Content:

EMRP – Earth Magnetism & Rock Physics

EMRP1.14 – Multiscale rock damage in geology, geophysics and geo-engineering systems

EGU21-15691 | vPICO presentations | EMRP1.14

Microfracture evolution leading to catastrophic failure observed by hearing and seeing

Maria-Daphne Mangriotis, Andrew Curtis, Alexis Cartwright-Taylor, Edward Andò, Ian Main, Andrew Bell, Ian B. Butler, Florian Fusseis, Roberto Rizzo, Martin Ling, Sina Marti, Derek Doug Vick Leung, Jonathan Singh, and Oxana Magdysyuk

Catastrophic failure is a critical phenomenon present in Earth systems on a variety of scales, and is associated with the evolution of damage leading to system-size failure. Laboratory testing of rock failure permits characterization of fracture network evolution at the micro-scale to understand the interaction of cracks, pores and grain boundaries to an applied stress field, and the relationship between deformation and seismic response. Previous studies have relied on acoustic emissions (hearing) or X-ray imaging (seeing) to study the process of localization, which involves spontaneous self-organization of smaller cracks along faults and fractures on localised zones of deformation. To combine hearing and seeing of the microscopic processes and their control of system-sized failure, a novel x-ray transparent cell was used for deformation experiments of rock samples, which permits integration of acoustic monitoring with fast synchrotron x-ray imaging. To increase temporal characterization of damage beyond the temporal resolution of the fast 3D synchrotron system, acoustic emission (AE) feedback control was used to regulate the applied stress and slow down the deformation processes. As a result, there is increased temporal resolution of the incremental deformation between successive x-ray scanned states allowing synchronized comparison of acoustic emissions to x-ray scans. Here, we present the seismic analysis used to characterize the velocity evolution of the rock samples, and the location and characteristics of individual AE events in relation to microscopic deformation processes. Time-lapse velocity measurements are linked to internal stress changes and structural damage corresponding to seismic and aseismic deformation processes, while acoustic emissions are a direct indication of local cracking.  We show that we can successfully locate AE events in 3D using only two sensors on either end of the sample, based on ellipsoid mapping, and x-ray image to event correlation. We explore temporal and spatial statistics of AE signatures and how those are linked to the strain field in the samples measured with incremental digital volume correlation between pairs of recorded x-ray tomograms. The direct observation of AE and X-ray images enables quantification of relevant seismic (local cracking leading to AE) to aseismic (elastic loading and silent irreversible damage) processes, with information extracted over fine temporal resolution throughout the deformation process through the AE-feedback control.

How to cite: Mangriotis, M.-D., Curtis, A., Cartwright-Taylor, A., Andò, E., Main, I., Bell, A., Butler, I. B., Fusseis, F., Rizzo, R., Ling, M., Marti, S., Leung, D. D. V., Singh, J., and Magdysyuk, O.: Microfracture evolution leading to catastrophic failure observed by hearing and seeing, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15691, https://doi.org/10.5194/egusphere-egu21-15691, 2021.

EGU21-2629 | vPICO presentations | EMRP1.14

From monodisperse to polydisperse: the influence of grain size distribution on the mechanical behavior of porous synthetic rocks

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

Sedimentary crustal porous rocks span a wide range of grain size distributions – from monodisperse to highly polydisperse. The distribution of grain size depends on the location and conditions of rock formation, the chemico-physical processes at play, and is influenced by subsequent geological processes. Well-sorted granular rocks, with a grain size distribution close to monodisperse, and granular rocks with a more polydisperse grain size distribution, have repeatedly been subjected to laboratory experiments. And yet the natural variability from sample to sample and structural heterogeneity within single natural samples all conspire to prevent us from constraining the effect of grain size polydispersivity. While a few studies have focused on the influence of grain size, the control of grain size distribution on the mechanical behavior of rocks has scarcely been studied, especially in the laboratory. In this study, we address this knowledge-gap using synthetic samples prepared by sintering glass beads with controlled polydisperse grain size distributions. When heated above the glass transition temperature, the beads act as viscous droplets and sinter together. Throughout viscous sintering, a bead pack evolves from an initial granular discontinuous state into a solid connected porous state, at which the microstructural geometries and final porosity are known. Variably polydisperse individual samples were prepared by mixing glass beads with diameters of 0.2, 0.5, and 1.15 mm in various proportions, which were sintered together to a final porosity of 0.25 or 0.35. Hydrostatic and triaxial compression experiments were performed for each combination of polydispersivity. The samples were water-saturated, deformed at room temperature, and deformed under drained conditions (with a fixed pore pressure of 10 MPa). Triaxial experiments were conducted at a constant strain rate at effective pressure corresponding to the ductile (compactive) regime. Our mechanical data provide evidence that polydispersivity exerts a significant control on the compactive behavior of porous rocks. Insights into the microstructure were gained using scanning electron microscopy on thin sections prepared from samples before and after deformation. These data allow for the observation of the different deformation features, and by extension the deformation micro-mechanisms, promoted by the different type and degree of polydispersivity. Overall, our data show that, at a fixed porosity, increasing polydispersivity decreases the stress required for compactant failure.

How to cite: Carbillet, L., Heap, M., Wadsworth, F., Baud, P., and Reuschlé, T.: From monodisperse to polydisperse: the influence of grain size distribution on the mechanical behavior of porous synthetic rocks, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2629, https://doi.org/10.5194/egusphere-egu21-2629, 2021.

EGU21-172 | vPICO presentations | EMRP1.14

Investigation of failure and damage processes of andesitic rocks

Özge Dinç Göğüş, Deniz Yılmaz, Elif Avşar, and Kamil Kayabalı

In this research, failure and deformation processes of andesitic rocks are investigated through laboratory and discrete element modeling (DEM) analysis to reveal the transition of the cracking, namely from microscale to mesoscale (lab scale). For this purpose, the mechanical properties of Ankara andesites were initially investigated by performing uniaxial - triaxial compressive and indirect tensile laboratory tests. Further, these properties were used as reference parameters for the calibration process in a numerical model, generated through a three-dimensional open source code (Yade) based on the discrete element method (DEM). Our results show that during the linear-elastic region of the stress-strain curve, the major mechanism of rock behavior is driven by tensile cracks. When the crack damage threshold is reached, as a result of plastic strain, the strength related to the inherent cohesion significantly decreases and damage in the rock cannot be prevented anymore. At the peak stress of the curve, both tensile and shear cracks accumulate, intensively. Even the mesoscale failure mechanism is controlled by shearing at the residual stage of the yielding, based on the micro-scale process in the DEM model, the number of micro shear cracks is very limited compared to the tensile ones. This finding shows that the friction takes the control of the damage process as the only driving force during residual phase time.  

How to cite: Dinç Göğüş, Ö., Yılmaz, D., Avşar, E., and Kayabalı, K.: Investigation of failure and damage processes of andesitic rocks , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-172, https://doi.org/10.5194/egusphere-egu21-172, 2021.

EGU21-9048 | vPICO presentations | EMRP1.14

Seeing and hearing quasi-static shear band localization in a sandstone

Alexis Cartwright-Taylor, Ian G. Main, Ian B. Butler, Florian Fusseis, Maria-Daphne Mangriotis, Andrew Curtis, Andrew Bell, Martin Ling, Edward Andò, Roberto Rizzo, Sina Marti, Derek Leung, and Oxana Magdysyuk

The localisation of structural damage, in the form of faults and fractures, along a distinct and emergent fault plane is the key driving mechanism for catastrophic failure in the brittle Earth. However, due to the speed at which stable crack growth transitions to dynamic rupture, the precise mechanisms involved in localisation as a pathway to fault formation remain unknown. Understanding these mechanisms is critical to understanding and forecasting earthquakes, including induced seismicity, landslides and volcanic eruptions, as well as failure of man-made materials and structures. We used time-resolved synchrotron x-ray microtomography to image in-situ damage localisation at the micron scale and at bulk axial strain rates down to 10-7 s-1. By controlling the rate of micro-fracturing events during a triaxial deformation experiment, we deliberately slowed the strain localisation process from seconds to minutes as failure approached. This approach, originally established to indirectly image fault nucleation and propagation with acoustic emissions, is completely novel in synchrotron x-ray microtomography and has enabled us to image directly processes that are normally too transient even for fast synchrotron imaging methods. Here, we first present the experimental apparatus and control system used to acquire the data, followed by damage localisation and shear zone development in a sample of Clashach sandstone viewed in unprecedented detail. Time-resolved microtomography images demonstrate a strong intrinsic correlation between shear and dilatant strain in the localised zone, with bulk shear strain accomodated by the nucleation and rotation of en-echelon tensile microcracks within a grain-scale shear band. Rotation is accompanied by antithetic to synthetic shear sliding of neighbouring crack surfaces as they rotate. The evolving 4D strain field, measured with incremental digital volume correlation between pairs of recorded x-ray tomographic volumes, independently confirm the correlation between shear and dilatant strain and show how strain localises spontaneously, first through exploration of several competing shear bands at peak stress before transitioning to failure along the optimally-oriented final fault plane. In order to ‘ground-truth’ inferences made from bulk measurements and seismic waves (the primary method of detecting deformation at the field-scale where direct imaging of the subsurface is impossible), we (a) compare rupture energy estimates from local slip measurements with those from bulk slip data, and (b) use AE source location estimates to identify individual cracks and other local changes in the microstucture that may explain the AE source.

How to cite: Cartwright-Taylor, A., Main, I. G., Butler, I. B., Fusseis, F., Mangriotis, M.-D., Curtis, A., Bell, A., Ling, M., Andò, E., Rizzo, R., Marti, S., Leung, D., and Magdysyuk, O.: Seeing and hearing quasi-static shear band localization in a sandstone, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9048, https://doi.org/10.5194/egusphere-egu21-9048, 2021.

