlantic paleointensity stack (SAPIS). Additionally, anomalous inclinations at about 41 ka and 35 ka, observed in core PS97-085, are coeval with the Laschamps and the Mono Lake excursions, respectively. This study provides new paleomagnetic records from the Southern Ocean, though further age constrains are needed to consolidate the paleomagnetic interpretations. The up to now obtained paleomagnetic records, together with previous studies from the Southern Ocean, are aiming to clarify the asymmetric pattern of non-dipole geomagnetic field between Northern and Southern Hemispheres.

How to cite: Liu, J., Nowaczyk, N., Zheng, X., Liu, Q., and Arz, H.: Preliminary paleomagnetic results from PS97 cores from the Drake Passage for the past 110 ka, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3292, https://doi.org/10.5194/egusphere-egu2020-3292, 2020.

A series of paleomagnetic works relying on the ocean sediments present some significant astronomic periods, such as a 100 kyr quasi-period and 41 kyr obliquity signal. These studies provide the new insights unscrambling what and how the earth magnetic field changed in despite of the ongoing debating. Numerical studies of recent years also reveal the possibility of the precession drive the dynamos and influence the magnetic field. However, the less of reliable high-resolution paleomagnetic records besides of relative paleointensity reduce its credibility. Here, we present some detailed rock magnetic and paleomagnetic studies on the continuous 40-m-thick sediments in two parallel cores retrieved from Tianyang Maar lake, southern China. The new results would contribute to discuss the correlation of paleomagnetic field with the astronomical factors.

Tianyang Maar lake  is located in the southern part of the Leizhou Peninsula.  The maar lake has a surface area of ~ 7.3 km2 surrounded by a 40 - 60 m high crater rim composed of basaltic breccia and tuff . Two new parallel cores, TY08 and TY15 (~ 10 m apart), were extracted from center of the crater in 2008 and 2015, respectively, using a rotary borer consisting of a stainless steel outer tube and a plastic inner tube to minimize sediment disturbances and contamination. The sediments of two cores can divided into three zones: about upper 15.59 m was composed of varying colors clay and the middle part (15.59-21.94 m), was dominated by the grey and greyish-brown fine to coarse sand with occasional gravels, embedded a thick grey clay layer; the lower part (21.94-40.0 m) shown as the dark grey and black organic-rich clay.

The paleomagnetic results show that the natural remanent magnetization (NRM) of the sediments is mainly contributed by magnetically soft minerals, and the sediments have fairly documented geomagnetic field variations. A chronology is constructed using multiple methods, including radiocarbon dating, optically stimulated luminescence dating and terrestrial-marine pollen correlation. The 340-kyr paleomagnetic inclination record displays patterns similar to those seen in regional records over a large spatial scale (> 3000 km), implying that these records may reflect large-scale core dynamics on timescales of 104 - 105 years in this low-latitude region. The Tianyang inclination record exhibits a negligible inclination anomaly (∆I = -0.08°) and features six anomalous inclination events, which are assigned to the Laschamp, Blake, Fram Strait II/6α, Iceland Basin, Mamaku and 9α excursions respectively. The spectral and singular spectrum analysis (SSA) exhibit that the inclination does not show the significant signal of 100-kyr periodicity, however, the closed precession period is obvious in the third components of inclination (PC3). PC3 component shows nearly synchronous variations with the precession parameter while the opposite correlation appeared under the condition of eccentricity minima strong. This corresponding pattern hint us that astronomical parameters have the essential influence to the earth magnetic field, however, the different moving may forc or constrain the earth magnetic behavior.

How to cite: Yang, X., Chen, C., and Zheng, Z.: High-resolution Geomagnetic Field Records Decipher the Possible Precession links since about 340-kyr in a maar lake sediment sequence in tropical Asia, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3785, https://doi.org/10.5194/egusphere-egu2020-3785, 2020.

EGU2020-19562 | Displays | EMRP3.5

Dipole strength during the Matuyama-Brunhes reversal reconstructed from the deconvolution of magnetic and climatic modulation of 10Be/9Be records

Ramon Egli, Tatiana Savranskaia, Jean-Pierre Valet, Franck Bassinot, Laure Meynadier, Quentin Simon, Didier Bourlès, and Nicolas Thouveny

The global production rate of the cosmogenic isotope 10Be by cosmic ray spallation is modulated by the activity of the sun and the intensity of the far-reaching component of the Earth magnetic field, which is in turn dominated by the dipolar term. Therefore, sedimentary 10Be records can be used to reconstruct past variations of the geomagnetic dipole moment. However, several environmental factors affect the transfer of 10Be atoms from the high atmosphere and soils, where it is produced, to the sediment, introducing a significant climatic modulation that can, in worst cases, completely obscure the paleomagnetic signal. These factors include variations of the continental runoff, oceanic circulation, sediment fluxes, and sediment scavenging effi­ciency. The latter is largely removed by normalizing the 10Be record with the concentration of authigenic 9Be, which is accumulated by sediment particles in the same manner as the cosmo­genic isotope. Even with this correction in place, individual 10Be/9Be records are significantly influenced by climate, to the point that only major geomagnetic events, such as the MB reversal, can be recognized. We present a model, which, for the first time, enables to deconvolve, at least partially, the climatic and magnetic components of 10Be/9Be records on a set of cores from the Atlantic, Indian, and Pacific Ocean. The climatic modulation is composed of an additive term, which reflects Be recycling through diagenetic release from sediments, and a multiplicative term, which is dominated by oceanic current patterns. Knowledge of these terms enables to remove, at least partially, site-specific environmental effects, obtaining a corrected 10Be/9Be stack that can be inverted to reconstruct variations of the dipole moment during the last geomagnetic reversal.

How to cite: Egli, R., Savranskaia, T., Valet, J.-P., Bassinot, F., Meynadier, L., Simon, Q., Bourlès, D., and Thouveny, N.: Dipole strength during the Matuyama-Brunhes reversal reconstructed from the deconvolution of magnetic and climatic modulation of 10Be/9Be records, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19562, https://doi.org/10.5194/egusphere-egu2020-19562, 2020.

The knowledge of the geomagnetic field intensity during the Cretaceous Normal Superchron, a long term of forty million years without polarity reversals, may have a large impact on our understanding of the dynamo process occurring in Earth’s outer core. How, it is difficult to get the geomagnetic field behavior during the Cretaceous Normal Superchron resulting from the inadequate sampling or data of variable qualities from igneous rocks and sedimentary. Here we examine 20 magnetic anomaly profiles across the Cretaceous magnetic quiet zone of the Central Atlantic Ocean in the African flank extracted from the EMAG2v3, and calculate a synthetical magnetization profile based on the forward modeling method. We suggest that this profile records the high strength of geomagnetic field at the beginning of ~30 million years and low signal during the late period, which could be correlated with the low-resolution relative paleointensity record from the sediment samples at the Falkland Plateau, and which also could be found the VDMs/VADMs averaged by a 7-Ma sliding window from the absolute intensity records mostly from the MagIC database. Our results support the hypothesis that the distribution of heat flow along the core-mantle boundary is positively correlative to the intensity of the dipole field.

How to cite: Li, Y. and Liu, Q.: Geomagnetic Field Paleointensity during the Cretaceous Normal Superchron from Marine Magnetic Anomalies, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12152, https://doi.org/10.5194/egusphere-egu2020-12152, 2020.

EGU2020-18463 | Displays | EMRP3.5

The strength of the Earth Magnetic field around the Cretaceous Normal Superchron: new data from Costa Rica

Anita Di Chiara, Lisa Tauxe, Hubert Staudigel, Fabio Florindo, Yongjae Yu, Marino Protti Quesada, and Kaj Hoernle

There has been an increasing effort toward the constraint of the average and long-term variability of the magnetic field strength, fundamental to better understand the characteristics and behaviour of the geomagnetic dipole field. Nonetheless, open questions remain about the value of the average dipole field, the relation between dipole strength and excursion reversal. Indeed, depending on the criteria adopted to analyse the current database, different long-term average values can be found, leading to different answers. The reason for the open debate can explained with the limited amount of data from key time intervals and geographical areas, due to both to complexities behind the method to obtain absolute paleointensities (several methods and experimental designs, selection criteria, high failure rate, etc..) and suitable materials.

Here, we focus on the Cretaceous Normal Superchron, a long period, from approximately 121 to 83 Ma, when the magnetic field was characterised by a stable polarity. Yet, few paleointensity data were available so far. In this study, we present new results from 48 Submarine Basaltic Glass sites from pillow lava margins, sampled on the upper crust sequence of the Costa Rica Ophiolite. Ar/Ar ages along with biostratigraphic age constraints from previous studies indicate ages ranging from from 139 to 94 Ma. After 473 samples were measured using the IZZI-Thellier protocol and analysed using strict selection criteria, 13 sites between 109 and 133 Ma gave reliable and robust results. Our new results from Costa Rica suggest that the strength of the Earth Magnetic field during CNS, 70.2 ± 21 ZAm2  are slightly lower than the pre-CNS and also lower than, for instance, at Troodos Ophiolite (81 ± 43 ZAm2; Tauxe and Staudigel 2004), consistent with the observations by Tauxe (2006) of an average dipole moment being substantially less than the present day value.

How to cite: Di Chiara, A., Tauxe, L., Staudigel, H., Florindo, F., Yu, Y., Protti Quesada, M., and Hoernle, K.: The strength of the Earth Magnetic field around the Cretaceous Normal Superchron: new data from Costa Rica, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18463, https://doi.org/10.5194/egusphere-egu2020-18463, 2020.

EGU2020-13546 | Displays | EMRP3.5

The first definition of paleointensity in the Early Cretaceous basalts from the Franz Josef Land

Evgeniy Vinogradov, Andrey Eliseev, Dmitriy Metelkin, Victor Abashev, Valery Vernikovsky, and Nikolay Mikhaltsov

We present the first definition of paleointesity of the Earth’s magnetic field that were obtained in the Early Cretaceous igneous rocks from the Franz Josef Land archipelago (Hooker and Scott Kelty Islands). The age of magmatism was determined by U-Pb method as the Early Cretaceous, about 125 Ma. A mean paleomagnetic direction for these rocks was calculated as D=40.2 deg, I=75.5 deg, a95=2.1 deg, k=89.3, N=52. A corresponding paleomagnetic pole is now located at Plat=69.0 deg; Plon=180.3 deg, A95=3.7 deg. An assessment of the domain structure of ferrimagnets using the Day plot diagram shows that the carriers of the natural remanent magnetization are pseudo-single-domain grains of titanomagnetites with varying Ti-content. Magnetic remanence was unblocked in temperatures of 350-400 °C. Some samples are characterized by unblocking temperatures of 560 °C. The determinations of the absolute values of paleointensity were obtained by the Thellier-Coe method with the implementation of the procedure "check-points". The values of Banc vary within 8.4–16 µT, which is noticeably lower than the current magnetic field at the sampling point ≈55 µT. The corresponding VDMs of 1.13–2.25 × 1022 Am2, with the current value of VDM ≈8 × 1022 Am2. Numerous basalt flows are well studied by paleomagnetic and rockmagnetic methods, together with a large number of geochronological definitions, this makes basalts from the Franz Josef Land promising for obtaining new qualitative determinations of paleointensity in the Early Cretaceous time.

This work was supported by the RSF (project no. 19-17-00091) and the RFBR (project nos. 18-35-00273, 18-05-70035).

How to cite: Vinogradov, E., Eliseev, A., Metelkin, D., Abashev, V., Vernikovsky, V., and Mikhaltsov, N.: The first definition of paleointensity in the Early Cretaceous basalts from the Franz Josef Land, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13546, https://doi.org/10.5194/egusphere-egu2020-13546, 2020.

EGU2020-5776 | Displays | EMRP3.5

New 1.72-1.76 GA paleointensity data obtained on Proterozoic volcanic rocks from the Ukrainian Shield

Valentina Shcherbakova, Vladimir Bakhmutov, Valeriy Shcherbakov, and Grigoriy Zhidkov

The Precambrian period occupies ≈ 85% of the Earth’s geological history and accommodates all the main formation stages of the Earth as a planet, including the emergence of its magnetic field. Variations in the time-averaged geomagnetic dipole moment have the potential to learn about the long-term development of the geodynamo and its response to mantle forcing and thermal evolution of the core. But determinations of paleointensity (Banc) of the geomagnetic field during this period are sparse and of limited reliability. Here we report detailed palaeomagnetic and paleointensity studies combined with comprehensive investigations of magnetic properties of Proterozoic volcanic rocks from the Ukrainian Shield.

The Ukrainian Shield comprises the crust of the Palaeoproterozoic protocraton Volgo-Sarmatia, which together with the Fennoscandian crustal segment constitutes the East European Craton (Baltica). The different megablocks of Ukrainian Shield can be treated as a coherent unit since 1.77 Ga.  Our studies has been performed on gabbro-anorthosite complexes from Ingul megablock within the Korsun-Novomigorodsky Pluton (ages 1.75-1.72 Ga) and North-Western megablock within the Korosten Pluton (age ca 1.76 Ga). The high-temperature stable ChRM component was isolated in the interval of blocking temperatures of 500-580°C by more than 300 samples from 7 sites. The presence of dual-polarity high-temperature component, lack of signs of metamorphism and good agreement of the mean palaeomagnetic pole position obtained from the Ingul block with age ca.1.75 Ga (Φ=22.5º, Λ=167.3º, dp/dm=4.0/7.7) with previous studies of anorthosites (Elming et al., 2001) of similar age suggests a primary origin of ChRM.

Comprehensive investigations of magnetic properties of rocks, the electron microscopic images of thin sections and X-ray diffractograms were performed. Rocks demonstrate thermally stable successive Msi(T) curves with clearly pronounced near-magnetite Tc. The carriers of remanent magnetization are fine magnetite isolated needle-like and/or lamellar ferromagnetic particles dispersed in plagioclas. According to the thermomagnetic criterion, high-temperature pTRMs show typical SD-PSD behavior. Palaeointensity determinations were successful on samples from 5 sites carrying well-identified ChRM components using the Thellier-Coe method with pTRM checks and the Wilson protocols. Reliable Banc values give generally low palaeofield (3.7-6.6 µT) with corresponding VDM values in the range (0.93-1.6)×1022 Am2. These findings agree with our previous results for Proterozoic rocks of Kola Peninsula (age 1.86 GA) and with the data reported in the World paleointensity databases (http://wwwbrk.adm.yar.ru/palmag/index_e.html and others data), which also provide a noticeably low paleofield intensity with mean VDM = 3.2×1022 Am2 for the Paleo-Proterozoic period. Thus, our new data support the Proterozoic dipole low hypothesize by Biggin et al., 2009. The work was supported by the state assignment 17-05-00259 and the RFBR grant 19-05-00433.

How to cite: Shcherbakova, V., Bakhmutov, V., Shcherbakov, V., and Zhidkov, G.: New 1.72-1.76 GA paleointensity data obtained on Proterozoic volcanic rocks from the Ukrainian Shield, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5776, https://doi.org/10.5194/egusphere-egu2020-5776, 2020.

We present absolute paleointensity results obtained from a collection of samples from ~250 Ma Kuznetsk Traps (Kuznetsk Depression, Southern Siberia). In addition to similar age these rocks display geochemical signatures similar to those reported for basalts of Siberan Traps and represent the southernmost affinity of the latter.

The primary nature of magnetic remanence in studied rocks was established by previous paleomagnetic studies Rock magnetic analysis indicates that the main magnetic mineral is titanomagnetite in a predominantly single-domain state with Curie temperatures between ~275 and 350⁰C. Scanning electron microscopy showed that titanomagnetite grains range in size from 0.5 to 1 μm. Individual grains are separated from each other and “sealed” within silicate matrix, which largely predetermined perfect preservation of primary mineral textures.