EGU21-14332 | vPICO presentations | EMRP1.14

Deformational characteristics of thermally treated sandstone from an underground coalmine fire region, India

Adarsh Tripathi, Noopur Gupta, Ashok Kumar Singh, Nachiketa Rai, and Anindya Pain

The Jharia region of lower Gondwana in India is one of the largest Underground Coalmine Fires (UCF) affected coalfield in the world. The UCF induced small scale as well as large-scale surface fracturing often creates the life-threatening conditions to coal miners and local surroundings. So, there is a need to understand the thermomechanical behaviour of coal measures rocks to predict the land disturbances in thermo-environmental conditions. It will provide an insight into the UCF induced subsidence mechanism and its preventive measures. The Jharia coal field predominantly consists of sandstone (75-80% by volume) and rest is composed of coal, shale and carbonaceous shale. The present study focuses on thermo-mechanical behaviour of Barakar sandstone (BS) under elevated temperatures. The cores of BS sample were prepared according to the ISRM standards. Further, samples were grouped and thermally treated in temperature range of 25°C, 100°C, 150°C, 300°C, 400°C, 500°C, 600 °C, 700°C and 800°C at a heating rate of 5°C/min for 24 hours in furnace.  Then, these thermally treated BS samples were subjected to laboratory test for stress-strain characteristics. In the process of deformational characteristics evaluation, effect of mineralogical changes and mode of fracture pattern were also studied at the mentioned elevated temperature. Based on the obtained results, the deformational behaviour of thermally treated BS specimens can be grouped into three zones, viz., zone 1 (25-300°C), zone 2 (300-500°C) and zone 3 (500-800°C). In zone 1, the characteristics of the stress-strain curve is similar to those under air dried sandstone specimen. However, small increment in stiffness were observed upto 300°C. The stress-strain curves in this zone shows dominantly brittle fracturing. The increment in stiffness may be related to evaporation of pore water that increases the cohesion between the mineral grains resulting higher stiffness value. In zone 2, the deformation pattern again shows brittle fracturing with continuous decrement in stiffness. The reduction in stiffness may be related to thermally induced porosity and increased microcrack density. In zone 3, the stress strain curve is observed to be concave upward. It indicates the pseudo-ductile behaviour of the thermally treated BS specimens. The observed results suggest a typical behaviour of deformation pattern under UCF induced rock fracturing which may be useful in predicting the land subsidence in UCF affected areas. Present research outcome may be used to design the support measures to reduce the associated hazards.

How to cite: Tripathi, A., Gupta, N., Singh, A. K., Rai, N., and Pain, A.: Deformational characteristics of thermally treated sandstone from an underground coalmine fire region, India, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14332, https://doi.org/10.5194/egusphere-egu21-14332, 2021.

EGU21-5959 | vPICO presentations | EMRP1.14

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

Anne Pluymakers, Auke Barnhoorn, and Richard Bakker

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. To date, investigation of sample heterogeneity, specifically tensile strength and strength anisotropy has focused on layered rocks, such as shales, sandstones and gneisses. Data is lacking on the effect of single planar heterogeneities, such as pre-existing fractures or stylolites, even though these frequently occur in geo-energy settings.

We have performed Brazilian Disc tests on limestone samples containing planar heterogeneities, investigating Brazilian test Strength (BtS) and the effects of orientation on strength. We used prefractured Indiana limestone to represent a planar heterogeneity without cohesion and Treuchtlinger Marmor samples with central stylolites to represent a planar heterogeneity of unknown strength (as an example of a sealed fracture). The planar discontinuity was set at different rotation angles of approximately 0–20–30–45–60–90⁰, where 90⁰ (steep angle) is parallel to the principal loading direction, and 0⁰ (low angle) to the horizontal axis of the sample. All experiments were filmed, and where possible Particle Image Velocimetry was used to determine internal particle motion. Moreover, we used a 2D Comsol model in which we simplified the stylolite surface as a sinusoid. The model was used to qualitatively determine how i) a different period of the sinusoid and ii) relative strength of sinusoid/matrix affect the results.

Our results show that all imperfect samples are weaker than intact samples. The 2D Comsol model indicates that the qualitative results remain unaffected by changing the period (assumed to be representative of roughness) of the cohesive heterogeneity, nor by the relative strength contrast: the location of the first fracture remains unaffected. For both heterogeneity types, the fracture patterns can be divided into four categories, with two clear endmembers, and a more diffusive subdivision in between.

For a cohesion-less heterogeneity:

  • steep angles lead to frictional sliding along the interface, and only a small hypothesized permeability increase.
  • Intermediate angles lead to a combination of tensile failure of the matrix and sliding along the interface, where for steeper angles more new fractures form which follow the path of the existing fracture.
  • Low angles lead to closure of the old fracture and new tensile failure.

For a cohesive heterogeneity of unknown cohesion:

  • Steep angles lead to intensive failure of the heterogeneous zone, attributed to the presence of a stress concentrator.
  • Intermediate angles lead to partial failure along the heterogeneous zone, and the formation of new fractures in the matrix, potentially instigated by mode II failure to accommodate motion.
  • Low angles lead to the formation of a new fracture plus opening within the heterogeneous zone.

These results imply that hydrofracture (i.e. creating tensile stresses) of a stylolite-rich zone will lead to more fractures than fractures in a homogeneous zone, where the orientation of the stylolites and bedding will control the orientation of the permeable pathways.

How to cite: Pluymakers, A., Barnhoorn, A., and Bakker, R.: Effect of a heterogeneity on tensile failure: interaction between fractures in a limestone, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5959, https://doi.org/10.5194/egusphere-egu21-5959, 2021.

EGU21-11553 | vPICO presentations | EMRP1.14

A postmortem approach via X-ray computed tomography and thresholding to investigate fracture network evolution in Onagawa shale

Eranga Jayawickrama, Jun Muto, Osamu Sasaki, and Hiroyuki Nagahama

A postmortem technique is introduced to investigate the fracture connectivity evolution under elevated confining pressures via a sensitivity analysis. Three Onagawa shale samples are deformed under brittle, ductile, and transition conditions, by increasing the confining pressures. Brittle deformation is characterized by longitudinal splitting of the sample at 3% axial strain, and the onset of transition from brittle to ductile deformation is between 4% ~ 5% axial strain. The ductile deformation is characterized by a distributed conjugate fracture network and strain hardening. In completion of the deformation, the samples are scanned in a commercially available X-ray CT machine. The grayscale values of the primary 2D images were reversed, stacked, and surface rendered to obtain the 3D volume distribution of the fractures. Reversing and surface rendering allowed the acquisition of volume and surface data of the fractures along with their direct visualization. Further, utilizing a residual analysis, the voxel value density distribution that fabricated the fracture network is extracted (Residual histogram). Thresholding of the residual histogram generated volume segments of the final fracture network demonstrating the sensitivity of the fracture network to the choice of threshold. Voxel volumes of fractures alone are obtained by thresholding post-peak voxel values of the residual histogram and consecutive post-peak thresholding shows that the generated volume segments of the fracture network can be utilized to interpret, possible nucleation sites after strain localization, propagation of fractures, and coalescence. Fracture connectivity is quantified by means of relative entropy from information theory, and the relative entropy of size distribution of fracture volumes showed that it is closer to zero with the fractures being well connected. Moreover, the cumulative fracture volume shows a power-law growth towards the failure after a unique threshold to each sample. These results have been validated by previous acoustic emission studies and a 4D tomographic investigation on strain localization of shale. Therefore, despite the postmortem nature of the investigation, the new technique has opened possibilities to investigate the fracture properties and their evolution under elevated confining pressures.

How to cite: Jayawickrama, E., Muto, J., Sasaki, O., and Nagahama, H.: A postmortem approach via X-ray computed tomography and thresholding to investigate fracture network evolution in Onagawa shale, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11553, https://doi.org/10.5194/egusphere-egu21-11553, 2021.

Large rock slopes instabilities form over long timescales through progressive rock mass strength weakening of initially stable slopes. Progressive rock mass damage is driven by environmental loads and is thus strongly dependent on the local setting and environmental conditions of the rock slope, which can vary over time. It is often assumed that the strong variations of the thermal and hydraulic boundary conditions during deglaciation in combination with unloading due to ice downwasting cause enhanced rates of rock mass damage. However, in-situ observations to quantify deformation, damage and the relevance of different drivers in such environments are rare. This presentation is related to the contribution of Oestreicher et al., presenting in the same session, addressing similar questions, but at different scales and based on different field data and analysis.

In this contribution we analyze continuous pore pressure, temperature and micrometer-scale deformation time series from a subsurface monitoring system comprised of three, 50 m deep, highly instrumented boreholes in a crystalline rock slope which is located beside the rapidly retreating glacier tongue of the Great Aletsch Glacier (Switzerland). We compare high-resolution reversible and irreversible deformation signals with potential drivers, including locally measured pore pressure fluctuation, rock temperature variations, and nearby earthquakes. We show that shallow (10 - 15 m deep) deformations in our rock slope are dominated by thermo-mechanical forcing, whereas deformation measured below this depth is mainly driven by hydro-mechanical effects related to pore pressure fluctuations. Both reversible deformation and irreversible damage events occur more frequently during the snow-free summer season, when we observe higher dynamics in thermal and hydraulic boundary conditions. In our 2.5 years long time series, we do not find any significant deformation event coinciding with a nearby earthquake. Additionally, we discuss differences in the deformation signal with respect to the stability state and the rock mass quality at the different monitoring locations. Also, we assess longer term impacts of glacier retreat and ice downwasting on rock slope deformation and damage. Such information is critical for an improved understanding and quantification of factors contributing to the formation of paraglacial rock slope instabilities.