Paleointensity estimates were obtained using the Coe-version of Thellier-Thellier double-heating protocol with partial TRM checks. 36 samples (5 sites taken along the Tom River) yielded straight Arai-Nagata diagrams within temperature interval between 100 to 275⁰C. The average paleointensity value obtained from these samples was calculated at 12.7 ± 1 µT (with a factor q of about 11) with corresponding VDM=2.1 ± 0.2 × 1022 Am2.

Arai-Nagata diagrams for 20 samples from two other sites (collected in quarries on the Karakansky ridge) display more complex behavior. Straight linear segment of Arai plots between ~100 and 300⁰C yielded an average paleointensity value of 44 ± 1 μT, which corresponds to a VDM=7.0 ± 0.1 × 1022 Am2. However on higher temperatures, NRM vs. TRM data have a trend of flattening that increase in artificial TRM is accompanied by no loss of NRM. We interpret this observation as a result of laboratory-induced thermochemical alteration, namely, unmixing of homogeneous Ti-magnetites into Ti-rich and Fe-rich phases, with the latter phase responsible for such NRM-lost vs. TRM-gained behavior. Thermomagnetic analyses on these samples indicated mineralogical changes that set approximately at 300⁰C, supporting our interpretation. However, reversible thermomagnetic curves and p-TRM checks within 10% from initial pTRM, below ~300⁰C suggest that paleointensities determined between ~100 and 300 ⁰C of Arai plots are trustworthy.

We will discuss our results and reasons for such radical differences between paleointensity estimates obtained from the same suite of rocks sampled at different locations.

The study was supported by the Russian Foundation for Basic Research, grant No. 18-05-00234 and the Russian Science Foundation, grant No. 19-17-00091.

How to cite: Eliseev, A., Mikhaltsov, N., and Kulakov, E.: New absolute paleointensity results from ~250 Ma Kuznetsk basalts. Weak versus strong geomagnetic field at the P-T boundary., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21196, https://doi.org/10.5194/egusphere-egu2020-21196, 2020.

EGU2020-9121 | Displays | EMRP3.5

Evaluating the anomalous palaeomagnetic field behaviour in the Ediacaran with new palaeointensity data from Laurentia and Baltica.

Daniele Thallner, Andy Biggin, Mimi Hill, Henry Halls, Phil J. A. McCausland, Valentina V. Shcherbakova, Valeriy P. Shcherbakov, and Vladimir G. Bakhmutov

Palaeomagnetic investigations from the Ediacaran period (635-541 Ma) give anomalous results, which might indicate unusual behaviour of Earth’s magnetic field. In contrast to the conventional geomagnetic dipole field, geocentric and aligned with Earth’s rotational axis, records of the palaeomagnetic field from several locations in Laurentia and Baltica indicate that the Ediacaran geomagnetic field might have been exceptionally weak and spent extended periods of time with its poles close to the geographic equator. Multi-method palaeointensity determinations have been performed on rocks from the Grenville Dykes (Canada, 584-598 Ma), Skinner Cove volcanics (Newfoundland, 550.5 Ma) and the Volyn Traps (Ukraine, 560-580Ma), confirming that the field was exceptionally weak, with VDM values between 4 and 15 ZAm2. These values could correspond to considerably lower VDM strengths predicted by geodynamo simulations for fields with low dipolarity before the onset of nucleation of the solid inner core. In contrast, preliminary evaluation of published directional data indicates that palaeosecular variation in the Ediacaran might not be distinguishable from palaeosecular variation predicted from palaeomagnetic data of the last ten million years.

These new palaeointensity results contribute to the elimination of data gaps in the Neoproterozoic palaeomagnetic record and will be used in combination with qualitatively assessed published directional and intensity data to capture the behaviour of the geomagnetic field of this time period in statistical field models. Comparisons of time averaged palaeosecular variation and VDM distribution with predictions of the geomagnetic field from geodynamo simulations will help to verify and improve models of deep Earth structures and dynamics.

How to cite: Thallner, D., Biggin, A., Hill, M., Halls, H., McCausland, P. J. A., Shcherbakova, V. V., Shcherbakov, V. P., and Bakhmutov, V. G.: Evaluating the anomalous palaeomagnetic field behaviour in the Ediacaran with new palaeointensity data from Laurentia and Baltica., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9121, https://doi.org/10.5194/egusphere-egu2020-9121, 2020.

EMRP3.8 – Paleomagnetism and magnetic fabric: Recent advances and applications to Earth evolution its environment

EGU2020-207 | Displays | EMRP3.8

Magnetic fabric in brittle faults and ductile shear-zones: Examples from cataclasites from the Iberian Peninsula

Marcos Marcén, Antonio Casas-Sainz, Teresa Román-Berdiel, Belén Oliva-Urcia, Ruth Soto, Cristina García-Lasanta, Pablo Calvin, Andrés Gil-Imaz, and Luca Aldega

Shear zones, or their counterparts in near-surface conditions, the brittle fault zones, constitute crustal-scale, narrow, planar domains where deformation is strongly localized. The variation with depth of deformation conditions (P-T), rheology and strain rates entails a wide range of fault rock types, characterized by different petrofabrics and classically grouped into mylonitic (fault rocks undergoing crystalline plasticity) and cataclasitic (fault rocks undergoing frictional deformation) series. Magnetic fabric methods (most frequently anisotropy of magnetic susceptibility, AMS) have been established as a useful tool to determine fault rock petrofabrics in shear/fault zones, being interpreted as kinematic indicators with a considerable degree of success. However, mylonites and cataclasites show remarkable differences in magnetic carriers, shape and orientation of the fabric ellipsoid. Here, we present a study of ten brittle fault zones (one of them at the plastic-brittle transition) located in various locations in the Iberian Plate, with an aim  to interpret patterns of AMS in cataclasites.

Reviewing AMS studies dealing with SC mylonites, three fundamental features can be drawn: i) the presence of composite magnetic fabrics with shape and lattice-preferred orientations, ii) the fabric is carried predominately by ferromagnetic minerals and iii) surprisingly in composite fabrics, the absolute predominance of magnetic lineations parallel to (shear) transport direction (88% of the reviewed sites), independently of fabrics being defined by paramagnetic or ferromagnetic carriers. Based on our study, magnetic fabrics in cataclasites: i) are mainly carried by paramagnetic minerals and ii) show a strong variability in magnetic lineation orientations, which in relation with SC deformational structures, are either parallel to transport direction (44% of sites) or parallel to the intersection lineation between shear (C) and foliation (S) planes (41%). Furthermore, changes between the two end-members can be frequently observed in the same fault zone. Sub-fabric determinations (LT-AMS; AIRM and AARM) also indicate that the type of magnetic lineation cannot be consistently related with a specific mineralogy (i.e. paramagnetic vs ferromagnetic minerals).

The wide range of deformation conditions and fault rocks covered in our study allowed us to analyse the factors that control these different magnetic lineation orientations, especially in brittle contexts. Plastic deformation results into a mineral stretching parallel to transport direction which can be directly correlated with the development of transport-parallel magnetic lineation. In brittle fault zones, the degree of shear deformation can be directly correlated with the type of magnetic lineation. The fault cores, where strain and slip are localized, show a predominance of transport-parallel magnetic lineations, most probably related with the development of lineated petrofabrics. Furthermore, the minor development of shear-related petrofabrics enhance the frequency of intersection-parallel magnetic lineations, also contributing the presence of inherited, host rock petrofabrics in the fault rocks.

How to cite: Marcén, M., Casas-Sainz, A., Román-Berdiel, T., Oliva-Urcia, B., Soto, R., García-Lasanta, C., Calvin, P., Gil-Imaz, A., and Aldega, L.: Magnetic fabric in brittle faults and ductile shear-zones: Examples from cataclasites from the Iberian Peninsula, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-207, https://doi.org/10.5194/egusphere-egu2020-207, 2020.

EGU2020-11399 | Displays | EMRP3.8

AMS of strained shales fragments: a fast way to quantify the matrix damage.

Francho Gracia-Puzo, Charles Aubourg, Antonio Casas-Sainz, and Tiphaine Boiron

With the objective of mapping strain around a thrust front in an orogenic context (Pyrenean Range), 757 shale fragments (0.7-6.2 g) have been collected in 49 sites. Scalar data (degree of anisotropy P and shape parameter T), together with ellipse of confidence of individual axes provide a proxy of strain acquired by shales in the footwall of the main thrust (Saur et al. 2020).

Normally, sampling is done by two methods: collecting oriented decimetric hand specimens; or drilling 2.5 cm diameter cylinders. This presents the advantage to deal with oriented samples. However, those techniques are time consuming and it is difficult to collect numerous samples in loose materials like shales. On the contrary, collecting rock fragments present the net advantage to have a much better statistical description of the site. We are restricted by the dimensions of AGICO holders (8cm3 for cubes, or 10 cm3 for cylinder). It is possible to use an empty 10 cm3 cylinder, which could be filled with smaller fragments of rock. The homogeneity of magnetic field of MFK2 Kappabridge (AGICO) allows to measure sample with no distortion due to irregular shape. In addition, the automatic rotator allow a fast and precise description of the AMS tensor.

All samples belong to the Hecho Group (Eocene from Jaca Basin), consisting of cleaved or stratified marls. Rock fragments are mostly fractured according to the bedding and/or cleavage surfaces. Then we set the rock “horizontally” with the main surface parallel to the bottom of the box, to keep a geometrical reference. We assume that the anisotropy parameters P and T will maintain their values, regardless the shape and size of fragments. Rock magnetism indicates that AMS is primarily governed by illite, with little contribution of magnetite. AMS provides therefore a proxy of illite organisation within the matrix.

It is noticeable the speed with which data can be acquired in a well-known regional geological setting (757 samples, 49 sites) during 5 field work days and 17 laboratory days. About 15 fragments per site, covering few square meters, display homogenous pattern of P, T, and ellipse of confidence. The data visualization is done thanks to Anisoft 5.1 Software (Chadima, M.). We removed from analysis low susceptibility samples which are carbonate-rich and with more varieties of magnetic minerals. All sites present homogenous results at the site scale, but with significant differences with respect to strain. P and T parameters are very sensitive to strain as illite is the dominant carrier. In addition, the ellipse of confidence of minimum AMS axis (K3) provide a sensitive proxy to characterize the competition between bedding and cleavage.

This new approach is very promising, and allows much more detailed sampling in difficult area, with much more robust statistical description of scalar AMS data. Aubourg et al. (EGU, TS7.3 session) will use these data to show the pattern of strain in a ramp-related fold.

How to cite: Gracia-Puzo, F., Aubourg, C., Casas-Sainz, A., and Boiron, T.: AMS of strained shales fragments: a fast way to quantify the matrix damage., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11399, https://doi.org/10.5194/egusphere-egu2020-11399, 2020.

EGU2020-2868 | Displays | EMRP3.8

Lattice preferred orientation of graphite in a graphite ore as investigated by the anisotropy of out-of-phase magnetic susceptibility

Frantisek Hrouda, Martin Chadima, Josef Ježek, Štěpánka Mrázová, and Michal Poňavič

Literature data on single crystals of graphite shows that this mineral is diamagnetic and strongly anisotropic. In addition, it possesses high electrical conductivity and extremely strong conductivity anisotropy. The AC magnetic susceptibility of graphite exhibits weak and negative in-phase component and relatively strong out-of-phase component, which is no doubt due to electrical eddy currents. Consequently, if the graphite crystals are oriented preferentially by crystal lattice (LPO) in graphite ore, one would expect strong anisotropy of magnetic susceptibility (AMS) of the ore. Unfortunately, the standard AMS, which is in fact the anisotropy of the in-phase component of susceptibility (ipAMS), reflects not only the LPO of graphite, but also the preferred orientation of paramagnetic and ferromagnetic admixtures. On the other hand, the anisotropy of out-of-phase susceptibility (opAMS) indicates LPO of graphite, free of the effects of non-conductive paramagnetic and ferromagnetic minerals.

The above theoretical expectations were tested on natural South Bohemian graphite ores occurring in the wide vicinity of the town of Český Krumlov in the Moldanubian Unit and being mined for pencil industry in the past. The ores are metamorphic in origin and one can therefore expect strong LPO of graphite in them. The graphite ores were sampled in two localities near the town of Český Krumlov, one being a road cut outcrop and the other one being a graphite mine. In both cases, the in-phase susceptibility is very low, in the order of 10-6 [in SI units], being positive in some specimens and negative in the others. This indicates simultaneous and more or less balanced control by graphite and paramagnetic and/or ferromagnetic minerals. On the other hand, the out-of-phase susceptibility is much higher, in the order of 10-4, and no doubt indicates its graphite control. The degree of opAMS is truly high, P = 2 to 3, and the opAMS foliation is closely related to the metamorphic foliation in ores and wall rocks. All this indicates a conspicuous LPO of graphite in the ore that was probably created during Variscan regional metamorphism and associated ductile deformation.

How to cite: Hrouda, F., Chadima, M., Ježek, J., Mrázová, Š., and Poňavič, M.: Lattice preferred orientation of graphite in a graphite ore as investigated by the anisotropy of out-of-phase magnetic susceptibility, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2868, https://doi.org/10.5194/egusphere-egu2020-2868, 2020.

Field impressed AMS fabric, although it has been recognized for a very long time, has been the subject of very few publications in the paleomagnetic literature. This effect has been mainly described in samples with magnetite as a main magnetic carrier. This fabric is usually of low magnitude and observed mainly in nearly isotropic rock after application of static AF demagnetization or after acquisition of an isothermal remanent magnetization (IRM). Forty four paleomagnetic sites have been sampled in a >2 km thick sequence of Cretaceous volcano-clastic rocks from the western Central Pamir mountain (Tadjikistan). These rocks present a medium grade level of metamorphism characterized by fine grained recrystallisation of biotite. The magnetic properties are very homogeneous across the sequence. Bulk magnetic susceptibilities vary between 150-250 μ SI. The AMS magnetic fabrics correspond to triaxial tensors with a well defined foliation plane and a steeply dipping magnetic lineation. The degree of anisotropy varies between 1.03 and 1.2. This fabric was likely acquired during the deformation associated with the emplacement of Middle Miocene gneiss domes. SEM/EDS data indicate that the main iron oxide mineral is hematite with up to 15% of ilmenite in solid solutions. This is in agreement with unblocking temperatures of SIRM around 630 °C, lower than the one of pure hematite. One of the most surprising magnetic characteristics of these rocks is the effect of strong-field remanent magnetizations upon the AMS. During the acquisition of an Isothermal Remanent Magnetization (IRM), the initial AMS is progressively obliterated by a new AMS fabric. The field-impressed AMS is characterized by a decrease of the magnetic susceptibility along the direction of the IRM and an increase in magnetic susceptibility in the orthogonal plane. The field-impressed AMS is thus mainly oblate with a degree of anisotropy usually between 1.2 and 1.4. As far as we know, such a strong effect has never been reported. In sandstone with detrital hematite as the main carrier, the degree of the induced AMS fabric is less than 1.02 suggesting that the ilmenite content in the metamorphic hematite is the main cause of the large observed field induced fabric in these rocks.

 

How to cite: Roperch, P., Aminov, J., Dupont-Nivet, G., Guillot, S., and Lagroix, F.: Large field impressed anisotropy of magnetic susceptibility (AMS) in metamorphic volcanoclastic rocks from the western Central Pamir with ilmeno-hematite as the main magnetic carrier., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2702, https://doi.org/10.5194/egusphere-egu2020-2702, 2020.

The dewatering and subsequent drainage of fluids from porous sediments in forearc regions controls heat flux and the frictional behavior of the plate boundary decollement and all other forearc faults. Here we present new rock magnetic datasets that help to depict the strain history and locus of fluid and gas migration across a shallow subduction thrust near the deformation front of the Hikurangi subduction margin (New Zealand). Site U1518 of International Ocean Discovery Program (IODP) Expedition 375 penetrated hanging-wall, the roughly 60 m thick fault-zone, and footwall sequences of the Pāpaku fault up to a maximum depth of 504 mbsf.

Rock magnetic investigations include the measurement of Anisotropy of Magnetic Susceptibility (AMS), static three-axis alternating field demagnetization (AFD), magnetic hysteresis, anhysteretic remanence acquisition (ARM) and S-ratio measurement. The datasets are presented for an interval between 275 and 375 mbsf, and encompass both fault-zone and directly adjacent sequences.

Throughout most of the sedimentary sequence, samples yield intensities of the natural remanent magnetization (NRM) between 10-5 and 10-6 Am2/kg. Magnetic coercivities range from 40 to 60 mT. During static AFD samples acquired a gyroremanent magnetization. These observations indicate the presence of authigenic greigite (Fe3S4). In two intervals, between 304 and 312, and 334 - 351 mbsf, samples yield distinctively lower remanence intensities (~ 10-7 Am2/kg) and lower coercivities around 20 mT. The upper interval coincides with the onset of brittle deformation at the top of the fault-zone. In the same interval AMS results change abruptly. We propose that the rock magnetic signature is due to the reduction of ferrimagnetic greigite to paramagnetic pyrite (FeS2). This is most likely caused by the drainage of methane-, and sulfide rich fluids/gas along the upper fault-zone and supports interpretations that the fault zone acts as effective conduit. A continued transport of fluids/gases could have promoted a self-sustaining weakening and strain decoupling with episodic high pore-fluid pressure within localized parts of the fault-zone.

How to cite: Greve, A., Kars, M., Stipp, M., and Dekkers, M.: Characterizing sediment dewatering and constraining spatially limited fluid flux in accretionary systems. A rock magnetic approach., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17918, https://doi.org/10.5194/egusphere-egu2020-17918, 2020.

EGU2020-2064 | Displays | EMRP3.8

New constraints on the role of late Variscan extension in the origin of the Eastern Galicia Magnetic Anomaly (NW Spain)

Puy Ayarza, Juan José Villalaín, Jose Ramón Martínez Catalán, Fernando Alvarez Lobato, Manuela Durán Oreja, and Clemente Recio

The Eastern Galicia Magnetic Anomaly (EGMA) is one of the most conspicuous and, definitively, the best studied of all the magnetic anomalies in the Central Iberian Arc (CIA). This is probably due to its location, on the thoroughly researched Lugo-Sanabria gneiss dome and to the unique fact that its source rocks crop out in the Xistral Tectonic Window. Multiple studies and models of this anomaly have been carried out in the last 25 years and still, new results keep on shedding more light on its understanding. Rock magnetic analyses, natural remanent magnetization, anisotropy of the magnetic susceptibility and stable isotopes geochemistry carried out on the rocks that produce this anomaly have provided new insights on the processes that led to magnetization and on its age. Results suggest that magnetization of source rocks is a consequence of the increase in oxygen fugacity underwent by metamorphic and magmatic rocks affected by late-Variscan extensional tectonics. Extensional detachments were the pathways that allowed the entrance of fluids that led to syn-tectonic crystallization of magnetite and hematite in S-Type granites. Accordingly, magnetization is not really linked to primary lithologies but mostly to extensional structures. This process took place in the late Carboniferous to earliest Permian, during the Kiaman reverse superchron. Natural remanent magnetization exhibited by hematite-bearing samples confirms the age of the magnetization and adds complexity to the interpretation of the EGMA, where remanence has been often largely ignored or underestimated. Understanding the origin of the EGMA contributes to the interpretation of other anomalies existing in the CIA, also located on thermal domes. Furthermore, it provides new hints to interpret magnetic anomalies located in extensional tectonic contexts worldwide

How to cite: Ayarza, P., Villalaín, J. J., Martínez Catalán, J. R., Alvarez Lobato, F., Durán Oreja, M., and Recio, C.: New constraints on the role of late Variscan extension in the origin of the Eastern Galicia Magnetic Anomaly (NW Spain), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2064, https://doi.org/10.5194/egusphere-egu2020-2064, 2020.

EGU2020-16795 | Displays | EMRP3.8

100 Ma palinspastic restoration of the Anemzi syncline from paleomagnetic results.

Angela Jimenez-Sanz, Pablo Calvín, Juan José Villalaín, and Antonio M. Casas-Sainz

The Atlas system is an ENE-WSW intracontinental chain that extends from Morocco to Tunisia. It is the result of the Cenozoic inversion of a set of intraplate extensional basins that started its development during the Triassic and continued during the Jurassic. The Central High Atlas (CHA) is located at the Moroccan part of the Atlas System, characterised by NE-SW to ENE-WSW tight anticlines that limit wide synclines with the same orientation.

In this work, we present a high resolution structural and paleomagnetic study in a representative area with a tectonic evolution characteristic of the CHA. The study area is formed mainly by the Anemzi syncline, a structure of about 28 km long and 12 km wide. This structure is filed in by lower to mid Jurassic marine carbonates, which gradually change upwards to continental red beds. Towards the south, the Anemzi syncline limits with a vertical set of Jurassic intrusive bodies together with Triassic shales and basalts. On the other hand, towards the north crops out Lower Jurassic carbonates in the north limb of the syncline, which overthrust Middle Jurassic rocks.

Alongside with other areas of the CHA, in the study area can be identified a widespread remagnetization that has been dated ca 100 Ma. This remagnetization happened after the extensional period, and before the Cenozoic deformation started. The fact that it is an inter-folding record, allows using an already proved method in the CHA to[o1]  restore the structures of the area, and so, erase the Cenozoic deformation to better understand all the structural evolution of the area.

Samples from 90 palaeomagnetic sites were collected from sedimentary rocks, together with 170 bedding sites. The paleomagnetic results can be divided depending on the lithology. (1)  Jurassic[o2]  limestones show, in addition to a viscous component, the remagnetization typical from the CHA: a component with maximum unblocking temperatures between 450⁰C and 550⁰C carried by magnetite. Also in this lithology, in few samples a component carried by pyrrhotite can be observed. (2) Red beds show also a Cretaceous overprint, but carried by hematite.

By applying Small Circles methods to the Cretaceous remagnetization, we have obtained the paleobedding at the remagnetization acquisition time (ca. 100 Ma). These results allow us to restore two geological cross-sections at the remagnetization time and compare their with the present-day geometry. Besides, two maps of dip domains have been done, one showing the present day structure and the other one using the dips at the remagnetization. This methodology is a remarkable tool to assess the evolution of singular structures and to separate the deformation related with the basinal period with those related with the subsequent inversion. This restoration is part of a bigger project, whose objective is to build two 3D model of the Anemzi syncline with both the present-day and the restored structure at ca. 100 Ma using the palaeomagnetic data.

How to cite: Jimenez-Sanz, A., Calvín, P., Villalaín, J. J., and Casas-Sainz, A. M.: 100 Ma palinspastic restoration of the Anemzi syncline from paleomagnetic results., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16795, https://doi.org/10.5194/egusphere-egu2020-16795, 2020.

EGU2020-4429 | Displays | EMRP3.8

The Bigger The Better? - Updates on the Statistical Limits of Nano-Palaeomagnetic Works and (sub)millimeter Samples

Thomas Berndt, Adrian Muxworthy, Karl Fabian, and Liao Chang

A recent trend in paleomagnetism is the study of samples of ever decreasing sizes, going down to (sub)millimeter scales and even microscopic scales (“nanopaleomagnetism”). These include studies of single-silicate-crystals, microscopic magnetic imaging of the cloudy zones in Iron meteorites, and recently even the determination of individual magnetic remanence carriers. As single-crystal and nanopalaeomagnetic methods are getting more adopted, it is getting increasingly important to assess the statistical reliability with which such small samples can record remanences from a physical perspective. We previously proposed a benchmark to assess small-scale samples of randomly oriented non-interacting single-domain (SD) particles and found that in most cases, the number of magnetic particles a sample must contain lies in the order of tens to hundreds of millions – or equivalently NRM strengths of the order of 10-12 Am2. In this talk, we present how this benchmark can be used as a simple yet indispensable tool to assess whether or not (sub)millimeter-size and nanopalaeomagnetic samples are able to statistically reliably record palaeomagnetic fields. Moreover, this talk will provide an outlook into future limitations but also opportunities of the statistical physics nature of microscopic magnetic particle systems. It will explore if multi-domain particles should ever be considered statistically reliable recorders, how interactions in SD particle clusters might affect statistical reliability, and will review the various challenges that Iron meteorites pose as a remanence recorder.

How to cite: Berndt, T., Muxworthy, A., Fabian, K., and Chang, L.: The Bigger The Better? - Updates on the Statistical Limits of Nano-Palaeomagnetic Works and (sub)millimeter Samples, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4429, https://doi.org/10.5194/egusphere-egu2020-4429, 2020.

EGU2020-577 | Displays | EMRP3.8

Wobbles in the Early Cambrian Earth's spin axis? New high-quality paleomagnetic data from NE Brazil

Paul Yves Jean Antonio, Ricardo Ivan ferreira da Trindade, Maria Helena B. M. Hollanda, and Bruno Giacomini

The Neoproterozoic-Paleozoic transition (~541 Ma) was a turning point in Earth’s history resulting in great biological changes between the microbial Precambrian life and the Ediacaran biotic revolution with the occupation of the sedimentary substrate, the dawn of biomineralization and the appearance of the earliest multicellular organisms. In parallel, this period is marked by a large plate reorganization leading to the assembly of Gondwana and by major climatic changes (extreme glacial events). Due in part to a poor paleomagnetic database for the different cratons in the Ediacarian-Cambrian times, the global paleogeography at that time still remains controversial. In this study we present a new paleomagnetic pole (Q= 6) for the Monteiro dike swarms in the Borborema Province (NE Brazil). They are fine-grained hornblende dolerite dated by U-Pb on zircon at ~538 Ma. Rock magnetic data indicate that magnetite and pyrrhotite are the main remanence carriers. Positive baked-contact tests support the primary remanence obtained for these dikes (19 sites). A positive reversal test (classified C) was also obtained from the 14 sites with normal polarity and the 5 sites with reversed polarity, indicating that the secular variations was eliminated with our sampling. Our new key pole is not consistent with the classical Apparent Polar Wander Path of the West Gondwana which consists of a long track from a southern polar position at ~590 Ma to an equatorial position at ~520 Ma. The Monteiro paleomagnetic pole suggest instead rapid and small oscillations of the APW, or wobbles, after 560 Ma. These rapid oscillations may be related to inertial readjustments in response to true polar wander (TPW) of the spin axis. TPW events have been suggested from 615 to 590 and then from 575 to 565 Ma in previous works. These TPWs are supposedly caused by changes in the inertia tensor of the Earth due to internal mass redistribution, related to rapid changes in subduction velocity. Possible links between these events and life evolution will also be discussed.

How to cite: Antonio, P. Y. J., da Trindade, R. I. F., Hollanda, M. H. B. M., and Giacomini, B.: Wobbles in the Early Cambrian Earth's spin axis? New high-quality paleomagnetic data from NE Brazil, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-577, https://doi.org/10.5194/egusphere-egu2020-577, 2020.

EGU2020-2001 | Displays | EMRP3.8

Rotational Evolution of Western Anatolia since the Miocene and Its Implications on the Subduction Dynamics of Eurasia-Africa Collision

Bora Uzel, Nuretdin Kaymakci, Elif Cakir, Levent Tosun, Murat Ozkaptan, Okmen Sumer, Klaudia Kuiper, Cor Langereis, and Ersin Koralay

The African-European convergent tectonic setting has resulted in a complex deformation history with several large-scale tectonic features in western Anatolia, where is dominated by a crustal-scale extension since the late Eocene. The Menderes metamorphic core complex, the İzmir-Balıkesir Transfer Zone, and the North Anatolian Fault Zone are some of these main tectonic features. To understand their spatio-temporal relationships we employ paleomagnetic, geochronologic and kinematic studies in the northernmost part of the western Anatolia, where these structures interacting with each other. 

Our results show that western Anatolia has experienced at least two separate rotational phases since the Miocene. The first rotational phase is clockwise and related volcanism is dated as 21–16 Ma. The second rotational phase is counterclockwise and related volcanic rocks are dated as 14–12 Ma. According to collected kinematic data, pervasive transcurrent tectonism was dominated during the first phase, while the second one was dominated by extensional (and/or transtensional) tectonism. Here, the mode of extension switched from distributed diffuse deformation to discrete local deformation, possibly due to tearing and retreating of the northward subducting African oceanic slab below the western  Anatolian crust. This interrelated process also led to the localization of the İzmir-Balıkesir Transfer Zone with the decoupling of strike-slip faults, and to the episodic exhumation of the Menderes metamorphic core complex. This study is supported by a Tübitak Project, Grant Number of 117R011.

How to cite: Uzel, B., Kaymakci, N., Cakir, E., Tosun, L., Ozkaptan, M., Sumer, O., Kuiper, K., Langereis, C., and Koralay, E.: Rotational Evolution of Western Anatolia since the Miocene and Its Implications on the Subduction Dynamics of Eurasia-Africa Collision, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2001, https://doi.org/10.5194/egusphere-egu2020-2001, 2020.

EGU2020-20258 | Displays | EMRP3.8

Unification of the Australian cratons before the formation of Nuna

Uwe Kirscher, Ross Mitchell, Yebo Liu, Adam Nordsvan, Grant Cox, Sergei Pisarevsky, and Zheng-Xiang Li

The paleogeography and chronology of the Paleoproterozoic supercontinent Nuna are highly debated. To further test the paleogeography of Australian cratons in the leadup to Nuna formation, we present new paleomagnetic results from two Paleoproterozoic rock formations in North Australia. First, we obtained paleomagnetic directions from the 1825±4 Ma, bimodal Plum Tree Creek Volcanics sequence located within the Pine Creek Inlier of the North Australian Craton. Second, we studied the 1855±2 Ma layered mafic-ultramafic ‘Toby’ intrusion from the Kimberley Craton (KC). Samples from both study areas reveal high quality, stable, magnetite related characteristic remanent magnetization directions. Combining within-site clustered mean directions, we obtained two paleopoles, which plot proximal to each other in the present day central Pacific Ocean, off the east coast of Australia. These results agree with previous interpretation that the Kimberly Craton was amalgamated with the rest of the North Australian Craton (NAC) prior to ca. 1.85 Ga. Comparing these new results with slightly younger poles from the NAC and slightly older, rotated poles form the West Australian Craton (WAC) reveal a high degree of clustering suggesting very minimal absolute plate motion between ca. 1.9-1.85 and 1.6 Ga before the final amalgamation of Nuna. All available paleomagnetic poles agree with an assembly, or close juxtaposition, of the two major Australian cratons (NAC and WAC) before 1.8 Ga. Furthermore, the individual virtual geomagnetic poles from the potentially slow cooled Toby intrusion show a non-fisherian distribution along a great circle. This spread might be related to previously interpreted major true polar wander events based on Laurentian data, which would be global if such an interpretation is correct. The assembly of proto-Australia prior to ca. 1.85 Ga roughly 250 to 300 Myr before the final stage of supercontinent Nuna’s amalgamation ca. 1.6 Ga suggests that assembling of major building blocks, such as Australia and Laurentia for the supercontinent Nuna and Gondwana for the supercontinent Pangea, is an important step in the formation of supercontinents.