How to cite: Hugentobler, M., Aaron, J., and Loew, S.: Drivers for micrometer-scale deformation and progressive rock mass damage in a deglaciating rock slope – insights from subsurface in-situ monitoring, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5192, https://doi.org/10.5194/egusphere-egu21-5192, 2021.

EGU21-8872 | vPICO presentations | EMRP1.14

Multiscale fracture density analysis at Stromboli Volcano, Italy: implications to flank stability

Thomas Alcock, Sergio Vinciguerra, Phillip Benson, and Federico Vagnon

Stromboli volcano has experienced four sector collapses over the past 13 thousand years, resulting in the formation of the Sciara del Fuoco (SDF) horseshoe-shaped depression and an inferred NE / SW striking rift zone across the SDF and the western sector of the island. These events have resulted in the formation of steep depressions on the slopes on the volcano where episodes of instability are continuously being observed and recorded. This study aims to quantify the fracture density inside and outside the rift zone to identify potential damaged zones that could reduce the edifice strength and promote fracturing. In order to do so we have carried out a multiscale analysis, by integrating satellite observations, field work and seismic and electrical resistivity analyses on cm scales blocks belonging to 11 lava units from the main volcanic cycles that have built the volcano edifice, ie. Paleostromboli, Nestromboli and Vancori. 0.5 m resolution Pleiades satellite data has been first used to highlight 23635 distinct linear features across the island. Fracture density has been calculated using Fracpaq based on the Mauldon et al (2001) method to determine the average fracture density of a given area on the basis of the average length of drawn segments within a predetermined circular area. 41.8 % of total fracture density is found around intrusions and fissures, with the summit area and the slopes of SDF having the highest average fracture density of 5.279  . Density, porosity, P- wave velocity in dry and wet conditions and electrical resistivity (in wet conditions) were measured  via an ultrasonic pulse generator and acquisition system (Pundit) and an on purpose built measuring quadrupole on cm scale blocks of lavas collected from both within and outside the proposed rift zone to assess the physical state and the crack damage of the different lava units.  Preliminary results show that P-wave velocity between ~ 2.25 km/s < Vp < 5km/s decreases with porosity while there is high variability electrical resistivity with 21.7 < ρ < 590 Ohm * m. This is presumably due to the lavas texture and the variable content of bubble/vesicles porosity and crack damage, that is reflected by an effective overall porosity between 0 and 9 %. Higher porosity is generally mirrored by lower p-wave velocity values. Neostromboli blocks show the most variability in both P-wave velocity and electrical resistivity. Further work will assess crack density throughout optical analyses and systematically investigate the UCS and elastic moduli. This integrated approach is expected to provide a multiscale fracture density and allow to develop further laboratory testing on how slip surfaces can evolve to a flank collapse at Stromboli.

How to cite: Alcock, T., Vinciguerra, S., Benson, P., and Vagnon, F.: Multiscale fracture density analysis at Stromboli Volcano, Italy: implications to flank stability, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8872, https://doi.org/10.5194/egusphere-egu21-8872, 2021.

The Fracture Induced Electromagnetic Radiation (FEMR) technique has gradually progressed in the past decade as a useful geophysical tool to determine the direction and magnitude of recent crustal stresses, visualize the modification and realignment of stresses inside tunnels thus proving to be an important precursor for geohazards, earthquake forecasting, as well as delineate landslide-prone slip planes in unstable regions. Its working principle is based on the generation of geogenic electromagnetic radiation emanating from the brittle rock bodies that are fractured being subjected to an incremental increase of the differential stress in the near-surface of the Earth’s crust. The “Process zone” at the fractured crack tip contains numerous microcracks which subsequently creates dipoles due to the polarization of charges on such microcrack tips which rapidly oscillates emitting FEMR waves of frequencies between KHz to MHz range. The coalescence of the microcracks eventually leads to a macro failure dampening the amplitude of the FEMR pulses. The attenuation of FEMR pulses is comparatively lesser than seismic waves making it a more efficient precursor to potential tectonic activities indicating an upcoming earthquake a few hours/days before the actual event. In the current study, we have attempted to exploit this technique to identify the locations of the potential active faults across the tectonically active Narmada-Son Lineament (NSL), Central India. Although the first tectonic stage involved rifting and formation of the NSL during the Precambrian time, the rifting continued at least till the time of Gondwana deposition. Later, tectonic inversion took place as a result of the collision between the Indian and the Eurasian plate resulting in reverse reactivation of the faults. Episodic reverse movement along NSL caused recurrent earthquakes and linear disposition of the sediments that were deposited at the foothills of the Satpura Horst. Although the origin of East-West trending NSL dates back to the Precambrian time, it is very much tectonically active as manifested by recent earthquakes. The study has been conducted by taking linear FEMR readings across 3 traverses along the NSL which on analysis provides an idea about the potential active faults, their locations, and frequency of occurrence. The accumulation of strain in the brittle rocks that can eventually lead to a macro failure is demarcated as an anomalous increase in the amplitude of the FEMR pulses indicative of an upcoming tectonic episode in the region. To further corroborate the analysis, we have attempted to determine the neo-tectonic activity in the region by calculating the morphometric parameters across the Khandwa-Itarsi-Jabalpur region, Central India. Finally, we attempt to comment on the tectonic evolution of Central India in the recent past. We also encourage researchers to adapt the novel technique of FEMR which is swift, affordable, and feasible compared to conventional techniques deployed to survey the active tectonics of a region.

How to cite: Das, S. and Mallik, J.: Delineating active faults across the Narmada-Son Lineament (NSL), India using the technique of Fracture Induced Electromagnetic Radiation (FEMR), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2370, https://doi.org/10.5194/egusphere-egu21-2370, 2021.

EGU21-9403 | vPICO presentations | EMRP1.14

Integrated geophysical and geotechnical monitoring for multiscale rock mass damaging investigation at the Acuto Field-Lab (Italy)

Guglielmo Grechi, Danilo D'Angiò, Matteo Fiorucci, Roberto Iannucci, Luca Lenti, Gian Marco Marmoni, and Salvatore Martino

Rock mass damaging has become a topic of great interest in the engineering-geology research community during the last decades as it can significantly influence slopes stability. In this sense, the study of mechanics and dynamics of jointed rock masses represents a challenge because it will allow to better understand how external continuous and transient stressors can influence the short- to long-term stability controlling their pre-failure behavior. Consequently, the detection of permanent changes in physical and mechanical parameters, due to periodic or transient stressors, is an important target to mitigate the related geological risk as it can potentially lead rock masses to failure, especially when infrastructures and natural or cultural heritages are exposed elements. In this framework, the Acuto field laboratory (Central Italy) has been designed and implemented in 2016 within an abandoned quarry by employing an integrated geotechnical and geophysical monitoring system, with the aim of investigating how natural and anthropic conditioning factors could lead fractured rock masses to failure. The integrated monitoring system, which is installed on a potentially unstable 20-m3 jointed rock block, is composed of several strain devices (i.e., strain gauges -SG- and jointmeters -JM-), one fully equipped weather station, one rock thermometer, eight high-sensitivity microseismic uniaxial accelerometers and optical and InfraRed Thermal cameras. The acquisition of long-term monitoring time-series, coupling multimethodological approaches, allowed to establish cause-to-effect relationships among different environmental stressors and induced strain effects, highlighting the continuous action of thermal stresses on rock mass deformations both at the daily and seasonal timescales. In fact, while the analysis of thermal and strain monitoring data allowed to characterize the cyclic contraction and relaxation response of major rock fractures and microcracks to temperature fluctuations, the microseismic monitoring array was able to detect during thermal transient (i.e., freezing conditions) the occurrence of microseismic emissions potentially related to the genesis or progressive growth of pre-existing cracks.

Starting from 2018, experimental activities at the Acuto field lab are supported by the “Dipartimento di Eccellenza” project of the Italian Ministry of Education Universities and Research funds attributed to the Department of Earth Sciences of the University of Rome “Sapienza”.  In this framework, the Acuto filed laboratory will undergo a structural upgrade that will be aimed at the investigation of two new sectors of the abandoned quarry. These new sectors will be instrumented with innovative thermal profiles probe, fiber Brag grating sensors and traditional SG and JM for detailed stress-strain monitoring, acoustic emission sensors and high-frequency and low-frequency geophones for ambient seismic noise monitoring and microseismic events detection as well as accelerometers for evaluating the rock mass response in the case of seismic shaking. The main goal of such an improvement will be both technical and methodological, and will shed light on the application of integrated geophysical and geotechnical monitoring approaches in investigating the multiscale rock mass damaging process as well as the detection of rock mass failure precursors by using non-conventional combinations and configurations of geotechnical and broad-band geophysical devices.

How to cite: Grechi, G., D'Angiò, D., Fiorucci, M., Iannucci, R., Lenti, L., Marmoni, G. M., and Martino, S.: Integrated geophysical and geotechnical monitoring for multiscale rock mass damaging investigation at the Acuto Field-Lab (Italy), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9403, https://doi.org/10.5194/egusphere-egu21-9403, 2021.

EGU21-8082 | vPICO presentations | EMRP1.14

Drivers of reversible and irreversible slope deformations in a paraglacial environment

Nicolas Oestreicher, Clément Roques, Marc Hugentobler, Jordan Aaron, and Simon Loew

Retreating glaciers around the world lead to rapid and profound changes in the surrounding landscapes. In the Alps, many glaciers are rapidly retreating and downwasting, substantially modifying stresses and hydro-thermal boundary conditions on the adjacent slopes. There is an increase in observations of bedrock responses and the formation of large-scale instabilities in paraglacial environments, but still a little knowledge about the underlying preparatory factors and drivers. This presentation is linked to the one from Hugentobler et al. in the same session. Both studies take place in the same catchment and address the same questions at different spatial scales, with other techniques and datasets.

We analyse surface deformation data monitored in a crystalline bedrock catchment, on the recently deglaciated slopes of the Great Aletsch Glacier (Valais, Switzerland). Our monitoring system has been in operation for six years and comprises 93 reflectors, 2 robotic TPS, and 4 cGPS stations distributed on both sides of the glacier tongue. This unique dataset allows studying the main processes involved at relevant spatial and temporal scales. The response of potential drivers for reversible and irreversible deformation is evaluated through combined multivariate (vbICA) and cross-correlation statistical analysis. We found that the variability in deformation near the glacier tongue is primarily controlled by glacier unloading through melting and seasonal groundwater fluctuations. At the catchment scale, the later effect is poroelastic and hence reversible, but we argue that it could also induce hydromechanical fatigue. By investigating the deformation's spatial pattern, we observed that the reversible deformation is mostly controlled by discrete structures such as hectometer-scale brittle-ductile shear zones striking subparallel to the valley axes and the main Alpine foliation. Field mapping and pressure monitoring during borehole drilling suggest that infiltration into the fractured rockmass is very heterogeneous and mainly controlled by the presence of interconnected tensile fractures.

How to cite: Oestreicher, N., Roques, C., Hugentobler, M., Aaron, J., and Loew, S.: Drivers of reversible and irreversible slope deformations in a paraglacial environment, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8082, https://doi.org/10.5194/egusphere-egu21-8082, 2021.

EGU21-5083 | vPICO presentations | EMRP1.14

The Luco dei Marsi deep-seated gravitational deformation: first evidence of a basal shear zone in the central Apennine mountain belt (Italy) 

Emiliano Di Luzio, Marco Emanuele Discenza, Maria Luisa Putignano, Mariacarmela Minnillo, Diego Di Martire, and Carlo Esposito

The nature of the boundary between deforming rock masses and stable bedrock is a significant issue in the scientific debate on Deep-Seated Gravitational Slope Deformations (DSGSDs). In many DSGSDs the deforming masses move on a continuous sliding surface or thick basal shear zone (BSZ) [1-3]. This last feature is due to viscous and plastic deformations and was observed (or inferred) in many worldwide sites [4]. However, no clear evidence has been documented in the geological context of the Apennine belt, despite the several cases of DSGSDs documented in this region [5-6].

This work describes a peculiar case of a BSZ found in the central part of the Apennine belt and observed at the bottom of a DSGSD which affects the Meso-Cenozoic carbonate ridge overhanging the Luco dei Marsi village (Abruzzi region). The NNW-SSE oriented mountain range is a thrust-related Miocene anticline, edged on the east by an intramountain tectonic depression originated by Plio-Quaternary normal faulting. The BSZ appears on the field as a several meters-thick cataclastic breccia with fine matrix developed into Upper Cretaceous, biodetritic limestone and featuring diffuse rock damage.

The gravity-driven process was investigated through field survey, aerial photo interpretation and remote sensing (SAR interferometry) and framed into a geological model which was reconstructed also basing on geophysical evidence from the CROP 11 deep seismic profile. The effects on slope deformation determined by progressive displacements along normal faults and consequent unconfinement at the toe of the slope was analysed by a multiple-step numerical modelling constrained to physical and mechanical properties of rock mass.

The model results outline the tectonic control on DSGSD development at the anticline axial zone and confirm the gravitational origin of the rock mass damage within the BSZ. Gravity-driven deformations were coexistent with Quaternary tectonic processes and the westward (backward) migration of normal faulting from the basin margin to the inner zone of the deforming slope.

References

[1] Agliardi F., Crosta G.B., Zanchi A., (2001). Structural constraints on deep-seated slope deformation kinematics. Engineering Geology 59(1-2), 83-102. https://doi.org/10.1016/S0013-7952(00)00066-1.

[2] Madritsch H., Millen B.M.J., (2007). Hydrogeologic evidence for a continuous basal shear zone within a deep-seated gravitational slope deformation (Eastern Alps, Tyrol, Austria). Landslides 4(2), 149-162. https://doi.org/10.1007/s10346-006-0072-x.

[3] Zangerl C., Eberhardt E., Perzlmaier S., (2010). Kinematic behavior and velocity characteristics of a complex deep-seated crystalline rockslide system in relation to its interaction with a dam reservoir. Engineering Geology 112(1-4), 53-67. https://doi.org/10.1016/j.enggeo.2010.01.001.

[4] Crosta G.B., Frattini P., Agliardi F., (2013). Deep seated gravitational slope deformations in the European Alps. Tectonophysics 605, 13-33. https://doi.org/10.1016/j.tecto.2013.04.028.

[5] Discenza M.E., Esposito C., Martino S., Petitta M., Prestininzi A., Scarascia-Mugnozza G., (2011). The gravitational slope deformation of Mt. Rocchetta ridge (central Apennines, Italy): Geological-evolutionary model and numerical analysis. Bulletin of Engineering Geology and the Environment,70(4), 559-575. https://doi.org/10.1007/s10064-010-0342-7.

[6] Esposito C., Di Luzio E., Scarascia-Mugnozza G., Bianchi Fasani G., (2014). Mutual interactions between slope-scale gravitational processes and morpho-structural evolution of central Apennines (Italy): review of some selected case histories. Rendiconti Lincei. Scienze Fisiche e Naturali 25, 161-155. https://doi.org/10.1007/s12210-014-0348-3.

How to cite: Di Luzio, E., Discenza, M. E., Putignano, M. L., Minnillo, M., Di Martire, D., and Esposito, C.: The Luco dei Marsi deep-seated gravitational deformation: first evidence of a basal shear zone in the central Apennine mountain belt (Italy) , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5083, https://doi.org/10.5194/egusphere-egu21-5083, 2021.

Rainfall-induced landslides pose a substantial risk to people and infrastructure worldwide, but their mechanical behavior is not well understood. As a result, hazard predictions for these landslides, especially for rainfall and slope-failure correlations, remain an active area of research. Many operational rainfall-induced landslide hazard maps still assume a classical Coulomb type failure criterion where slope-failure must occur either before or at peak subsurface pore pressure reached during a precipitation event. Using satellite-derived surface precipitation data and soil infiltration simulations over a 15 day period preceding 121 rainfall-induced landslides across India, we find that these events occurred systematically 2-12 days after the simulated peak pore pressures on the inferred failure slope nucleating between 0.5 and 5 m depth. These observations cannot be explained with the Coulomb failure criterion, since failure on these slopes is significantly delayed behind the occurrence of the inferred strength minimum. Instead, in this study, we investigate whether a slope failure model with time- and slip-variable shear strength, governed by the rate-state friction (RSF) equations widely used in earthquake mechanics, can explain the observed ranges of time-delays between slope failure and inferred peak pore pressure.

To concentrate on the role of the constitutive behavior of the failure surface, we examine spring-slider dynamics under a classical RSF framework driven by variable on-slope and far-field pore pressure and flux time histories. We derive analytical expressions for the time-to-failure of such a spring-slider under simple pore pressure perturbation histories and find that the delay-times can vary significantly depending on the laboratory derivable RSF parameters, soil bulk properties, and particulars of the pressure history. We further examine the roles of dilatancy strengthening and pore compaction in determining the time-lag between peak pore pressure and slope failure. We find that dilatancy can have either a stabilizing or a destabilizing effect on slope failure depending on the hydrological and mechanical properties of the failure plane and the soil column. Finally, we show with numerical simulations that periodic pore pressure or flux oscillations can also drive asynchronous repeated slope failures in both the presence and absence of the coupling of pore pressure and shear deformation. Our results show that the observed rainfall-landslide correlations for these 121 landslides can be explained with inherently time- and slip-dependent shear strength prescriptions like the RSF equations. This, in turn, implies that realistic landslide hazard monitoring might require the examination of soil shear strength under the experimental protocols widely used in rock friction experiments to determine whether the constant friction assumption inherent in the Coulomb criterion needs to be revised in favor of RSF or similar constitutive equations for shallow landslides.

How to cite: Paul, K., Bhattacharya, P., and Misra, S.: Investigation of The Observed Time-lag Between the Simulated Peak Pore Pressures and Slope Failures in Rainfall Induced Landslides: A Numerical Approach, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15352, https://doi.org/10.5194/egusphere-egu21-15352, 2021.

In 2009, a large-scale landslide was triggered by typhoon rainfall and buried an entire village, which named Hsiaolin and located in Taiwan. 
After that, Soil and Water Conservation Bureau (SWCB) has promoted a national project for the prevention work of large-scale landslide. The national project includes with the investigation of potential area, the design of monitoring system, and the design of warning system, etc. 
The investigation of potential large-sclae landslide was based on the  digital elevation model with 1 meter resolution. However, the investigation of the underground was lack and not clear enough. Therefore, the specific landslide's body is hardly to estimate and it causes difficulty in follow-up works. 
This study applied two methods to investigate the scenario of slope failure. The first method is based on the limited equilibrium method, which proposed by Yoshino and Uchida (2019). The method was used to search the specific region of unstable slope based on a series of high-resolution digital elevation models. After the specific region of unstable slope was confirmed, the landslide can be simulated by a numerical model, which this study proposed to represent the entire landslide process from occurrence to post-failure . 
These proposed methods were applied at Baolai area, south Taiwan to track the evolution of the potential area. The failure scenario could be evaluated by the proposed numerical model. By this study, the investigation of underground can be evaluated and these results are very important information for the design of monitoring system.