How to cite: Kirscher, U., Mitchell, R., Liu, Y., Nordsvan, A., Cox, G., Pisarevsky, S., and Li, Z.-X.: Unification of the Australian cratons before the formation of Nuna, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20258, https://doi.org/10.5194/egusphere-egu2020-20258, 2020.

EGU2020-21920 | Displays | EMRP3.8

New paleomagnetic data on marine sediments from the Central Arctic Ocean

Daria Elkina, Thomas Frederichs, Walter Geibert, Jens Matthiessen, Frank Niessen, Alexey Piskarev, and Rüdiger Stein

Accurate dating of marine sediments from the Arctic Ocean remains a subject of great debate over the last decades. Due to the lack of adequate materials for biostratigraphy and stable isotope analyses, paleomagnetic reconstructions came into play here but though yielded ambiguous interpretations. Moreover, sedimentation rates in the Quaternary, determined for isolated morphological features in the Arctic Ocean, are often applied to the entire Arctic Ocean realm resulting in an inappropriate oversimplification of probably diverging regional depositional regimes.

Paleomagnetic studies on four long sediment cores, collected from the Mendeleev Ridge and the Lomonosov Ridge, complemented by the results from one core from the Podvodnikov Basin, have provided an opportunity to compare the sedimentation history of these profound structures in the Arctic Ocean. Cores PS72/396-5 and PS72/410-3 (Mendeleev Ridge), PS87/023-1, PS87/030-1 (Lomonosov Ridge) and PS87/074-3 (Podvodnikov Basin) were retrieved during expeditions of RV Polarstern in 2008, and 2014. Paleomagnetic, rock magnetic and physical properties measurements were carried out at the Center for Geo-Environmental Research and Modeling (GEOMODEL) of the Research Park in St. Petersburg State University, at the University of Bremen, and the Alfred Wegener Institute.

According to the results on the Mendeleev Ridge’s cores, complemented with 230Th excess study on core PS72/396-5, the Brunhes Matuyama boundary (0.78 Ma) is observed at the first meters below the seafloor. That, together with the Matuyama Gauss transition (2.58 Ma) recorded in both cores, implies the mean sedimentation rate in this area to be in the order of mm/kyr.

In contrast to the Mendeleev Ridge, the cores from the Lomonosov Ridge and the Podvodnikov Basin have shown a more complex paleomagnetic record with a relevant shift to negative inclinations significantly deeper downcore. This could signify a relevant difference in the sedimentation regimes between both ridges during the Quaternary.    

How to cite: Elkina, D., Frederichs, T., Geibert, W., Matthiessen, J., Niessen, F., Piskarev, A., and Stein, R.: New paleomagnetic data on marine sediments from the Central Arctic Ocean, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21920, https://doi.org/10.5194/egusphere-egu2020-21920, 2020.

In the paleogeographic reconstruction of the Rodinia supercontinent, the Tarim Craton is placed either on the periphery of the supercontinent to the northwestern Australia or in the heart of the supercontinent between Australia and Laurentia. The mystery of the Tarim Craton is caused by the lack of paleomagnetic data, especially during the Rodinia assembly. We present here new primary paleomagnetic data from ca. 900 Ma volcanic strata in the Aksu region of the northeastern Tarim Craton. Rock magnetic investigations reveal magnetite and hematite as the main magnetic carriers. Characteristic remanent magnetizations isolated from 15 sites show both normal and reverse polarities. A site-mean direction is calculated at Dg/Ig = 155.2°/47.5° (kg = 11.6, α95g = 11.7°) in geographic coordinate and Ds/Is = 205.2°/64.0° (ks = 24.4, α95s = 7.9°) after tilt-correction. The site-mean direction passes fold tests and a ~900 Ma paleomagnetic pole is calculated at λ/φ = -0.5°N/62.3°E (A95 = 11.8°) corresponding to a paleolatitude of 45.7° N. The data reveal a ~20° latitude difference between the northern Tarim (N-Tarim) and southern Tarim (S-Tarim) terranes. Together with the late Meso- to early Neo-proterozoic arc magmatism identified both in the central Tarim Basin and along the north margin of the Tarim Craton, a post-900 Ma cratonization of the Tarim Craton resulting from a dual subduction system is proposed. Finally, a new paleogeographic reconstruction of the Rodinia supercontinent is made with the Tarim Craton being placed to the northwestern Australia and cratonization of the Tarim Craton may occur during the Rodinia assembly.

How to cite: Zhao, P. and He, J.: Neoproterozoic (post-900Ma) cratonization of the Tarim Craton and its role in assembly of the Rodinia supercontinent, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11742, https://doi.org/10.5194/egusphere-egu2020-11742, 2020.

EGU2020-12816 | Displays | EMRP3.8

Enigma of the Jurassic monster shift of the North China block

Shihong Zhang, Yangjun Gao, and Qiang Ren

Accumulation of the global paleomagnetic data, from both continental and oceanic plates, may suggest a true polar wander (TPW) event in Jurassic, with a rotation axis located in the present northwestern Africa, but no consensus has been reached regarding to the initiation, duration and velocity of the TPW. As one of the eastern Asian blocks, the north China block (NCB) is then located far from the rotation axis of the TPW and the plate convergence between Siberia and the Amur-NCB, known as the subduction in the Mesozoic Okhotsk-Baikal ocean, did exist. Paleogeographic changes observed of the eastern Asian blocks in Jurassic thus should contain the TPW component and plate moving component. To better estimate the influence of the TPW in the Eastern Asia blocks, we carried out a new paleomagnetic and precision U-Pb geochronological study on the middle Jurassic lavas in the NCB. Being profoundly different to the recent paleogeographic model (Yi et al., 2019, https://doi .org/10.1130/G46641.1) that suggest that the NCB experienced a large latitudinal displacement (monster-shift) responding to the TPW event between ~174 and ~157 Ma, we suggest that the NCB, as well as other blocks already connected with it, do not record any monster-shift between ~170 and ~160 Ma. The strata, ranging from 160 to 145 Ma, however, yield considerable paleomagnetic variations and need further investigation.

How to cite: Zhang, S., Gao, Y., and Ren, Q.: Enigma of the Jurassic monster shift of the North China block, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12816, https://doi.org/10.5194/egusphere-egu2020-12816, 2020.

The magnetic fabric of strongly magnetic materials originates from (1) self-demagnetization in bodies of non-isometric geometry, and (2) magnetostatic interactions between bodies with non-random distribution. These contributions, termed shape anisotropy and distribution anisotropy, carry information about a rock’s formation or deformation history. Both may be important when magnetite grains control the anisotropy of a rock, or when the pore space of a rock is impregnated with strongly magnetic fluid. The relative importance of each contribution to the overall anisotropy is debated, partly because it is influenced by many factors, including the body shape, orientation, or spacing. Another challenge is that existing models of distribution anisotropy consider infinite regular arrangements of equal bodies and take into account nearest neighbour interactions only. These simplifications make it difficult to predict distribution anisotropy in real rocks, where particles or pores are distributed irregularly, and display a range of sizes, shapes and orientations. A new model is presented here, which calculates both shape and distribution anisotropy for irregular assemblages of diverse bodies – differing in their size, shape, and orientation. The model assumes ellipsoidal bodies of equal intrinsic magnetic susceptibility in a non-magnetic matrix. Input parameters include the coordinates of each body centre, dimensions and orientation vectors of the three principal axes. These can be derived from imaging and X-ray computed tomography, or be pre-defined parameters of man-made samples. Calculations were verified against magnetic pore fabric measurements performed on synthetic samples with known pore parameters. The model is expected to advance our understanding of the interplay between shape and distribution anisotropies in natural samples. Hence, it will facilitate structural interpretations in samples whose magnetic fabrics are predominantly controlled by magnetite, as well as the interpretation of magnetic pore fabrics in future studies.

How to cite: Biedermann, A. R.: Shape and distribution anisotropy of irregular arrangements of diverse bodies – a 3D computational model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1619, https://doi.org/10.5194/egusphere-egu2020-1619, 2020.

EGU2020-21596 | Displays | EMRP3.8

Magnetic fabric of Lamas de Olo Pluton: AMS and AARM fabrics comparison

Cláudia Cruz, Helena Sant'Ovaia, Maria Irene Bartolomeu Raposo, and Fernando Noronha

The Lamas de Olo Pluton (LOP) is a small outcrop located in the Northern part of Central Iberian Zone from the Iberian Variscan belt. The LOP is a post-tectonic (ca. 297.19 ± 0.73 Ma) pluton composed of different granites: Lamas de Olo (LO; medium to coarse-grained porphyritic granite, ilmenite, and magnetite-type), Alto dos Cabeços (AC; medium to fine-grained porphyritic, ilmenite-type granite), and Barragem (BA; leucocratic fine- to medium-grained, slightly porphyritic, ilmenite-type granite). The magnetic fabric was characterized by measurements of anisotropy of magnetic susceptibility (AMS), and anisotropy of anhysteretic remanent magnetization (AARM). Both techniques are based on the magnetic properties of rock minerals, but while AMS consider the contribution of all rock minerals (paramagnetic, diamagnetic and ferromagnetic s.l.), in the AARM, the fabric is exclusively given by the ferromagnetic s.l. minerals. A correlation between AMS and AMR tensor was established, in order to compare both fabrics. The magnetic lineation is Kmax or AARMmax and the magnetic foliation is perpendicular to Kmin or AARMmin. Considering the global magnetic fabric for all samples from all the granite set, the magnetic foliations (AMS: N166°, 82°NE; AARM: N167°, 83°NE) and the magnetic lineations (AMS: 23°- N166°; AARM: 68°- N163°) are coaxial in both tensors. On the other hand, the analysis of each site sampling shows some differences in the ilmenite-type granites. Magnetic lineations and foliations given by both tensors (AMS and AARM) are coaxial in the magnetite-type granites, meaning that the magnetite and paramagnetic (or diamagnetic) minerals have the same orientation. The coaxial AMS and AARM magnetic foliations are due to magnetite grains imitating the fabrics of paramagnetic phases, through preferred collage, or crystallization of magnetite along grain boundaries, or exsolutions of magnetite along biotite cleavage planes. However, in the ilmenite-type granites, the AMS and AARM foliations are parallel, but the AMS and AARM lineations are not coaxial. Previous magnetic mineralogy studies (e.g. thermomagnetic experiments and isothermal remanent magnetization) pointed out the presence of magnetite/Ti-poor magnetite in all LOP granites, even in the ilmenite-type, but in different proportions. The petrographic observations also showed that, in the ilmenite-type granites, the magnetite is often oxidized to hematite (martite). The presence of martite may justify non-coaxility linear fabrics. Regarding the LOP emplacement, WSW-ENE opening structures provided the space for magma ascending, with an NNW-SSE magmatic flow controlled by regional structures, as shown by the magnetic foliations and lineations ca. N170º trending. The absence of outcrop deformation and the lack of solid-state microstructures precludes the substantial deformation after full crystallization of LOP.

How to cite: Cruz, C., Sant'Ovaia, H., Raposo, M. I. B., and Noronha, F.: Magnetic fabric of Lamas de Olo Pluton: AMS and AARM fabrics comparison, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21596, https://doi.org/10.5194/egusphere-egu2020-21596, 2020.

EGU2020-20879 | Displays | EMRP3.8

Magnetic fabrics in Portuguese Variscan granites: structural markers of the Variscan orogeny

Helena Sant Ovaia, Ana Gonçalves, Claudia Cruz, and Fernando Noronha

This work focuses on the magnetic fabric of 20 variscan granitic massifs from northern and central Portugal and considers the Anisotropy of Magnetic Susceptibility (AMS) results obtained in about 750 sampling sites. In the northern and central Portugal, three main ductile deformation phases were recognized and described: D1, D2 and D3, being the variscan magmatism events mainly related to D3 phase. D3 produced wide amplitude folds with NW-SE subhorizontal axial plane and subvertical dextral and sinistral ductile shear zones, forming obtuse angles with the maximum compression direction, σ1, NE-SW oriented. The post-D3 brittle phase was responsible for the development of conjugate faults (NNW-SSE, NNE-SSW and ENE-WSW), related to a N-S maximum compression. The studied granites were subdivided according to U-Pb dating, field observations and considering the chronology of their emplacement relative to the D3 phase of Variscan orogeny. Therefore, the studied granites are subdivided into: (1) syn-D3 two-mica granites, ca. 311 Ma; (2) late-D3 monzogranites, biotite-rich and two-mica granites, ca. 300 Ma; (3) post-D3 monzogranites and biotite-rich granites, ca. 299 – 297 Ma. Magnetic fabric gives two types of directional data, magnetic foliations and magnetic lineations, which provide important information regarding the orientation of the magmatic flow, feeder zone location, relationship between the magma emplacement and tectonics and, also, the stress field. The data obtained for the magnetic fabric, based on AMS technique, allowed concluding: (i) syn-D3 granites show magnetic foliations and lineations consistent with the syn-D3 variscan structures ca. N110°-120°E, related to a NE-SW maximum stress field . The foliations are, mainly, subvertical (> 60º), which may indicate a high thickness of the granitic body and deep rooting; on the other hand, the magnetic lineations exhibit variables plunges. (ii) Late-D3 granites are characterized by foliations and lineations, dominantly NNW-SSE to NNE-SSW oriented. The foliations are subvertical dips (> 60º) and the lineations have, generally, soft plunges. (iii) Post-D3 granites have, in general, magnetic foliations and lineations associated with important regional post-D3 brittle structures, which display NNE-SSW and ENE-WSW trending. The subhorizontal fabric may suggest a small thickness of the granitic bodies. In all granite sets under study there is a dominance of weakly dipping lineations (slope <60º), indicating that the feeding zones are deep, which supports the idea of an emplacement at high structural levels.

Acknowledgments: The authors thank Department of Geosciences, Environment and Spatial Planning at Faculty of Sciences of the University of Porto and the Earth Sciences Institute (Porto Pole, Project COMPETE 2020 (UID/GEO/04683/2013), reference POCI-01-0145-FEDER-007690).

How to cite: Sant Ovaia, H., Gonçalves, A., Cruz, C., and Noronha, F.: Magnetic fabrics in Portuguese Variscan granites: structural markers of the Variscan orogeny, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20879, https://doi.org/10.5194/egusphere-egu2020-20879, 2020.

The paleo-position of the North China Craton (NCC) within the Supercontinent Nuna/Columbia is controversial. Hindered by ubiquitous alteration of the very ancient rocks, paleomagnetic studies have not been able yet to conclusively solve this puzzle. Comprehensive analysis on the relatively limited Precambrian records is essential to understand the geological history of these cratons. Within the NCC, the tectonic setting of a ~1.78 Ga large igneous province (LIP) is long debated. It is considered to be related to a paleoplume, post-collision extension, or an Andean continental margin. Knowing its mode of formation constrains the geological evolution of the NCC and its paleo-position within the Supercontinent Nuna/Columbia. Here we conduct a study into the anisotropy of magnetic susceptibility (AMS) in the dykes and lavas of the ~1.78 Ga LIP, together with systematic rock magnetic experiments, to constrain the geological background of the igneous event(s), to understand the tectonic evolution of the NCC, as well as its paleo-position within the assembly of the Nuna/Columbia supercontinent.