How to cite: Tsai, Y. and Lee, W.: Application of the high-resolution digital elevation model in an integrated numerical method for the occurrence mechanism and post-failure behavior of the landslide, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10764, https://doi.org/10.5194/egusphere-egu21-10764, 2021.

In the pre-feasibility study stage, only a small amount of borehole data can be obtained. Since the available geological information is insufficient, the engineering geological conditions of the project can only be preliminarily and approximately estimated during this stage. In this study, we attempt to seek a method to make a preliminary analysis and evaluation of the stability of the surrounding rock masses of an underground rock carven project, which makes full and optimum use of the limited borehole data to accomplish the assessment of the investigated site. The basic information on rock fractures is extracted from the borehole Television logging data and the fracture extension directions are also determined. Providing that the cracks detected in the borehole would extend to the cavern area, the cracks with appropriate direction, larger width and larger hydraulic conductivity can be selected. These selected cracks are considered in the numerical model established using ANSYS, and the stability of surrounding rock of cavern is analyzed under this situation. In the absence of large amount of borehole data, this method, which set up an extreme case, can be used to analyze possible failure of rock mass under extreme adverse conduction in advance. In general, the proposed method for stability analysis could contribute to the design and construction practice of a tunnel project constructed in fractured rock masses.

How to cite: Tan, L. and Xiang, W.: Stability analysis of the surrounding rock of cavern under extreme situation based on limited borehole data., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-230, https://doi.org/10.5194/egusphere-egu21-230, 2021.

EGU21-9667 | vPICO presentations | EMRP1.14

Cosmic-ray muography applications in underground tunnelling

Pasi Kuusiniemi, Marko Holma, and Zongxian Zhang

The novel geophysical remote imaging method of muography is based on cosmic-ray induced muon particles that are detected after passing through the media of interest. If the studied objects are solid, their sizes can vary from meters to up to kilometres. In terms of penetration capability, muography can be placed between methods based on X-rays and those using seismic waves. The most famous objects imaged with muography are pyramids (e.g., Khufu's Pyramid at Giza in Egypt) and volcanoes (e.g., Mt Etna in Italy). One clear advantage of muography compared with seismic methods is that muons, unlike seismic waves, do not reflect from geological interfaces. In addition, the scattering phenomenon is a minor issue and needs consideration only at low-energy muons. Raw data must be corrected according to topography. On the basis of extensive numeric simulations of Hivert et al. (2017), the lowest density variations observable for muography with a significant level of 3σ (a typical significance level in physics) are around 2% at 150 m, 4% at 300 m, and 10% at 700 m of depth, respectively. If these numbers are extrapolated to depths below 100 m, the mean density differences in the range of 1% are likely within the observation capability of muography. It is also worth to note that the 1% difference in a mean rock density results in an approximately 3% difference in the muon flux. This indicates that muon flux measurements are very sensitive to the density variations of rocks.

In underground tunnelling, muography has at least four applications: (1) muography can be used to detect a potential risk (such as a water reservoir, a weak zone with loose rocks, boulders, etc.) before or during tunnelling, (2) muography can be employed to monitor overburden rock behaviour during tunnelling operation to avoid risks like the roof cave-ins, (3) muography can be applied to monitor the overburdening rock masses in tunnels after they are excavated to predict and avoid the collapse of rock mass, and (4) muography can be used to estimate the size and volume of a rock mass collapse in a tunnel since the volume of the collapsed rocks must have markedly smaller density than original overburden rock mass. In an excavating tunnel project using a tunnel boring machine (TBM), a muon detector can be installed in the TBM during tunnelling. If there occurs a tunnel cave-in, muography can be employed in undamaged tunnels nearby (sideways or below) the collapse. If possible, the collapse can also be approached safely via an undamaged part of the collapsed tunnel. If none of these are available, borehole muography can be applied as a substitute solution. Whereas an undamaged underground tunnel is either filled by air or water, a collapsed tunnel segment is characterized by air and rock, or water and rock. In either case, the average density of the tunnel segment is increased. We are currently planning simulations and real-world tests to validate these assumptions.

How to cite: Kuusiniemi, P., Holma, M., and Zhang, Z.: Cosmic-ray muography applications in underground tunnelling, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9667, https://doi.org/10.5194/egusphere-egu21-9667, 2021.

EMRP1.15 – Petrophysics and rock-physics across the scales: integrating models, laboratory experiments and field geophysical studies

Experimental rock physics allows the study of specific geological phenomena in a controlled manner. The experimental data are used to develop and calibrate predictive numerical models, which ultimately improve our understanding of natural processes and interpretation of field scale datasets. However, upscaling laboratory geophysical datasets to explain large scale geological complexes is challenging and by definition imprecise, as core-scale experiments are not fully representative of the events occurring in the field. This challenge gains in complexity with the increasing number of involved parameters and with changing environmental conditions. Nowadays, one of the most challenging rock physics areas is Carbon Capture Utilization and Storage (CCUS).

CCUS is a realistic global scale mitigation solution to tackle the excess of CO2 expelled from industrial production and sequestering it into deep reservoir formations, while attempting at generate energetic benefits from this process (e.g., enhanced oil recovery and geothermal energy). When CO2 is injected in rocks, the parental pore fluid is displaced by CO2 and therefore the effective bulk modulus of the fluid (and the rock) changes; if the rock and/or the fluid are reactive to CO2, then the rock frame and fluid composition are also changing parameters, affecting both the elastic and hydrodynamic properties of the original rock.

In this contribution, I examine the complexity of the geophysical interpretation of a specific CCUS-related process: CO2-induced salt precipitation in saline aquifers. In essence, injected CO2 dry out the brine and salt precipitates in the pores, leading to some degree of clogging that depends on, but not exclusively, the pore size, salt volume and pore connectivity. This phenomenon affects the injectivity of the CO2 storage site, which could lead to pressure built-up events and ultimately compromise the geomechanical integrity of the reservoir. Therefore, an early warning of CO2-induced salt precipitation is essential to apply specific mitigation strategies in a timely manner.

Salt precipitation is a rapid, self-enhanced process, with a footprint in the geophysical record that has to be isolated from that of the CO2 fluid replacement and other potential CO2-induced rock reactions. When the geophysical data include seismic and electromagnetic surveys, S-waves provide information about the changes in the rock frame, while P-waves and electrical resistivity would help to distinguish between CO2, brine and salt fractions. Here, I use a dataset generated in the laboratory using ultrasonic P- and S-waves attributes, resistivity and axial deformation, during a CO2-brine flow-through experiment with salt precipitation, to discuss the potential of the rock physics to detect and quantify dynamic changes in the bulk properties of reservoir rocks.  

How to cite: Falcon-Suarez, I. H.: Joint elastic-electrical monitoring for detecting and quantifying CO2-induced salt precipitation during geological carbon and storage, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9920, https://doi.org/10.5194/egusphere-egu21-9920, 2021.

EGU21-9578 | vPICO presentations | EMRP1.15

Pore-scale hydrodynamic evolution within carbonate rock during CO2 injection and sequestration

Chi Zhang, Siyan Liu, and Reza Barati

The continuously rising threat of global warming caused by human activities related to CO2 emission is facilitating the development of greenhouse gas control technologies. Subsurface CO2 injection and sequestration is one of the promising techniques to store CO2 in the subsurface.  However, during CO2 injection, the mechanisms of processes like injectant immobilizations and trapping and pore-scale geochemical reactions such as mineral dissolution/precipitation are not well understood. Consequently, the multi-physics modeling approach is essential to elucidate the impact of all potential factors during CO2 injection, thus to facilitate the optimization of this engineered application. 

Here, we propose a coupled framework to fully utilize the capabilities of the geochemical reaction solver PHREEQC while preserving the Lattice-Boltzmann Method (LBM) high-resolution pore-scale fluid flow integrated with diffusion processes. The model can simulate the dynamic fluid-solid interactions with equilibrium, kinetics, and surface reactions under the reactive-transport scheme.  In a simplified 2D spherical pack, we focused on examining the impact of pore sizes, grain size distributions, porosity, and permeability on the calcite dissolution/precipitation rate. Our simulation results show that the higher permeability, injection rate, and more local pore connectivity would significantly increase the reaction rate, then accelerate the pore-scale geometrical evolutions. Meanwhile, model accuracy is not sacrificed by reducing the number of reactants/species within the system.

Our modeling framework provides high-resolution details of the pore-scale fluid-solid interaction dynamics. To gain more insights into the mineral-fluid interfacial properties during CO2 sequestration, our next step is to combine the electrodynamic forces into the model. Potentially, the proposed framework can be used for model upscaling and adaptive subsurface management in the future.  

How to cite: Zhang, C., Liu, S., and Barati, R.: Pore-scale hydrodynamic evolution within carbonate rock during CO2 injection and sequestration, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9578, https://doi.org/10.5194/egusphere-egu21-9578, 2021.

EGU21-13987 | vPICO presentations | EMRP1.15

The Effects of Surface Roughness on Fluid Displacement Mechanisms and Residual Residual Trapping - A Pore Scale Investigation

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

Immiscible fluid displacement and the trapping of residual oil and gas phases in the pore spaces of reservoir rocks is critical to geological operations such as carbon geo-sequestration and enhanced oil recovery. In carbon geo-sequestration, residual trapping is advantageous because it ensures long-term storage security of carbon dioxide (CO2). In contrast, residual trapping can pose significant challenges during waterflooding in oil recovery operations where large volumes of oil may remain trapped in the interstitial spaces of the porous reservoir rock and cannot be extracted, thereby reducing the efficiency of the recovery process. In such operations, residual trapping is strongly influenced by the inherent surface roughness of the solid rock matrix amongst many factors. Surface roughness occurs in natural reservoir rocks as a result of geological processes that physically, chemically or biologically convert sediments into sedimentary rock (known as diagenesis) and weathering.

The effects of surface roughness on immiscible two-phase flow are currently not well understood. Previous investigations into residual trapping in porous media have mainly focused on the influence of factors such as pore geometry, wettability, fluids interfacial tension, mobility ratio and injection scenarios. Although some of these studies acknowledge the potential effect of surface roughness, there is still a lack of quantitative characterization and understanding of the influence of surface roughness on immiscible two-phase displacements in porous media.

In this study, the impacts of surface roughness on immiscible two-phase displacement are quantified. Immiscible two-phase displacement of air by water was conducted in a custom laser-manufactured glass microfluidic chip (micromodel). The glass chip comprised a 2.5D micro-structure analogous to the pore network pattern (micro-structure) of a natural reservoir rock, Oolitic limestone. The pore network pattern consisted of cylindrical pillars 400 µm in diameter arranged in a rhombohedra type of packing, generated on to a glass substrate using an ultrafast, pulsed picosecond laser. Surface roughness is an innate characteristic of laser machined surfaces and as a result, small variations in depth of the porous micro-structure were observed (50 ± 8 µm). The average surface roughness (Sa) of the laser-machined structure was measured to be 1.2 μm.

Experimental results for the rough micromodel exhibit high repeatability of fluid displacement patterns (preferential flow pathways) demonstrating that surface roughness has a strong influence on fluid invasion patterns and sweep efficiency and its effects must not be ignored. To ascertain the effects of surface roughness on the fluid displacement process, a direct numerical simulation (DNS) of the fluid displacement process was performed in OpenFoam using the Volume of Fluid (VOF) method assuming zero surface roughness. Comparing the experimental results with the numerical simulations, we show that surface roughness can significantly enhance residual trapping in porous media by up to 49.2%.

 

How to cite: Nhunduru, R., Shahrokhi, O., Wlodarczyk, K., Jahanbakhsh, A., Garcia, S., and Maroto-Valer, M.: The Effects of Surface Roughness on Fluid Displacement Mechanisms and Residual Residual Trapping - A Pore Scale Investigation, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13987, https://doi.org/10.5194/egusphere-egu21-13987, 2021.

EGU21-16072 | vPICO presentations | EMRP1.15

Seismic wave attenuation due to fluid pressure diffusion at the mesoscopic scale: an experimental and numerical study

Samuel Chapman, Jan V. M. Borgomano, Beatriz Quintal, Sally M. Benson, and Jerome Fortin

Monitoring of the subsurface with seismic methods can be improved by better understanding the attenuation of seismic waves due to fluid pressure diffusion (FPD). In porous rocks saturated with multiple fluid phases the attenuation of seismic waves by FPD is sensitive to the mesoscopic scale distribution of the respective fluids. The relationship between fluid distribution and seismic wave attenuation could be used, for example, to assess the effectiveness of residual trapping of carbon dioxide (CO2) in the subsurface. Determining such relationships requires validating models of FPD with accurate laboratory measurements of seismic wave attenuation and modulus dispersion over a broad frequency range, and, in addition, characterising the fluid distribution during experiments. To address this challenge, experiments were performed on a Berea sandstone sample in which the exsolution of CO2 from water in the pore space of the sample was induced by a reduction in pore pressure. The fluid distribution was determined with X-ray computed tomography (CT) in a first set of experiments. The CO2 exosolved predominantly near the outlet, resulting in a heterogeneous fluid distribution along the sample length. In a second set of experiments, at similar pressure and temperature conditions, the forced oscillation method was used to measure the attenuation and modulus dispersion in the partially saturated sample over a broad frequency range (0.1 - 1000 Hz). Significant P-wave attenuation and dispersion was observed, while S-wave attenuation and dispersion were negligible. These observations suggest that the dominant mechanism of attenuation and dispersion was FPD. The attenuation and dispersion by FPD was subsequently modelled by solving Biot’s quasi-static equations of poroelasticity with the finite element method. The fluid saturation distribution determined from the X-ray CT was used in combination with a Reuss average to define a single phase effective fluid bulk modulus. The numerical solutions agree well with the attenuation and modulus dispersion measured in the laboratory, supporting the interpretation that attenuation and dispersion was due to FPD occurring in the heterogenous distribution of the coexisting fluids. The numerical simulations have the advantage that the models can easily be improved by including sub-core scale porosity and permeability distributions, which can also be determined using X-ray CT. In the future this could allow for conducting experiments on heterogenous samples.

How to cite: Chapman, S., Borgomano, J. V. M., Quintal, B., Benson, S. M., and Fortin, J.: Seismic wave attenuation due to fluid pressure diffusion at the mesoscopic scale: an experimental and numerical study, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16072, https://doi.org/10.5194/egusphere-egu21-16072, 2021.

EGU21-9230 | vPICO presentations | EMRP1.15

Laboratory experiments of water injection coupled with ultrasonic monitoring reveal wave-induced fluid flow in microporous carbonate rock

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

Monitoring of fluid movements in the crust is one of the most discussed topics in oil & gas industry as well as in geothermal systems and CO2 storage, but still remains a challenge. The seismic method is one of the most common ways to detect the fluid migration. However, the use of ultrasonic monitoring at the sample scale in laboratory experiments persists as the most effective way to highlight large scale observations in which the boundary conditions are not well constrained.

To unravel the fluid effect on P-wave and S-wave velocity, we performed mechanical experiments coupled with ultrasonic monitoring on Obourg chalk from Mons basin (Belgium). Water injection tests under critical loading, imbibition tests and evaporation tests provided a full spectrum of observations of fluid-induced wave alteration in term of propagation time and attenuation.

The analysis of these experimental results showed that significant velocity dispersion and attenuation developed through variations in water saturation, and that these processes are linked to the presence of patches of water and air in the pore space.

We used the White’s formulation to model the relaxation effects due to spherical pockets of air homogeneously distributed in a water-saturated medium. In this framework, the pressure induced by the passing wave, produces a fluid flow across the water-air boundary with consequent energy loss.

This model reproduces both qualitatively and quantitatively the experimental results observed on the water injection tests. Indeed, it is shown that the progressive water saturation or desaturation of this chalk, generates a shift of the critical frequency (from the undrained relaxed towards unrelaxed regimes) which at some point matches the resonance frequency of the piezoelectric transducers used in the experimental setup (0.5 MHz). This phenomenon allowed us to get a continuous recording of the relaxation processes induced by saturation variations.

The outcomes of this work can significantly improve the actual knowledge on coupled effects of waves and fluids which is a crucial aspect of fluid monitoring in the context of reservoir evaluation and production.

How to cite: Geremia, D., David, C., Barnes, C., Menéndez, B., Dautriat, J., Esteban, L., Sarout, J., Vandycke, S., and Descamps, F.: Laboratory experiments of water injection coupled with ultrasonic monitoring reveal wave-induced fluid flow in microporous carbonate rock, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9230, https://doi.org/10.5194/egusphere-egu21-9230, 2021.

EGU21-12197 | vPICO presentations | EMRP1.15

Capillary suction effects on surface-wave dispersion in partially saturated soils

Santiago Solazzi, Ludovic Bodet, Klaus Holliger, and Damien Jougnot

Estimation of water content in the shallow subsurface using seismic data is a complex task of increasing importance in the overall field of hydrogeophysics. In this context, the velocities of compressional (Vp) and shear (Vs) waves can be used to infer strong water content variations in unconsolidated soils, such as, the presence of the water table, by means of Vp/Vs ratio estimations. This approach, which is based on first-arrival time data, generally does not permit a proper quantification of the water content distribution in the partially saturated zone. Conversely, field and laboratory measurements indicate that surface-waves are indeed remarkably sensitive to both the water table depth and the saturation characteristics in the overlaying capillary fringe. This apparent difference in sensitivity between body and surface waves cannot be explained using conventional models. Observations and experiments show that the effective stress of unconsolidated porous media is not only affected by the overburden stress and pore pressures, as classic models assume, but also by capillary forces, which tend to stiffen the soil at relatively low saturations. In this work, we extend seminal rock physics models to include capillary suction effects in the effective stress of the soil. This approach provides effective elastic moduli and, thus, Vp and Vs, which are depth- and saturation-dependent. Then, we solve the quasi-static fluid flow equations in a porous medium and obtain saturation profiles for a given water table depth. This information, combined with the proposed rock physics model, permits to simulate simple seismic data sets, that is, body-wave first-arrival times and surface-wave phase velocities, for different water table depths and soil textures. Our results clearly show that capillary effects allow to explain the apparent difference in sensitivity between body- and surface-wave signatures in response to small water content variations in the partially saturated zone. Capillary effects are primarily relevant in porous media composed by relatively small characteristic grain sizes. We conclude that the proposed framework has the potential to fundamentally improve our characterization of near-surface environments using both active and passive seismic methods.

How to cite: Solazzi, S., Bodet, L., Holliger, K., and Jougnot, D.: Capillary suction effects on surface-wave dispersion in partially saturated soils, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12197, https://doi.org/10.5194/egusphere-egu21-12197, 2021.