Thirty-three dykes in the northern and middle parts and thirty lavas in the southern part of the NCC were collected. Detailed rock magnetic analyses indicate PSD magnetite to be the dominant magnetic mineral in the samples, occasionally with pyrrhotite in the dykes and hematite in the lavas. The often observed relatively weak anisotropy degree suggests that the AMS ellipsoids probably portray magma flow-related fabrics. The inferred directions from the AMS fabrics of the lavas reveal a radial flow pattern with an eruption center located on the south margin of the NCC. The studied dykes show a predominance of horizontally to subhorizontally northward magma flow, with only few vertical intrusions. These observations imply that the ~1.78 Ga LIP may have formed by magma source(s) at the south margin of the NCC. Some localized magma sub-chambers may have formed during the propagation of the magma and could have been responsible for the less common vertically intruded dykes and the EW-trending dykes. Therefore, we favor a plume-related tectonic setting for the ~1.78 Ga LIP with the eruption center along the margin of the NCC. It can serve as an essential criterion to search for possible neighbour(s) of the NCC within Nuna/Columbia, which should preserve the relics of the ~1.78 Ga LIP. Our study, in combination with extant geological and paleomagnetic results suggests a close linkage of the NCC with the São Francisco-Congo, Rio de la Plata and Siberia cratons in the Nuna/Columbia supercontinent.

How to cite: Xu, H., Yang, T., Dekkers, M., Peng, P., Ge, K., and Deng, C.: Magnetic fabric and rock magnetic studies of the ~1.78 Ga large igneous province in the North China Craton and its implication for the configuration of the supercontinent Nuna/Columbia, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9703, https://doi.org/10.5194/egusphere-egu2020-9703, 2020.

The Capinha area is located in the Central Iberian zone and is characterized by several Variscan granites intruded in the Neoproterozoic–Cambrian metasedimentary rocks. The main goal of the study is to identify the deformation patterns and provide crucial information to investigate the evolution of the magnetic fabrics in a post-Variscan granite emplaced during the crustal thinning, at the end of the Variscan orogeny. In order to achieve these purposes, fieldwork, petrography, microstructures and anisotropy of magnetic susceptibility (AMS) analysis were undertaken. The AMS was measured in 160 oriented cores, collected from 20 sampling sites homogeneously distributed, allowing the quantification of scalar (magnetic susceptibility, K; paramagnetic anisotropy, Ppara; magnetic ellipsoid shape, T) and directional data (magnetic lineation, //K1; magnetic foliation, perpendicular to K3). The Capinha granite (CG), exposed over an area of about 7 km2, is a small circular circumscribed outcrop in the NE-SW contact between the regional Belmonte–Caria granite (301.1±2.2 Ma) and the metasedimentary sequences. The CG is cut by two main fracturing systems: N30º-40ºE and N110º-120ºE, both subvertical. The contact is sharp, intrusive and discordant with the general trending of the D1 and D3 Variscan structures registered in the metasedimentary rocks. The CG is homogeneous in the whole area and consists of a fine- to medium-grained, muscovite-biotite leucogranite. The CG exhibit a paramagnetic behaviour with a K mean of 73 µSI, belonging to the ilmenite-type granites. At several scales, the CG does not show any magmatic flow or ductile deformation patterns displaying Ppara of about 1.6%, which corresponds to dominant magmatic to submagmatic microstructures. The Ppara highest values are concentrated in the NE border associated to prolate ellipsoids (linear fabric). Based on the interpretation of the magnetic fabric, is possible to observe that the orientation of the magnetic foliation is variable ranging from NNW-SSE to NNE-SSW. Generally, the magnetic foliations are sub-horizontal, being the vertical dips observed in the NE border, near the intersection of the N100º-120ºE and the N30º-40ºE fractures. The arrangement of the magnetic foliations follow concentric trajectories, with the symmetry axe parallel to the major axis of the outcrop (roughly NNE-SSW). The magnetic lineations are mainly sub-horizontal NNE-SSW parallel to the granite major axis; although, in the SW border the lineations tend to be parallelized to the contact. The magnetic lineation arrangement develops linear trajectories converging to the NE zone, where the dip is strong. The common gently magnetic fabric suggests the roof of the CG intrusion. During the late stages of the Variscan orogeny (D3, 321-300 Ma), ductile extensional detachments promoted the thinning of a previously thickened crust, providing the opening of pre-existing structures and the production of new ones. These structures act as conduits for a passive magma ascending and emplacement at shallow levels. Therefore, it is suggested that the CG magma ascent and emplaced in the intersection of pre-existing fractures, located in the NE zone, and flowed to the SW, developing a small asymmetric laccolith, poorly eroded, with a tongue-shaped body.

How to cite: Gonçalves, A., Sant'Ovaia, H., and Noronha, F.: Deformation and magnetic fabric of the Capinha granite (Fundão, Central Portugal): ascent and emplacement mechanisms during the late-Variscan crustal thinning, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4940, https://doi.org/10.5194/egusphere-egu2020-4940, 2020.

Volcano eruption forecasting typically links ground deformation patterns to sub-surface magma movement. Injection and inflation of magmatic intrusions in the shallow crust is commonly accommodated by roof uplift, producing intrusion-induced forced folds that mimic the geometry of underlying igneous bodies. Whilst such forced folds have previously been described from field exposures, seismic reflection images, and modelled in scaled laboratory experiments, the dynamic interaction between progressive emplacement of hot magma, roof uplift, and any associated fracture/fault development remains poorly understood. For instance, analysis of ancient examples where magmatism has long-since ceased only provides information on final geometrical relationships, while, studies of active intrusions and forced folding only capture brief phases of the dynamic evolution of these structures. If we could unravel the spatial and temporal evolution of ancient forced folds, we could therefore acquire critical insights into magma emplacement processes and interpretation of ground deformation data at active volcanoes.

 

We put forth and aim to test a new hypothesis suggesting that thermoremanent magnetization (TRM) records progressive deflection of the host rock during incremental laccolith construction and that these measurements can be used to measure the rate of laccolith construction. Here, we integrate palaeomagnetic techniques with semi-automated, UAV-based photogrammetric structural mapping to test: (1) whether we can identify variations in Natural Remanent Magnetisation (NRM), TRM, and magnetic mineralogy across an intrusions structural aureole; and (2) whether measured magnetic variations can be related to deflection caused by incremental sheet emplacement. Our test site is located within the basaltic lava pile of the ~800 m wide structural aureole of the rhyolitic Sandfell Laccolith in SE Iceland, which intruded <1 Km below the palaeosurface at ~11.7 Ma. We discuss whether palaeomagnetic backstripping can be an effective resource to constrain the rate and magnitude of intrusion-induced forced fold evolution, and thus an effective tool in volcanic hazard assessment.

How to cite: McCarthy, W., Twomey, V., Magee, C., and Petronis, M.: Unravelling magma emplacement through palaeomagnetic backstripping of an intrusion-induced forced fold: A case study from the Sandfell Laccolith, SE Iceland, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20895, https://doi.org/10.5194/egusphere-egu2020-20895, 2020.

EGU2020-2986 | Displays | EMRP3.8

Strain partitioning in a collapsing hot orogeny

Filipe Temporim, Ricardo Trindade, Eric Tohver, Marcos Egydio-Silva, and Tiago Valim

Large, hot orogens are characterized by an orogenic plateau supported by a zone of weak ductile flow. During the collision phase, the magnitude of the belt and the temperature increase as radioactive crustal material is accreted, buried and heated. After convergence ends, no material is added to the orogenic system and the orogen undergo gravitational (or extensional) collapse that results from the lateral flow of the hot orogenic infrastructure. In the Araçuaí-West Congo orogen (AWO), the high temperatures, slow cooling, and excessive amount of melt in the hinterland, in the northern part of the belt, imply that a high temperature was maintained for a long time. Geochronologic results suggest that this internal domain was hot for a long time, cooling at < 3°/Myr since 600 Ma until 500 Ma, and cooling through the Ar/Ar retention temperature for biotite occurred around 470 Ma. In the south the collapse of the orogen is marked by the widespread intrusion of bimodal, composite plutons at ~500 Ma. Here we use the magnetic fabric (i.e. low-field anisotropy of magnetic susceptibility) of intrusions in the north and south sectors to track the kinematics and rheological changes across the belt. In the northern part of the AWO we studied the Padre Paraíso Charnockite and the southern part of the AWO we studied the Conceição de Muqui and Santa Angélica plutons. The Padre Paraíso charnockite has a coherent magnetic fabric, with magnetic foliations trending N-S, following the general structure of the belt in that sector. In turn, Conceição de Muqui and Santa Angélica plutons show a concentric distribution of foliations and lineations, in starking contrast with the general NE-SW trend of the belt in the south. This contrasting structural pattern for coeaval plutons along the AWO belt reveal the strain partitioning at the scale of the orogenic belt during the cooling of the AWO. At 500 Ma the hot northern sector remains warm enough to allow a coherent deformation of intrusions and host rocks. At the same time, more material was being added to the margins of the hot orogen, which already cold, with the diapire-like plutons structure being dominantly controlled by the forces of magma ascent and emplacement.

How to cite: Temporim, F., Trindade, R., Tohver, E., Egydio-Silva, M., and Valim, T.: Strain partitioning in a collapsing hot orogeny, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2986, https://doi.org/10.5194/egusphere-egu2020-2986, 2020.

As shown in the literature several times, the calculation of the anisotropy of magnetic susceptibility (AMS) of hematite single crystals using standard linear AMS theory reveals that the calculated minimum principal susceptibility is parallel to the crystallographic c-axis, but is negative, which is however not due to diamagnetism as evidenced by direct measurements of susceptibility along the principal directions.

Susceptibility of a few hematite single crystals from Minas Gerais, Brazil, was measured in 320 directions using a special 3D rotator and the measurements were processed through AMS calculation by means of standard linear theory and through constructing contour diagram in equal-area projection. In addition, the deviations of the measured directional susceptibilities from the directional susceptibilities calculated from the fitted AMS tensor were calculated. The crystals show extremely high anisotropy, the susceptibility measured along the basal plane is several hundred to several thousand times higher than that along the c-axis and the AMS ellipsoids are very oblate, nearly rotational. The contour diagrams show relatively simple patterns of directional susceptibilities, similar to those of the second-rank tensor. However, the calculated AMS ellipsoids are slightly more eccentric than is the surface connecting the directional susceptibility values. The present study is assessing whether, realizing that the susceptibility along the c-axis is about three orders lower than that along the basal plane and taking into account the directional distribution of the fitting errors, one can ascribe the existence of the negative minimum susceptibilities calculated through standard linear theory to imperfect techniques of second-rank tensor fitting rather than to complicated magnetization mechanisms.  

How to cite: Jezek, J., Chadima, M., and Hrouda, F.: On the origin of apparently negative minimum susceptibilities of hematite single crystals calculated from low-field anisotropy of magnetic susceptibility, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2993, https://doi.org/10.5194/egusphere-egu2020-2993, 2020.

A growing interest in isolating ferromagnetic fabric, i.e. magnetic fabric carried solely by ferromagnetic (sensu lato) grains, creates a need for optimization of laboratory protocols used to acquire the array of magnetic remanence vectors necessary to calculate the anisotropy of magnetic remanence (AMR) tensors. Before a laborious and tedious process of measuring large sample collections, several aspects shall be experimentally assessed, namely: (1) what type of magnetic remanence should be applied (e.g. isothermal, anhysteretic), (2) how magnetizing fields (AC and DC) control the acquired magnetization and its anisotropy, (3) how the viscous decay influences the measured remanence, and (4) how many and which magnetizing directions are necessary to obtain reliable and statistically sound AMR tensors. A careful examination of these factors considerably influences the quality of acquired AMR data. To facilitate the AMR tensor calculation, we present AREM – a simple and user-friendly toolbox embedded into Anisoft software. Prior to the tensor calculation, AREM provides a graphical visualization of a set of measured remanence vectors as spherical projections of unit vectors compared to the respective magnetizing directions and their intensity compared to the intensity of demagnetized state (background magnetization). The correction for the background magnetization is optionally done by a direct subtraction of measured background or by a mutual subtraction of antipodally magnetized vectors, if available. The AMR tensor is fitted by a least-square algorithm using a set of pre-selected full-vectors (full-vector method) or their projections to their magnetized directions (projection method). The calculated AMR tensors, including their confidence limits, can be immediately reviewed and processed using all features of the Anisoft software.

How to cite: Chadima, M.: AREM: A user friendly toolbox for calculating anisotropy of magnetic remanence, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17562, https://doi.org/10.5194/egusphere-egu2020-17562, 2020.

EGU2020-10284 | Displays | EMRP3.8

Anisotropy of magnetic susceptibility (AMS) of lava and volcanoclastic flows of the Stephano-Permian Cadi basin (central-eastern Pyrenees)

Ana Simon-Muzas, Antonio M Casas-Sainz, Ruth Soto, Josep Gisbert, Teresa Román-Berdiel, Belén Oliva-Urcia, Emilio L Pueyo, and Elisabet Beamud

The aim of this work is to apply the anisotropy of magnetic susceptibility (AMS) to determine the primary and tectonic fabrics of lava flows and volcanoclastic materials in one of the Pyrenean Stephano-Permian basins.

The Pyrenean Range is a double vergence orogen located at the northern end of the Iberian Peninsula. During Carboniferous-Early Permian times the extensional or transtensional regime dominant during the progressive dismantling of the Variscan belt resulted in the development of E-W elongated intra-mountainous basins. This process was coeval with an exceptional episode of magmatic activity, both intrusive and extrusive. The Cadí basin represents a good example of these structures were Stephano-Permian rocks are aligned along an E-W continuous outcrop and reach thickness of several hundreds of meters. The stratigraphy of the study area is characterized by fluviolacustrine sediments changing laterally to volcanoclastic and pyroclastic rocks with interbedded andesitic lava flows.  

A total of 75 sites (733 standard specimens) were studied and analysed throughout the volcanoclastic, volcanic and intrusive materials of the Stephano-Permian outcrops in the Cadí basin. Samples were drilled in the field along 5 sections with N-S or NW-SE direction in the Grey and Transition Unit. Afterwards, standard specimens were measured in a Kappabridge KLY-3 (AGICO) at the Zaragoza University to characterise the magnetic fabric. The susceptibility bridge combined with a CS-3 furnace (AGICO) was used for the temperature-dependent magnetic susceptibility curves (from 20 to 700 °C) to recognize the magnetic mineralogy. In addition, textural and mineralogical recognition in thin-sections of the samples was carried out in order to recognize the relationship between magnetic and petrographic fabrics.

The results shows that the bulk magnetic susceptibility of the specimens ranges between 118 and 9060·10-6 SI but most of the values are bracketed between 160 to 450·10-6 SI. Taking into account magnetic parameters (Km, Pj and T) there is no correlation between magnetic fabrics and magnetic mineralogy and there is a dominance of triaxial and prolate ellipsoids. Thermomagnetic curves indicate the dominance of paramagnetic behaviour in all the samples and except in one case there is a ferromagnetic contribution due to the generalised presence of magnetite.

Magnetic ellipsoids can be divided into four main types depending on the orientation of the main axes and associated with the lithologic types: 1) Kmax vertical and Kint and Kmin horizontal for small intrusive bodies (no restoring); 2) Kmax horizontal or subhorizontal and Kint and Kmin included in a subvertical plane (before and after restitution) for volcanic breccias; 3) Kmin vertical (after restoring) and three directional maxima for lava flows and 4) non-defined fabric for the explosive materials (probably due to their complex depositional mechanisms). In general, a dominant E-W magnetic lineation is observed in many sites, resulting either from dominant flow direction, or to secondary processes. This is the case for some of the magnetic ellipsoids, that seems to be affected by deformation, Kmin is not normal to bedding and therefore, they do not become vertical after bedding restitution.