EGU21-12483 | vPICO presentations | EMRP1.15

Seismic anisotropy in metamorphic rocks from the COSC-1 borehole, Sweden: A cross-scale investigation from thin section analysis to seismic scales

Felix Kästner, Simona Pierdominici, Alba Zappone, Luiz F. G. Morales, Anja M. Schleicher, Franziska D. H. Wilke, and Christian Berndt

Metamorphic and deformed rocks in thrust zones show particularly high seismic anisotropy causing challenges for seismic imaging and interpretation. A good example is the Seve Nappe Complex in Jämtland, Sweden, an exhumed orogenic thrust zone characterized by a strong but incoherent seismic reflectivity and considerable seismic anisotropy. However, only little is known about the origin of the anisotropy in relation to composition, structural influences, and implications for measurements at different seismic scales. We present an integrative study of the seismic anisotropy at different scales combining mineralogical composition, microstructural analyses and seismic laboratory experiments from samples of the 2.5 km-deep COSC-1 borehole. While there is a pronounced crystallographic preferred orientation in most of the core samples, variations in anisotropy correlate strongly with bulk mineral composition and dominant core lithology. Based on three major lithologic different facies (felsic gneiss, amphibole-rich rocks, and mica schists), we propose an anisotropy model for the full length of the borehole, which indicates two prevailing anisotropic units. Comparison of laboratory seismic measurements and electron-backscatter diffraction (EBSD) data reveals a strong scale-dependence, which is more pronounced in the highly deformed, heterogeneous samples. This highlights the need for comprehensive cross-validation of microscale anisotropy analyses with additional lithological data when integrating seismic anisotropy through seismic scales.

How to cite: Kästner, F., Pierdominici, S., Zappone, A., Morales, L. F. G., Schleicher, A. M., Wilke, F. D. H., and Berndt, C.: Seismic anisotropy in metamorphic rocks from the COSC-1 borehole, Sweden: A cross-scale investigation from thin section analysis to seismic scales, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12483, https://doi.org/10.5194/egusphere-egu21-12483, 2021.

EGU21-10820 | vPICO presentations | EMRP1.15

Estimation of fracture compliance based on group delays inferred from full-waveform sonic log data

Zhenya Zhou, Eva Caspari, Nicolás D. Barbosa, Andrew Greenwood, and Klaus Holliger

Fractures, which are ubiquitous in the Earth’s upper crust, have significant impacts on a wide range of human activities, and, hence, their adequate characterization is of wide interest and importance. Seismic methods have significant potential for effectively addressing this objective. When a seismic wave propagates across a fluid-filled fracture, its amplitude is diminished and its travel time is increased. Based on the linear slip theory, the associated amplitude decays and phase delays can be used to estimate the mechanical compliance of fractures. Full-waveform sonic (FWS) log data are particularly well-suited for this purpose. While the amplitudes of FWS data acquired during standard continuous logging runs (tool being moved uphole at a constant logging speed) can be somewhat unstable, the associated first-arrival travel times are generally quite robust. In this work, we exploit the relation between the time delay that seismic waves experience across fractures and relate them to the associated compliances. Specifically, we estimate fracture compliance from the differences in group time delay of the refracted P-wave between fractured and non-fractured sections along a borehole. Numerical simulations indicate that the proposed method provides reliable compliance estimates not only for individual fractures, but also for sets of multiple discrete fractures. This finding is corroborated by applying our approach to FWS log data acquired in the course of standard logging runs in the Bedretto Underground Laboratory (www.bedrettolab.ethz.ch). Our estimates are comparable to previously inferred compliance values in a closely comparable geological environment (Grimsel test site, www.grimsel.com). The latter were inferred under rather ideal conditions, involving the quasi-static acquisition of the FWS data as well as the combination of amplitude and travel time information for their interpretation. An interesting and important open question, which we plan to address in the following, concerns the influence of the heterogeneity of the host rock embedding the fractures on compliance estimation in general and on the proposed method in particular.

How to cite: Zhou, Z., Caspari, E., D. Barbosa, N., Greenwood, A., and Holliger, K.: Estimation of fracture compliance based on group delays inferred from full-waveform sonic log data, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10820, https://doi.org/10.5194/egusphere-egu21-10820, 2021.

EGU21-3466 | vPICO presentations | EMRP1.15

Constrained Magnetic Vector Inversion of Scanning Magnetic Microscopy Data for Modelling Magnetization of Multidomain Mineral Grains

Peter Lelièvre, Zeudia Pastore, Nathan Church, Madeline Lee, Hirokuni Oda, and Suzanne McEnroe

We are using 3D magnetic vector inversion (MVI) of scanning magnetic microscopy (SMM) data to investigate the fine‐scale magnetization of rock samples, and particularly of their remanence carriers, which can record geologically meaningful information. Previous investigations of magnetite grains suggest variable remanence intensities and directions coherent with multidomain behaviour. This research seeks to improve our understanding of the contribution of different microstructures on remanence acquisition.

SMM offers a spatial resolution down to tens of micrometers, allowing detailed investigation of discrete magnetic mineral grains, or magnetic textures and structures. However, all magnetic measurements are, at some scale, bulk measurements. Further analysis of the data is required to extract information about the magnetization within the samples: for this, we employ state-of-the-art MVI methods. The MVI problem suffers from a high degree of nonuniqueness. Additional constraints are required to obtain accurate, reliable and interpretable results. Such constraints are readily available for this application.

SMM instruments use magnetic shields or Helmholtz coils to allow collection of data in controlled magnetic fields, enabling the removal of induced magnetization effects. Measurements can be taken both above and below the sample. Individual magnetized mineral grains are easily outlined through optical and electron microscopy. The internal geometry of the oxide mineral phases and compositions can also be constrained. Physical property information constrains the range of magnetization intensity. As such, there is a tremendous amount of constraining information invaluable for reducing the nonuniqueness of the inverse problem. We use a highly flexible and functional inversion software package, MAGNUM, developed jointly at Mount Allison University and Memorial University of Newfoundland, that allows incorporation of all available constraints.

We take a multitiered approach for investigating specific magnetized grains. First, coarse regional inversions are performed to assess and remove any effects of other magnetized grains in the vicinity. The entire grain is then modelled with a homogeneous magnetization to obtain an approximate but representative bulk magnetization. The grain is then modelled as a collection of independent subdomains, each with a different homogeneous magnetization direction. Subsequently, more heterogeneous scenarios are considered by relaxing inversion constraints until the data can be fit to the desired degree.

Obtaining reliable information about the magnetic mineralogy of rock samples is vital for an understanding of the origin of rock bulk behaviour in both the laboratory and larger scale magnetic surveys. This work is among the first to simultaneously invert SMM data collected above and below a thin sample, which is critical for improving depth resolution on thicker samples. It is also the first time we have been able to incorporate all available constraints into inverse modelling to improve results.

How to cite: Lelièvre, P., Pastore, Z., Church, N., Lee, M., Oda, H., and McEnroe, S.: Constrained Magnetic Vector Inversion of Scanning Magnetic Microscopy Data for Modelling Magnetization of Multidomain Mineral Grains, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3466, https://doi.org/10.5194/egusphere-egu21-3466, 2021.

EGU21-5887 | vPICO presentations | EMRP1.15

Seismic moduli dispersion and attenuation obtained using Digital Rock Physics

Simón Lissa, Matthias Ruf, Holger Steeb, and Beatriz Quintal

Seismic waves are affected by rock properties such as porosity, permeability, grain material and by their heterogeneities as well as by the fluid properties saturating the rocks. Consequently, seismic methods are a valuable tool for the indirect characterization of rocks. For example, at the microscale, the presence of compliant pores (cracks or grain contacts) in fluid-saturated rocks can cause strong seismic attenuation and velocity dispersion. In this case, the deformation caused by a passing wave induces a fluid pressure gradient between compressed compliant pores and much less compressed pores (stiff isometric pores or cracks having a different orientation than the most compressed ones) if they are hydraulically connected. The consequent fluid pressure diffusion (FPD) dissipates seismic energy due to viscous friction in the fluid.

Digital rock physics (DRP) aims to reproduce experimental measurements using numerical simulation in models derived from high resolution rock images. We developed a DRP workflow to calculate the frequency dependent seismic moduli dispersion and attenuation in fluid-saturated models derived from micro X-Ray Computed Tomography (µXRCT) images. Filtering, segmentation and meshing procedures are applied on sub-volumes of different rock images to create 3D numerical models. We apply our workflow to calculate seismic moduli attenuation due to FPD at the microscale (squirt flow). We consider a µXRCT image of a cracked (through thermal treatment) Carrara marble sample. A detailed visualization of the fluid pressure as well as of the energy dissipation rate in the 3D model helps to understand the squirt flow attenuation process at different frequencies.

How to cite: Lissa, S., Ruf, M., Steeb, H., and Quintal, B.: Seismic moduli dispersion and attenuation obtained using Digital Rock Physics, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5887, https://doi.org/10.5194/egusphere-egu21-5887, 2021.