How to cite: Simon-Muzas, A., Casas-Sainz, A. M., Soto, R., Gisbert, J., Román-Berdiel, T., Oliva-Urcia, B., Pueyo, E. L., and Beamud, E.: Anisotropy of magnetic susceptibility (AMS) of lava and volcanoclastic flows of the Stephano-Permian Cadi basin (central-eastern Pyrenees), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10284, https://doi.org/10.5194/egusphere-egu2020-10284, 2020.

EGU2020-4598 | Displays | EMRP3.8

Paleomagnetism on salt-detached syndiapiric overburden rocks from the Northern margin of the Basque-Cantabrian extensional Basin (N Spain)

Elisabet Beamud, Ruth Soto, Charlotte Peigney, Eduard Roca, and Emilio Luis Pueyo

The Basque-Cantabrian Basin is a hyperextended extensional basin that formed as the result of the opening of the Bay of Biscay at latest Jurassic-middle Cretaceous times. It is formed by upper mantle and crustal rocks affected by both high- and low-angle faults that die against an Upper Triassic salt layer that decouples the deformation and generate salt diapirs. From late Santonian (Late Cretaceous), the Basque-Cantabrian Basin was involved in the Pyrenean orogeny which reactivated the previous faults and salt décollement.

In this scenario, our study focuses at the northern margin of this basin where the salt overburden (Jurassic to Eocene in age) is displaced several km northwards and appears compartmentalized by several salt walls (Bakio, Bermeo, Guernica and Mungia diapirs). These walls are linked by narrow stripes of variable orientations in which the overburden appears strongly deformed by tight detachment folds and minor thin-skinned thrusts. The piercing salt is composed of Upper Triassic evaporites, red clays and volcanic ophites, and is flanked by Aptian-Albian syn-diapiric carbonate to terrigenous halokinetic sequences limited by angular unconformities that become conformable as distance to the diapir edges increases. Using a paleomagnetic study, we seek to better understand the kinematics of suprasalt deformation trying to detect and quantify vertical axis rotations recorded during both the extensional and later contractional reactivation of the basin margin. For that, 52 paleomagnetic sites have been analyzed in the overburden sequence. Of these, 50 sites were sampled in Aptian, Albian and Cenomanian marls, marly limestones and fine grained sandstones, and 2 sites were sampled in Eocene sandstones and marly limestones. Characteristic components are usually defined between 200-450 ºC pointing to titanomagnetite as the main remanence carrier. They show predominant anticlockwise rotations with some anomalous clockwise and larger anticlockwise rotations near the salt diapirs. All these components yield normal polarity, as expected by the age of the rocks, which (except the Eocene sites) coincide with the Cretaceous superchron C34n. However, some of the sites are clearly remagnetized as they yield negative fold tests, whereas some other sites show a prefolding magnetization. These results are also supported by several hysteresis analyses and back field experiments that confirm a clear remagnetization signal in the Day diagram in part of the studied rocks. However, the spatial location of remagnetized rocks does not show a distinct structural pattern. With the current data, the origin and age of this remagnetization is difficult to assess and further analysis will be necessary. It could be either an earlier Albian-Maastrichtian remagnetization or a remagnetization linked to the Pyrenean compression. Although these uncertainties, the obtained results allow establishing a preliminary kinematic model for the suprasalt deformation together with the underlying decoupled autochtonous materials.

How to cite: Beamud, E., Soto, R., Peigney, C., Roca, E., and Pueyo, E. L.: Paleomagnetism on salt-detached syndiapiric overburden rocks from the Northern margin of the Basque-Cantabrian extensional Basin (N Spain), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4598, https://doi.org/10.5194/egusphere-egu2020-4598, 2020.

EGU2020-13734 | Displays | EMRP3.8

Contribution of magnetic fabric to the knowledge of Mesozoic and Cenozoic kinematic evolution in the Central High Atlas

Teresa Román-Berdiel, Belén Oliva-Urcia, Antonio M. Casas-Sainz, Pablo Calvín, Bennacer Moussaid, Esther Izquierdo, Vicente Carlos Ruiz, Andrés Pocoví, Andrés Gil-Imaz, Sara Torres, Juan José Villalaín, Hmidou El Ouardi, Tania Mochales, Pablo Santolaria, Marcos Marcén, María Felicidad Bógalo, Elisa M. Sánchez-Moreno, Ángela Herrejón, Ángela Jiménez-Sanz, and Irene Falcón

Magnetic fabric has become a first-order tool for the study of the evolution of inverted sedimentary basins, as has been demonstrated in the last decade (García-Lasanta et al. 2018 and references therein). Its application is based on its broad and reliable applicability to characterize the structural context of a region where structural markers are often punctually located or scarce. Determining the contribution of basinal (extensional) and compressional (inversion) deformation to the total magnetic fabric is a major issue in understanding the internal deformation underwent by the basin fill.

The main goal of this work is to integrate the available data of anisotropy of magnetic susceptibility (AMS) performed during the last ten years in the Mesozoic series of the Central High Atlas. It has a total of 645 sites (7477 standard specimens), 484 of them (5657 standard specimens) are measured in the framework of the actual CGL2016-77560-C2-P research project (Spanish Ministry of Science and Innovation), and it has been integrated with 161 sites (1820 standard specimens) obtained in the precedent research projects (CGL2012-38481, CGL2009-08969 and CGL2009-10840). Samples were measured in a KLY3-S Kappabridge (AGICO) susceptometer at the Zaragoza University. Magnetic subfabric analysis were also done (AMS-LT and AARM) for representative selected sites, that allow us to identify anomalous fabrics. Magnetic carriers were determined by carrying out temperature-dependent susceptibility curves (from 40 to 700ºC) combining the susceptibility bridge with a CS-3 furnace, an also by means of the acquisition curves of isothermal remanent magnetization (IRM), backfield curves and hysteresis loops using a variable field translation balance MMVFTB at the Paleomagnetic Laboratory of the Burgos University. Rock magnetic experiments indicate the presence of paramagnetic behavior in most samples, the presence of magnetite as main ferromagnetic contribution, and of hematite in the red beds.

The application of the ASM has made it possible to obtain data of well-defined foliations and magnetic lines from the analysis of a large number of samples, and therefore representative of the Mesozoic rocks that emerge in the High Central Atlas. Viewing the data as a whole, magnetic ellipsoids can be divided into three main types depending on the orientation of the main axes, and can be related with the kinematic evolution of the Central High Atlas: 1) kmin normal to bedding and sub-horizontal kmax with a NW-SE main maximum, which is mainly associated with gentle synclines and can be related to Mesozoic extensional tectonic; 2) kint normal to bedding and sub-horizontal kmax with a NE-SW main maximum, which can be interpreted as modified by compressional tectonics; 3) kmax normal to bedding, which are located near thrust planes or near the core of narrow and tight anticlines and can be interpreted as related with transport direction or salt tectonics and re-tightening of structures. The predominance of one or another type of fabric varies spatially; so that in the Western and Eastern sectors type 1 fabric dominates (more tan 60% of the samples), whereas in the central sector this percentage  decreases to 48% of the samples.

How to cite: Román-Berdiel, T., Oliva-Urcia, B., Casas-Sainz, A. M., Calvín, P., Moussaid, B., Izquierdo, E., Ruiz, V. C., Pocoví, A., Gil-Imaz, A., Torres, S., Villalaín, J. J., El Ouardi, H., Mochales, T., Santolaria, P., Marcén, M., Bógalo, M. F., Sánchez-Moreno, E. M., Herrejón, Á., Jiménez-Sanz, Á., and Falcón, I.: Contribution of magnetic fabric to the knowledge of Mesozoic and Cenozoic kinematic evolution in the Central High Atlas, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13734, https://doi.org/10.5194/egusphere-egu2020-13734, 2020.

EGU2020-17600 | Displays | EMRP3.8

Palinspastic restorations using interfolding remagnetizations. The case of the Cretaceous widespread remagnetization of the Central High Atlas (Morocco)

Pablo Calvín, Juan J. Villalaín, Antonio M. Casas-Sainz, Teresa Román-Berdiel, Pablo Santolaria, Tania Mochales, Irene Falcón, Bennacer Moussaid, Belén Oliva-Urcia, Sara Torres-López, Esther Izquierdo, María F. Bógalo, Andrés Gil-Imaz, Vicente C. Ruíz, Elisa M. Sánchez-Moreno, Marcos Marcén, Ángela Herrejón, Ángela Jimenez-Sanz, Hmidou El Ouardi, and Andrés Pocoví

The Jurassic carbonates of the Central High Atlas (CHA) are affected by a widespread and homogeneous chemical remagnetization. This is an interfolding remagnetization (dated in ca. 100 Ma by comparison with the GAPWP of the African plate) that separates two deformational events; the first one is related to the basinal period in the Atlas (Triassic and Jurassic times) that is responsible of the thick Jurassic sequence that crops out in the CHA. The second one is associated with the tectonic inversion during the Cenozoic caused by the African and European plates convergence, which resulted in the uplift of the cordillera.

Using the Small Circle tools, it is possible (i) to obtain the remagnetization direction and then (ii) use it as reference to obtain the paleodips of each site (i.e. the paleodips at the remagnetization time). This methodology applied to interfolding remagnetizations allows restoring the present-day geometry to the one at ca. 100 Ma and therefore separating the structure associated to the extensional and compressional periods.

This work is framed on an ambitious research project (CGL2016-77560-C2-P, Spanish Ministry of Science and Innovation) in which 700 AMS/paleomagnetic sites and additional 1000 bedding data are integrated to unravel the Mesozoic and subsequent Cenozoic evolution of the CHA. Based on the aforementioned datasets, field work, geophysical (gravimetric and magnetic surveys) constraints and the construction and restoration (at the remagnetization time) of cross-sections, the ultimate goal of the project is to reconstructed and restored the 3-D geometry of the CHA fold-and-thrust belt.  

Here in particular, we present the paleomagnetic data supporting the calculated paleodips. We also analyze how the deformation associated with each of the two deformational events is distributed along the study area. The comparison of dip-domains maps showing the present day attitude and also the pre-inversional one allows analyzing how extensional deformation is more or less associated with particular structures and to understand the importance of the different processes that acted during this period (i.e. deformation associated with extensional faults, salt walls, igneous intrusions, etc.).

How to cite: Calvín, P., Villalaín, J. J., Casas-Sainz, A. M., Román-Berdiel, T., Santolaria, P., Mochales, T., Falcón, I., Moussaid, B., Oliva-Urcia, B., Torres-López, S., Izquierdo, E., Bógalo, M. F., Gil-Imaz, A., Ruíz, V. C., Sánchez-Moreno, E. M., Marcén, M., Herrejón, Á., Jimenez-Sanz, Á., El Ouardi, H., and Pocoví, A.: Palinspastic restorations using interfolding remagnetizations. The case of the Cretaceous widespread remagnetization of the Central High Atlas (Morocco), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17600, https://doi.org/10.5194/egusphere-egu2020-17600, 2020.

EGU2020-19781 | Displays | EMRP3.8

Links between remagnetizations and superchrons. New experiments and new results.

Juan José Villalaín, Pablo Calvín, María Felicidad Bógalo, Irene Falcón, and Antonio M. Casas-Sainz

Chemical widespread remagnetizations are especially frequents during superchrons. An interesting issue is whether this relationship is due to especial requirements of the mechanism of acquisition of the remagnetizations and their timing. One example of this type of remagnetizations during a superchron is the one recorded by the Jurassic carbonates from the Central High Atlas (CHA) in Morocco. This normal polarity overprint has been dated ca 100 Ma, comparing the remagnetization direction with the African APWP, i.e. during the Cretaceous Normal Superchron (CNS) and also during the extensional stage of these inverted basins.

After several paleomagnetic studies performed in this area in the framework of a big research project, paleomagnetic and rock magnetic data from a set of more than 600 paleomagnetic sites distributed over an area of 10000 km2 are available. The CHA cretaceous remagnetization has been observed in all these sites with the same magnetic properties: a viscous paleomagnetic component with maximum unblocking temperatures of 200-250ºC and the remagnetization normal polarity component up to 450–500ºC. Both are carried by authigenic uniaxial SSD magnetite. The paleomagnetic direction calculated by small circle intersection method (SCI) is also similar in the different locations of this wide area.

The mechanism proposed for this type of widespread remagnetization is the generation of magnetite grains due to the heating related with burial. The homogeneous direction of the remagnetization seems to suggest an acquisition for a short event at 100 Ma. However, the extensional stage of these basins lasts tens of millions years keeping the necessary burial conditions for growth of magnetite grains covering several polarity chrons including the CNS.

In this work we address the question of timing under with these processes happened, i.e. short vs. long remagnetization periods. We propose the hypothesis that the ca. 100 Ma paleomagnetic direction shows by the remagnetization is just the average of magnetic moments of the entire SSD magnetite population that grow from the Middle Jurassic up to the Cenozoic. Grains block the magnetic moments when they grow above their critical volume, keeping the magnetic polarity generating over time a distribution of grains in normal and reverse polarity groups. To test this hypothesis we develop 1) simulations for the calculation of magnetization directions assuming a homogeneous and constant growth of magnetite crystals and 2) rock magnetic experiments to demonstrate the presence of SSD magnetite grains with opposed magnetic moments. These experiments intend to assess the effectiveness of the SSD grains carrying the remagnetization by comparing the NRM and the ARM signal through the pseudo-Thellier approach.

How to cite: Villalaín, J. J., Calvín, P., Bógalo, M. F., Falcón, I., and Casas-Sainz, A. M.: Links between remagnetizations and superchrons. New experiments and new results., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19781, https://doi.org/10.5194/egusphere-egu2020-19781, 2020.

EGU2020-10371 | Displays | EMRP3.8

New constraints on the tectonic activity of the “Tellian Domain” from Northern Tunisia: preliminary paleomagnetic results.

Philippe Robion, Marwen Arfaoui, Riadh Ahmadi, Mohamed El Messaoud Derder, Mohamed Amena, Said Maouche, and Farhat Rekihiss

In this study, we present preliminary results on paleomagnetic data collected in the Tunisian Tellian domain in both magmatic and sedimentary rocks of middle to lat Miocene ages from the Nefza-Mogods province, North-West of Tunisia. About 320 cores distributed over twenty one sites were collected both in magmatic rocks (16 sites) and in sedimentary rocks (5 sites), in order to obtain geometric constraints to establish a kinematic model along the North-East African margin. The sampled rocks are distributed between basanites, rhyodacites and microgranites. Some samples were taken from host sedimentary rocks host rocks in lacustrine limestones and jaspilites. Demagnetization process and Rock-Magnetism studies revealed a diversified magnetic mineralogy. In basalts, magnetite with an unblocking temperature of 580 °C is identified. In rhyodacites, the mineralogy is mixed with three types of minerals: a mineral with an unblocking temperature around 350°-400°C attributed either to a sulfide or to titanomagnetite, magnetite with unblocking temperature at 580°C, and a high temperature mineral with unblocking temperature between 600°C and 680°C attributed to hematite or titanohematite. The limestones, having a low magnetization intensity, are characterized by the presence of magnetite and the jaspilites by hematite. Basalts, which have been mainly demagnetized by AF process , show a characteristic component demagnetized between 20mT and 100mT. For rhyodacites, some sites have a characteristic component demagnetized between 400°C and 580°C and others up to 670°C. Although their low magnetization intensity, the lacustrine limestones show a magnetic component between 20mT and 140 mT. The first result indicate that the mean directions associated to the younger magmatic (basalts and rhyodacite) rocks (8 Ma, Tortonian) and their sedimentary host deposits are very close to the expected magnetic field after tilting in paleogeographic coordinates. By contrast, the older microgranites and rhyodacites(-12 Ma) display a vertical axis clockwise rotation of about 30°. This result suggests a significant tectonic phase between 12 Ma and 8 Ma, linked to the implementation of the Tell nappes.

How to cite: Robion, P., Arfaoui, M., Ahmadi, R., Derder, M. E. M., Amena, M., Maouche, S., and Rekihiss, F.: New constraints on the tectonic activity of the “Tellian Domain” from Northern Tunisia: preliminary paleomagnetic results., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10371, https://doi.org/10.5194/egusphere-egu2020-10371, 2020.

EGU2020-2063 | Displays | EMRP3.8

Unravelling the Remagnetization of the Oman Ophiolite

Louise Koornneef, Antony Morris, Michelle Harris, and Christopher MacLeod

The Oman ophiolite is a natural laboratory for the study of processes operating above a nascent subduction zone. It formed in the Late Cretaceous by supra-subduction zone spreading and shortly afterwards was emplaced onto the Arabian continental margin. Twelve massifs in the ophiolite expose complete sections of the Neotethyan oceanic lithosphere, including upper mantle peridotites, lower crustal gabbros, and upper crustal sheeted dykes and lava flows.

 

Previous palaeomagnetic studies have suggested that the southern massifs of the ophiolite were affected by a large-scale remagnetization event during emplacement, that completely replaced original remanences acquired during crustal accretion. In contrast, primary magnetizations are preserved throughout the northern massifs. This study aimed to: (i) apply palaeomagnetic, magnetic fabric and rock magnetic techniques to analyse crustal sections through the southern massifs of the Oman ophiolite to investigate further the extent and nature of this remagnetization event; and (ii) use any primary magnetizations that survived this event to document intraoceanic rotation of the ophiolite prior to emplacement.

 

Our new data confirms that remagnetization appears to have been pervasive throughout the southern massifs, resulting in presence of shallowly-inclined NNW directions of magnetization at all localities. An important exception is the crustal section exposed in Wadi Abyad (Rustaq massif) where directions of magnetization change systematically through the gabbro-sheeted dyke transition. Demagnetization characteristics are shown to be consistent with acquisition of a chemical remanent overprint that decreased in intensity from the base of the ophiolite upwards. The top of the exposed Wadi Abyad section (in the sheeted dyke complex) appears to preserve original SE-directed remanences that are interpreted as primary seafloor magnetizations. Similar SE primary remanences were also isolated at a control locality in the Salahi massif, outside of the region of remagnetization. Net tectonic rotation analysis at these non-remagnetised sites shows an initial NNE-SSW strike for the supra-subduction zone ridge during spreading, comparable with recently published models for the regional evolution of the ophiolite.

How to cite: Koornneef, L., Morris, A., Harris, M., and MacLeod, C.: Unravelling the Remagnetization of the Oman Ophiolite, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2063, https://doi.org/10.5194/egusphere-egu2020-2063, 2020.

EGU2020-6878 | Displays | EMRP3.8 | Highlight

Time constraint on Danangou and Dongyaozitou mammalian faunas in the Nihewan Basin, North China

Ping Liu, Huafeng Qin, Shihu Li, and Baoyin Yuan

Nihewan Basin is one of a series of well-developed East Asian Cenozoic basins, located in Hebei Province, North China. It has abundant gullies developed along both banks of the Sanggan River during and after the demise of Nihewan paleo-lake, creating a number of outcrops of the Nihewan Beds of fluvio-lacustrine origin, which are underlain by the Pliocene eolian Red Clay and overlain by the late Pleistocene loess. The fluvio-lacustrine sequence is rich sources of mammalian faunas and Paleolithic sites, thus providing unique insights into our understanding of land mammal biochronology and early human settlements in East Asia. Among the Nihewan Fauna (sensu lato), the Danangou (DNG) and Dongyaozitou (DZ) faunas are two of the important Pleistocene and Pliocene mammalian faunas in the Nihewan Basin. Except for a biostratigraphy, precise age control on the DNG and DZ faunas remains unavailable. Here we report a high-resolution magnetostratigraphic results that stringently constrain their ages. Rock magnetism and thermal demagnetization results show that magnetite and hematite dominate the remanence carriers in the DNG and DZ fluvio-lacustrine sequences. High-resolution magnetic polarity stratigraphy indicates that the DNG sequence recorded the Brunhes normal chron, the Matuyama reverse chron and the late Gauss normal chron, yielding the fossil-rich layers of DNG fauna with an age of ca. 1.95 Ma to 1.78 Ma during the Olduvai normal subchron. The DZ sequence was located at the late Gauss normal chron, leading an age of ca. 3.04−2.58 Ma before the termination of the Kaena reverse subchron. This result, together with previously published magnetochronology data obtained in the eastern basin, constructs a precise age constraints on the chronological framework of the Nihewan faunas and Paleolithic sites, especially during the Plio-Pleistocene transition.

How to cite: Liu, P., Qin, H., Li, S., and Yuan, B.: Time constraint on Danangou and Dongyaozitou mammalian faunas in the Nihewan Basin, North China, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6878, https://doi.org/10.5194/egusphere-egu2020-6878, 2020.

EGU2020-9315 | Displays | EMRP3.8

Paleomagnetic data of the Siberian Mesoproterozoic rocks (Udzha Uplift, Northern Siberia)

Aleksandr Pasenko, Aleksandr Savelev, and Sergey Malyshev

In spite of the fact, that during the last two decades some number of new paleomagnetic poles, more or less meeting the modern standards of quality [Van der Voo, 1993], have been obtained for Mesoproterozoic of Siberia [Evans et al., 2016]. The problem of the Precambrian segment of the apparent polar wander path (APWP) for Siberia, rests still to be far from its solution.

The latter, obviously, hampers the elaboration of Precambrian paleogeographic reconstructions, solution of numerous other important tasks of the Earth Sciences.

The Late Precambrian key section of the Udzha Uplift seemed to be one of the most promising object to elaborate the Mesoproterozoic segment of APWP of the Siberian platform. Until recently, the rocks composing this section have been considered to be of the Mesoproterozoic and Vendian age.

As a result of isotope studies in recent years, the age of formations of the Udzha Uplift has been significantly increased (1386±30 Ma, apatite, U-Pb, [Malyshev et al., 2018]). In particular, age of the Udzha Fm, which forms the uppermost part of the Udzha riphean sequence is considered currently to be Mesoproterosoic. On the base of our new paleomagnetic data this formation has been formed about the same time as the Khaypakh Fm from the Olenek Uplift (NE Siberia), whose Mesoproterozoic age has been established earlier from independent isotopic data [Zaitseva et al., 2017].

During last several years we have carried out the paleomagnetic studies of Late Precambrian rocks of the Udzha Uplift including the Mesoproterozoic Udzha and Unguokhtakh formations as well as intrusions representing two Mesoproterozoic magmatic events.

In this abstract we present new paleomagnetic poles for the Mesoproterosoic rocks (1500 Ma, ca.1400 Ma, 1385 Ma) of the Siberian platform.

These paleomagnetic poles significantly complement the Mesoproterozoic segment of APWP of the Siberian Platform.

The studies were supported by the Russian Science Foundation project № 19-77-10048.

How to cite: Pasenko, A., Savelev, A., and Malyshev, S.: Paleomagnetic data of the Siberian Mesoproterozoic rocks (Udzha Uplift, Northern Siberia), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9315, https://doi.org/10.5194/egusphere-egu2020-9315, 2020.

EGU2020-11392 | Displays | EMRP3.8

Prospects of paleomagnetic studies of the Riphean intrusive bodies of the Bashkirian megazone (Southern Urals)

Maiia Anosova, Anton Latyshev, and Alexey Khotylev

         The studied objects are located in the core of the Bashkirian megazone and related to the Riphean stage of rift magmatism of the East European craton. Paleomagnetic studies of the Bashkirian megazone intrusive bodies can be a source of new information on the East European platform position in the Riphean, as well as on the process of remagnetization during the Late Paleozoic folding on the Southern Urals. At this moment, 42 thin basic intrusions and the Main Bakal dyke were investigated.

         According to the results of our previous paleomagnetic studies two remanence components were isolated in Bashkirian megazone intrusions. First, the primary remanence component of Middle Riphean age was isolated in 8 thin bodies. Pole for the boundary of the Early and Middle Riphean of the East European Craton was calculated from high-temperature component of remanence of 8 sheet intrusions. This pole is close to the known paleomagnetic poles of East European craton for close ages and agrees with U-Pb age of one of the studied bodies (1349 ± 11 Ma). Also, arguments in favor of the primary origin of the remanence and the absence of significant tectonic dislocations in the sampling area are discussed. In other 4 intrusive bodies, paleomagnetic directions that are close but slightly different from the Middle Riphean directions were found. Second, the Late Paleozoic directions were found in the studied objects. These directions are widespread in the Bashkirian megazone rocks and have been reported by other researchers. Presumably it is the result of the Late Paleozoic syn-collisional remagnetization.

            According to the new results another component of remanence was detected in the intrusive bodies of the Bashkirian megazone. In 2 sheet bodies and the Main Bakal dyke a component close to the Late Riphean identified earlier in sedimentary rocks of the same region was found (Pavlov, Gallet, 2009; Danukalov et al., 2019). Furthermore, in total 20 thin intrusive bodies and the Main Bakal dyke have paleomagnetic directions close to the Late Paleozoic directions. The comparison of mean paleomagnetic directions for the different studied regions demonstrates the absence of any traces of essential rotation of blocks within the Bashkirian megazone in the Later Paleozoic.

            At this moment the origin of the remanence of 8 thin bodies is unclear, the nature of the other components of remanence requires additional research. It is planned to sample more intrusive bodies and to perform the isotopic dating of the key objects.

References:

  • 1) Pavlov V.E., Gallet Y. Katav limestones: A unique example of remagnetization or an ideal recorder of the Neoproterozoic geomagnetic field. Izvestiya, Physics of the Solid Earth, 2009, vol. 45, no. 1, pp. 31-40
  • 2) Danukalov K. N., Salmanova R. Y., Golovanova I. V., Parfiriev N. P. New paleomagnetic data on sedimentary rocks of the Inzer and Zilmerdak formations in the Southern Urals// Materials of the XXV anniversary All-Russian School-Seminar on problems of paleomagnetism and magnetism of rocks. – IPE RAS Moscow, 2019. – P. 108-113

How to cite: Anosova, M., Latyshev, A., and Khotylev, A.: Prospects of paleomagnetic studies of the Riphean intrusive bodies of the Bashkirian megazone (Southern Urals), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11392, https://doi.org/10.5194/egusphere-egu2020-11392, 2020.

EGU2020-12382 | Displays | EMRP3.8

Magnetic fabric and flow directions in magmatic rocks of the Franz Josef Land, Arctic Ocean

Anna Chernova, Viktor Abashev, Dmitry Metelkin, Valery Vernikovsky, and Nikolay Mikhaltsov

Here, we present the results of a study of the anisotropy of magnetic susceptibility (AMS) completed in the Early Cretaceous magmatic complexes from the Franz Josef Land (FJL). AMS was measured in the framework of paleomagnetic research as a leading indicator of the rock magnetic fabric to help in understanding the lava flow directions and forming mechanisms. The three types of magmatic bodies were available in these studies: dolerite sills, dykes and basaltic lava flows from several islands (Alexandra, Hall, Ziegler, Jackson and Heiss Islands) among FJL. During the experiments the different parameters of AMS ellipsoids were obtained which have a good correlation with the igneous body shapes and also could illustrate lava flows direction parameters. The degree of anisotropy P is 1.01-1.06 for most sites that is typical for the primary igneous magnetic fabric. The form factor T characterizing the shape of the AMS ellipsoids demonstrates both planar and linear magnetic fabric in studied magmatic bodies. What is remarkable the part of the dykes is characterized strictly oblate magnetic fabric and another dykes have the prolate AMS ellipsoids. The linear magnetic structure is also more typical for lava flows with the maximum axes K1 lying in the flow plane that is obviously could point to the flow direction. The part of the igneous bodies are characterized by the inverse type of magnetic fabric, when the principal axis K1 of the ellipsoid is oriented perpendicularly to the plane of the flow or the sill, that was likely caused by the effect of secondary processes. The previous studies (Abashev et al., 2019) demonstrated that the primary orientation of the AMS ellipsoid could be recovered after temperature demagnetization. Noticeable changes were revealed at heating up to ~450 deg C, which generally corresponds to deblocking temperatures of titanomagnetites identified in the rocks by rock-magnetic methods. The degree of anisotropy was decreased after heating in 2-3 times. The heating also resulted to the redistribution of magnetic axes and in several cases the axes becomes more grouped. Analysis of the AMS results from the basaltic lava flows of the Aleksandra Island defined the magma flow direction to be NW-SE. Similar behavior of the AMS ellipsoids and lava flow orientation is typical for Ziegler Island. Generally our results show that complex analysis of AMS data in basaltic rocks is promising for identifying magma flow direction and it can give more detailed information about forming mechanisms of the different magmatic bodies.

This work was supported by the RSF (project no. 19-17-00091) and the RFBR (project nos. 18-35-00273, 18-05-70035).

How to cite: Chernova, A., Abashev, V., Metelkin, D., Vernikovsky, V., and Mikhaltsov, N.: Magnetic fabric and flow directions in magmatic rocks of the Franz Josef Land, Arctic Ocean, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12382, https://doi.org/10.5194/egusphere-egu2020-12382, 2020.

Products of the Permian-Triassic magmatic activity in the Kotuy river valley consist of two contrasting in composition groups: 1) tholeiitic basalts, similar to the main volume of the Siberian Traps; 2) alkaline-ultramafic rocks which are extremely rare in other regions of the Siberian platform. Alkaline lavas and tuffs in the Kotuy river valley are exposed only in limited area (Arydzhangsky and Khardakhsky formations), however, multiphase circular plutons (Kugda, Odikhincha) and swarms of radial and parallel dikes marks the essentially wider territory of the manifestation of alkaline magmatic activity.

Here we present the preliminary results of the investigation of AMS in the dike complex of alkaline lamprophyres from the Kotuy river valley. The majority of dikes demonstrate I-type of the magnetic fabric, when the medium axes K2 of AMS ellipsoid is orthogonal to the contact of intrusion. In dikes where the minimal axis K3 is subvertical and maximal axis K1 is flat, we interpret this magnetic fabric as a result of cooling of the static magma column after the emplacement in the setting of horizontal extension (Park et al., 1988; Raposo and Ernesto, 1995). Also, N-type and R-type of magnetic fabric were identified as well. In some intrusions, the orientation of the axes of AMS ellipsoid changes from the contact zones to the inner part if intrusion. In this case, we used data from the contact zones for the magma flow reconstruction.

Analysis of the maximal axis K1 orientation in different dikes showed that in majority of bodies it shallowly plunges to the west. This corresponds to the lateral magma flow from west to east during the emplacement. Consequently, formation of the studied dikes is not directly related to Kugda pluton, which is located 8 km eastward. The emplacement of dikes occurred from the magmatic center located westward from the Kotuy river valley and is not associated with any known large massifs. Petrographic similarity of the studied dikes to the lavas of Arydzhangsky formation allows us to suggest that they are coeval. This implies the wider area of manifestation of the Arydzhangsky magmatic stage.

This work was supported by RFBR (projects 18-35-20058, 18-05-70094, 17-05-01121 and 20-05-00573).

How to cite: Latyshev, A., Chmerev, V., and Zaitsev, V.: Anisotropy of magnetic susceptibility in alkaline-ultramafic dikes of the Kotuy river valley: Reconstruction of magma transport during the Siberian Traps emplacement, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15594, https://doi.org/10.5194/egusphere-egu2020-15594, 2020.

EGU2020-21720 | Displays | EMRP3.8

New petro-paleomagnetic data of Kandalaksha and Onega Bay Islands in the White Sea

Natalia Kosevich, Ivan Lebedev, and Tanya Bagdasaryan

We have studied the AMS of metamorphic rocks (gneiss, granitoids, dykes) and soft sediments (mainly marine sediments or reworked diamicton) from the Kandalaksha and Onega Bay’s Islands of the White sea. The objects of research are located within the White sea mobile belt, represented by large tectonic nappes.

The magnetic susceptibility in soft sediment samples ranges from 78.6 E-6 to 1525E-6 (Km), and the degree (P) from 1.8% to 4.1%. Ellipsoids have a predominantly flattened type, such a distribution of AMS is typical for sedimentary rocks. At the same time, in a number of samples (from the Islands of Joker, Ipanchinikha and Olenevsky), the maximum axis is directed in a North-Westerly direction, which may indicate the flow direction. This is especially evident in flattened-triaxial ellipsoids (T=0.2-0.3). Values that have a T greater than 0.5 have a predominantly northerly direction and the orientation of minerals of the magnetic fraction and the direction of paleoflow is less pronounced.

The study of the anisotropy of the magnetic susceptibility of the Archean complexes composing the Islands of the Kandalaksha Bay of the White sea showed a high magnetic susceptibility-5E-6-1E-3 (Km), which confirms the change in the petrographic composition of gneiss. The degree of anisotropy (P) is 9% on average. It was found that the distribution of the main axes of the magnetic susceptibility ellipsoid coincides with shale and banding in the root outlets, while the maximum axis of the ellipsoid coincides with the West-North-West stretch of the regional fault. In the Onega Bay we sampled paleoproerozoic dykes, and there are AMS is coincided as contacts of studied dykes.

We done alternating field and thermal demagnetization of pilot collection which contains samples from all studied complexes. And it gives us not good results because of bad paleomagnetic record. Most of samples contain only low coercive or low temperature components and it mainly has modern direction.

How to cite: Kosevich, N., Lebedev, I., and Bagdasaryan, T.: New petro-paleomagnetic data of Kandalaksha and Onega Bay Islands in the White Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21720, https://doi.org/10.5194/egusphere-egu2020-21720, 2020.

EGU2020-17266 | Displays | EMRP3.8

The final closure of the Vardar Ocean: paleomagnetic, AMS and structural results

Emö Márton, Marinko Toljić, Vesna Lesić, and Vesna Cvetkov

The Vardar zone divides units of African affinity from units of the European margin. It is characterized by extensional opening of an oceanic domain during the Triassic and Jurassic followed by divergent simultaneous obduction of the oceanic litoshphere over the continental units in the Upper Jurassic. However, a stripe of the oceanic domain persisted till the Cretaceous and Paleogene convergence. The remnants of the last closing part of the Vardar ocean are found in the Sava zone.

In this paper recently published and new paleomagnetic, AMS results in combination with structural observations will be presented from Upper Cretaceous sediments and Oligocene –Lower Miocene igneous rocks representing the areas bordering the Sava zone from the western and eastern sides, respectively and from the upper Cretaceous flysch deposited in the Sava zone.

In the areas W and E of the Sava zone, respectively, the primary remanences of the igneous rocks point to post-Oligocene CW rotation of about 30°. The sediments carry secondary magnetizations, imprinted during magmatic activity. Compared to the areas flanking it, the sediments of the Sava zone were intensively folded during the Upper Cretaceous and Paleogene and the paleomagnetic signals, which exhibit smeared distribution close to the present N, are of post-folding age. The AMS foliation and bedding planes are sub-parallel, thus the deformation must have been weak. Fold axes and AMS lineations are roughly N-S oriented, pointing to the deformational origin of the AMS lineations. These observations form the Sava zone will be discussed in the context of the post-Oligocene CW rotation of the flanking areas and the general NE-SW orientation of the compressional stress field outside of the zone.

Acknowledgement. This work was financially supported by the National Development and Innovation Office of Hungary, project K 128625 and by the Ministry of Education and Science of the Republic of Serbia, project 176015.

How to cite: Márton, E., Toljić, M., Lesić, V., and Cvetkov, V.: The final closure of the Vardar Ocean: paleomagnetic, AMS and structural results, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17266, https://doi.org/10.5194/egusphere-egu2020-17266, 2020.

The Marmara region is located on the Alpine Himalayan orogenic belt which experienced a active tectonic deformation. The region consists of tectonic units such as the Istanbul Zone, the Strandja Zone and the Sakarya Continent. It is reported in the previous geological studies that the Istanbul Zone began to move southwards appart from the Moesia Platform with the effect of West Blacksea Fault in the west and West Crimea Fault in the east after the the opening of the Black Sea in the Cretaceous. It is known that the Intra Pontide suture is formed after the closure of the Intra-Pontide ocean during the Early Eocene due to the collision between İstanbulzone and the Sakarya continent which moved northwards. As a result of the continental collision, the region has completed its evolution under the influence of basin formation and the emplacement of North Anatolian Fault Zone from Miocene to the present.

 

In this study, Upper Cretaceous-Oligocene sedimentary and volcanic rocks were sampled at 103 sites to investigate the tectonic deformation of the area. As a result of rock magnetism studies, it was shown that magnetic minerals in sedimentary and volcanic rocks are defined by titanium-rich titanomagnetite showing low coercivity, while in limestone samples, magnetization is defined by hematite showing high coercivity. As a result of anisotropy of magnetic susceptibility (AMS) measurements, it was observed that most of the samples show magnetic foliation and a deformation ellipsoid which is oblate. Paleomagnetic results show counterclockwise rotation of 19.9°±10.9° for the Sakarya continent, 27.4°±11.6°for the Pontides and 15.6°±11.8°for the Strandja Zone from Eocene to present. The results indicate that the region has completed the collision in Eocene and rotated counterclockwise as a large block. Deformation due to basin development or fault bounded block rotations which developed after Miocene could not been detected in this study. Miocene paleomagnetic data from previous studies in the study area are compatible with counterclockwise rotations in Upper Cretaceous-Oligocene which shows that different blocks emplaced in the study area moved together as a single plate during Eocene-Miocene time.

How to cite: Cabuk, B. S. and Cengiz, M.: A Paleomagnetic Study of the Tectonic Deformation in Circum-Marmara Region, NW Anatolia, during the Late Cretaceous and Cenozoic Period, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17635, https://doi.org/10.5194/egusphere-egu2020-17635, 2020.

EGU2020-803 | Displays | EMRP3.8

New paleomagnetic data for Ochotsk-Chukotka volcanic belt

Ivan Lebedev, Olesya Usanova, Tanya Fadeeva, Florian Lhuillier, Baha Eid, Louisa Murray-Bergquist, Alexander Pasenko, and Dmitriy Gavrushkin

The Okhotsk-Chukotka volcanic belt (OChVB),  located in the north-eastern part of Russia, is a unique volcanic structure, which has been formed over a wide time interval from Aptian (K1) to Cenomanian (K2) [Tihomirov, 2018]. Age of its formation nearly coincides with the occurrence of the Cretaceous geomagnetic superchron of normal polarity. Thus, the volcanic formations of the OChVB represent a promising object to study the characteristics of the geomagnetic field during the Cretaceous superchron (direction, paleointensity, secular variations) needed to test various models explaining superchrons’s existence .

During the reconnaissance field work of the summer 2019 we have sampled volcanic rocks in 9 sections each includes from 8 to 30 sites corresponding either to lava flow or to tuff layers.

Up to date we have carried out AF demagnetization, petromagnetic and AMS studies. Demagnetisations studies demonstrate that the rocks contain paleomagnetic record of the ancient (primary?) magnetization of good to excellent quality. Petromagnetic experiments indicate that the main magnetic mineral in majority of studied volcanics is titanomagnetite with pseudo-single domain grain size. We use the magnetic fabric derived from AMS studies to test either the modern attitude (slight dipping up to 10-15˚) of studied rocks is due to primary paleorelief or the rocks have experienced some tectonic deformations.

How to cite: Lebedev, I., Usanova, O., Fadeeva, T., Lhuillier, F., Eid, B., Murray-Bergquist, L., Pasenko, A., and Gavrushkin, D.: New paleomagnetic data for Ochotsk-Chukotka volcanic belt, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-803, https://doi.org/10.5194/egusphere-egu2020-803, 2020.

This paleomagnetic study is located at the north-west extremity of the Tarim Basin and has aimed to constrain the style of Neotectonic deformation where indentation of the Pamir Orogen into the southward-verging Tian Shan frontal zone has produced a complex zone of thrusting, folding and strike-slip. Sampling focused on two Pliocene to Pleistocene sedimentary formations folded across the Mingyaole Anticline, the major structural feature between the two frontal zones, has yielded well-grouped characteristic remanent magnetizations at 18 of 24 sites and a positive fold test. Together with fabric evidence, the results indicate a probable post-depositional detrital origin for the remanence. The results show that only small inter-locational vertical-axis rotations have occurred within the Kashi-Atushi fold and thrust system since the Miocene and imply that the Kashi depression has behaved as a quasi-rigid block. A common 15-30º counterclockwise (CCW) rotation relative to Eurasia since the Miocene of the Kashi Depression and the bordering Tian Shan range proves to be unrelated to the right lateral motion along the Talas-Ferghana intracontinental transform fault to the north west. This contrast is provisionally interpreted as taking place along a transfer fault between different segments of the thrust belt. Ongoing CCW rotation of the Tarim Basin is interpreted as a regional response to impingement by northward movement of the larger Tibetan Block to the south east.

How to cite: Qiao, Q.: Neotectonic deformation in the Southwestern Tian Shan, Western China: evidence from paleomagnetic study of Quaternary sediments from the Mingyaole Anticline, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12530, https://doi.org/10.5194/egusphere-egu2020-12530, 2020.

EGU2020-19033 | Displays | EMRP3.8

Paleomagnetic constrains on the assembly processes of the Antarctic Peninsula

Liang Gao, Junling Pei, Zhenyu Yang, Xiaochun Liu, Shuan-Hong Zhang, and Yue Zhao

The mid-Cretaceous paleo-Pacific ocean witnessed increased mantle plume activity, high oceanic crust production rate, enhanced subduction-related magmatism, and widespread short-lived intense deformation. The Antarctic Peninsula located at the Pacific margin of Gondwana and strongly influenced by the exceptionally pan-Pacific tectonic events during the mid-Cretaceous. Therefore, plate reconstruction of the Antarctic Peninsula and its implication to the global geodynamics, paleo-ocean circulation and paleoclimate have become one major subject for pan-Pacific geoscience studies. However, this is difficult because of the small number of reliable paleomagnetic data of the Antarctic Peninsula at the early stage of mid-Cretaceous. In this study, we obtained a key ca. 120-105 Ma paleopole from the Byers Peninsula, Livingstone Island, South Shetland Island, during the global ocean crust peak production period. Plate reconstruction reveals that the Western-Central Domain of the Antarctic Peninsula experienced clockwise rotation between ca. 155 Ma and 120-105 Ma, and large-scale anticlockwise rotation from ca. 120-105 Ma to 90 Ma. This anticlockwise rotation was ascribed to induce the final formation of the Antarctic Peninsula. This assembly age coincides with the global-scale plate reorganization at 105-100 Ma, which might associate with the eruption of mantle superplume in the southern Atlantic region.

Acknowledgments

This research was funded by the National Key R&D Program of China (2018YFC1406904), the Natural Science Foundation of China (NSFC) (41930218, 41706222, 41372082), the open foundation project (KLPTR-03) of Key laboratory of paleomagnetism and Tectonic Reconstruction, Ministry of Land and Resources.

How to cite: Gao, L., Pei, J., Yang, Z., Liu, X., Zhang, S.-H., and Zhao, Y.: Paleomagnetic constrains on the assembly processes of the Antarctic Peninsula, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19033, 2020.

EGU2020-2 | Displays | EMRP3.8

Magnetostratigraphy and Kinematic Characteristics of Datca Graben (Mugla, SW Turkey)

Meryem Dilan İnce, Nuretdin Kaymakcı, Ökmen Sümer, Bora Uzel, Seçkin Şiş, Levent Tosun, Cor Langereis, and Marius Stoica

This study involves kinematic development and magnetostratigraphy of infill of the Datça Graben located at the southwesternmost corner of Anatolia (Turkey). The study comprises kinematic analysis based on fault slip data collected from the margins of the Datça Graben and the magnetostratigraphic analysis of infill of the Datça Graben. For the kinematic analysis, 977 fault-slip data were collected from 44 sites. The data are analyzed using a software which is based on Angelier’s reduced stress tensor algorithm. For the magnetostratigraphic analysis, 344 samples are used and the paleomagnetic measurements of those samples are performed in the Fort Hofddjik Paleomagnetism Laboratory, University of Utrecht.

The results of the kinematic analysis have shown that the Datça Basin has developed under the effects of N-S-directed tensional stress regime manifested by WNW-ESE- striking normal faults. As a result of paleomagnetic measurements, the infill sediments of the Datça Graben can be represented by a reversed-normal-reversed polarity pattern, which can be correlated to C2r.1r-C2r.1n-C2r.2r subchrons within the C2r chron of the Early Matuyama in Geomagnetic Polarity Time Scale. This means that the graben filling sediments deposited between 2.3 Ma to 1.9 Ma, in the Late Pliocene.

This age interval suggests that the Datça Graben has completed its development from half-graben to full-graben geometry by the effects of syn-sedimentary WNW-ESE-striking faults in the Late Pliocene.

This thesis is supported by TUBITAK (Grant No: 117R012).

How to cite: İnce, M. D., Kaymakcı, N., Sümer, Ö., Uzel, B., Şiş, S., Tosun, L., Langereis, C., and Stoica, M.: Magnetostratigraphy and Kinematic Characteristics of Datca Graben (Mugla, SW Turkey), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2, https://doi.org/10.5194/egusphere-egu2020-2, 2020.

EGU2020-20186 | Displays | EMRP3.8

Paleomagnetism of the Kola Peninsula's dykes, NE Fennoscandia: the review

Roman Veselovskiy, Alexandra Stepanova, Alexander Samsonov, and Ivan Lebedev

In this work we present the overview of comprehensive research on paleomagnetism and rock magnetism of more than 120 Paleoproterozoic and Neoarchean dykes from the Kola Peninsula, NE Fennoscandia. We discuss our results in a context of Precambrian paleotectonic reconstructions of Fennoscandia and Murmansk craton in particular for the time intervals corresponding to the dyke magmatism. We also show our progress in dating of the remanent magnetization components using some approaches of isotope geochronology, such as U-Pb dating of baddeleyite and Ar/Ar dating of mica, feldspar and amphibole. In total, we have been able to calculate virtual geomagnetic poles for the selected dykes and to make some conclusions about paleotectonics of the Murmansk craton at the corresponding time. New paleointensity data for some 2.50 and 2.68 Ga dykes will be also presented and commented. The study is supported by the grant of RSF #16-17-10260.

How to cite: Veselovskiy, R., Stepanova, A., Samsonov, A., and Lebedev, I.: Paleomagnetism of the Kola Peninsula's dykes, NE Fennoscandia: the review, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20186, https://doi.org/10.5194/egusphere-egu2020-20186, 2020.

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