EGU21-6492 | vPICO presentations | EMRP1.15

Assessing elastic properties of cracks in rock samples subjected to thermo-hydro-chemo-mechanical damage

Anthony Clark, Tiziana Vanorio, Andrey Radostin, and Vladimir Zaitsev

An understanding of micro- and macrofracture behavior in low porosity rocks is pertinent to several areas of energy and environmental science such as petroleum production, carbon sequestration, and enhancement of technologies based on geothermal energy, etc. For example, the carbonate reservoirs in dolomitic or micritic formations with matrix porosities below 6% suggest the importance of fracture-augmented permeability in production. Similarly, hydrocarbons have been found on nearly every continent in tight basement rocks, all of which have little matrix porosity and their permeability therefore rely solely on hydraulic connectivity from fractures. For geothermal energy, various igneous and sedimentary rocks (granites, basalts, and limestones) are being exploited across the globe, with some of the lowest porosity and permeability. In all these cases, fractures are necessary to improve rock permeability and thermal exchange between the rock and working fluid, which can be enabled by hydraulic stimulation, as well as by secondary cracking due to extreme temperature gradients from the injection of cold water. The fracture geometry, density, and distribution all together control not only fluid and thermal transport in the rocks, but also their seismic attributes that can be used to extract information about the fractures. 
In order to accurately interpret the seismo-acoustic data (usually, the velocities of compression and shear waves) reliable rock physics models are required. Here, we report the results of interpretation of such experimental data for both as-cored rock samples and those subjected to thermo-hydro-chemo-mechanical damage (THCMD) in the laboratory. For interpretation, we use a convenient model of fractured rock in which fractures are represented as planar defects with decoupled shear and normal compliances. The application of such an approach makes it possible to assess and compare the elastic properties of fractures in the rocks before and after application of THCMD procedures. For the analyzed samples of granites, basalts, and limestones it has been found that for a significant portion of rocks, the ratio of normal-to-shear compliances of cracks significantly differ from the value typical of conventionally assumed penny-shape cracks. Furthermore, for some samples, this ratio appears to be noticeably different for fractures existing in the as-cored rock and arising in the same rock after THCMD procedures. These results indicate that damage to a rock typically changes its compliance ratio since the old and new cracks are likely to have different elastic properties. Our results are also consistent with the notion that a specific damage process occurring for a given microstructure will consistently create cracks with a particular set of elastic properties. The proposed methodology for assessment of elastic properties of cracks in rock samples subjected to thermo-hydro-chemo-mechanical damage has given previously inaccessible useful information about the elastic properties of fractures and can be extended to interpretation of seismic attributes of rocks for a broad range of other applications.

How to cite: Clark, A., Vanorio, T., Radostin, A., and Zaitsev, V.: Assessing elastic properties of cracks in rock samples subjected to thermo-hydro-chemo-mechanical damage, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6492, https://doi.org/10.5194/egusphere-egu21-6492, 2021.

EGU21-4147 | vPICO presentations | EMRP1.15

Digital rock physics: A geological driven segmentation workflow for Ruhr sandstone

Martin Balcewicz, Mirko Siegert, Marcel Gurris, David Krach, Matthias Ruf, Holger Steeb, and Erik H. Saenger

Over the last two decades, Digital Rock Physics (DRP) has become a complementary part of the characterization of reservoir rocks due to, among other things, the non-destructive testing character of this technique. The use of high-resolution X-ray Computed Tomography (XRCT) has become widely accepted to create a digital twin of the material under investigation. Compared to other imaging techniques, XRCT technology allows a location-dependent resolution of the individual material particles in volume. However, there are still challenges in assigning physical properties to a particular voxel within the digital twin, due to standard histogram analysis or sub-resolution features in the rock. For this reason, high-resolution image-based data from XRCT, transmitted-light microscope, Scanning Electron Microscope (SEM) as well as inherent material properties like porosity are combined to obtain an optimal spatial image of the studied Ruhr sandstone by a geologically driven segmentation workflow. On the basis of a homogeneity test, which corresponds to the evaluation of the grayscale image histogram, the preferred scan sample sizes in terms of transport, thermal, and effective elastic rock properties are determined. In addition, the advanced numerical simulation results are compared with laboratory tests to provide possible upper limits for sample size, segmentation accuracy, and a calibrated digital twin of the Ruhr sandstone. The comparison of representative grayscale image histograms as a function of sample sizes with the corresponding advanced numerical simulations, provides a unique workflow for reservoir characterization of the Ruhr sandstone.

How to cite: Balcewicz, M., Siegert, M., Gurris, M., Krach, D., Ruf, M., Steeb, H., and Saenger, E. H.: Digital rock physics: A geological driven segmentation workflow for Ruhr sandstone, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4147, https://doi.org/10.5194/egusphere-egu21-4147, 2021.

EGU21-4691 | vPICO presentations | EMRP1.15

Numerical determination of pressure-dependent effective thermal conductivity in Berea sandstone

Mirko Siegert, Marcel Gurris, and Erik Hans Saenger

Within the scope of the present work, the pressure-dependent effective thermal conductivity of rock samples is simulated. Our workflow can be assigned to the field of digital rock physics. In a first step, a 3D micro-CT scan of a rock sample is taken. Subsequently, the resulting greyscale images are analysed and segmented depending on the occurring phases. Based on this data set, a computational mesh is created and the corresponding thermal conductivities are assigned to each phase. Finally the numerical simulations can be carried out.
For the representation of the pressure dependency we use the approach proposed by Saenger [1]. By making use of the watershed algorithm, boundaries between the individual grains of the rock sample are detected and assigned to an artificial contact phase. In the course of several simulations, the thermal conductivity of the contact phase is continuously increased. Starting with the thermal conductivity of the pore phase and ending with the thermal conductivity of the grain phase. A linear correlation is used to match the thermal conductivity of the contact phase with the pressure of a given experimental data set. This enables a direct comparison between simulation and measurement.
In a further step, the numerical model is calibrated to optimise the agreement between experimental data and simulation results. In particular, starting from two calibration points of the experimental data set, an adjustment of the thermal conductivities in the numerical model is carried out. While the thermal conductivity of the pore phase is held constant during the whole calibration process, thermal conductivities of the grain and contact phase are adjusted.

References
[1] Saenger et al. 2016. Analysis of high-resolution X-ray computed tomography images of Bentheim sandstone under elevated confining pressures. Geophysical Prospecting, 64(4), 848–859.

 

How to cite: Siegert, M., Gurris, M., and Saenger, E. H.: Numerical determination of pressure-dependent effective thermal conductivity in Berea sandstone, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4691, https://doi.org/10.5194/egusphere-egu21-4691, 2021.

EGU21-14273 | vPICO presentations | EMRP1.15

Textural and mineralogical controls on temperature dependent SIP behavior during freezing and thawing

Jonas K. Limbrock, Maximilian Weigand, and Andreas Kemna

Geoelectrical methods are increasingly being used for non-invasive characterization and monitoring of permafrost sites, since the electrical properties are sensitive to the phase change of liquid to frozen water. Here, electrical resistivity tomography (ERT) is most commonly applied, using resistivity as a proxy for various quantities, such as temperature or ice content. However, it is still challenging to distinguish between air and ice in the pore space of the rock based on resistivity alone due to their similarly low electrical conductivity. Meanwhile, geoelectrical methods that utilize electrical polarization effects to characterize permafrost are also being explored. For example, the usage of the spectral induced polarization (SIP) method, in which the complex, frequency-dependent impedance is measured, can reduce ambiguities in the subsurface conduction properties, considering the SIP signature of ice. These measurements seem to be suitable for the quantification of ice content (and thus the differentiation of ice and air), and for the improved thermal characterization of alpine permafrost sites. However, to improve the interpretation of SIP measurements, it is necessary to understand in more detail the electrical conduction and polarization properties as a function of temperature, ice content, texture, and mineralogy under frozen and partially frozen conditions.

In the study presented here, electrical impedance was measured continuously using SIP in the frequency range of 10 mHz to 45 kHz on various water-saturated solid rock and loose sediment samples during controlled freeze-thaw cycles (+20°C to -40°C). These measurements were performed on rock samples from different alpine permafrost sites with different mineralogical compositions and textures. For all samples, the resistance (impedance magnitude) shows a similar temperature dependence, with increasing resistance for decreasing temperature. Also, hysteresis between freezing and thawing behavior is observed for all measurements. During freezing, a jump within the temperature-dependent resistance is observed, suggesting a lowering of the freezing point to a critical temperature where an abrupt transition from liquid water to ice occurs. During thawing, on the other hand, there is a continuous decrease in the measured resistance, suggesting a continuous thawing of the sample. The spectra of impedance phase, which is a measure for the polarization, exhibit the same qualitative, well-known temperature-dependent relaxation behaviour of ice at higher frequencies (1 kHz - 45 kHz), with variations in shape and strength for different rock texture and mineralogy. At lower frequencies (1 Hz - 1 kHz), a polarization with a weak frequency dependence is observed in the unfrozen state of the samples. We interpret this response as membrane polarization, which likewise depends on the texture as well as on the mineralogy of the respective sample. This polarization response partially vanishes during freezing. Overall, the investigated SIP spectra do not only show a dependence on texture and mineralogy, but mainly a dependence on the presence of ice in the sample as well as temperature. This indicates the possibility of a thermal characterization, as well as a determination of the ice content, of permafrost rocks using SIP.

How to cite: Limbrock, J. K., Weigand, M., and Kemna, A.: Textural and mineralogical controls on temperature dependent SIP behavior during freezing and thawing, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14273, https://doi.org/10.5194/egusphere-egu21-14273, 2021.

EGU21-13527 | vPICO presentations | EMRP1.15

Correlations of magnetic pore fabrics with pore fabrics derived from high-resolution X-ray computed tomography and with permeability anisotropy in sedimentary rocks and synthetic samples

Yi Zhou, Michele Pugnetti, Anneleen Foubert, Pierre Lanari, Christoph Neururer, and Andrea Biedermann

Magnetic pore fabrics (MPF) are an indirect measure of the 3D pore structure. They are defined by measuring anisotropy of magnetic susceptibility after samples have been impregnated with ferrofluid. Previous studies proposed that MPFs target pores down to 10 nm. Therefore, the method complements X-ray computed tomography (XRCT) datasets, with resolution on the order of 1-10 µm. Empirical relationships exist between MPF and pore fabric, and between MPF and permeability anisotropy. This study investigates quantitative correlations between these three properties, and between measured quantities and digital-rock-model-simulations of permeability anisotropy and MPF. Samples used for this study include natural sedimentary rocks and synthetic samples. Sediments are Plio-Pleistocene calcarenite (Apulia, Italy) with ~50% porosity and complex pore structure, and Upper Marine Molasse sandstone (Belpberg, Switzerland) with 10-20% porosity and relatively homogeneous pore space properties. Synthetic samples were made from quartz sand and calcite powder in different proportions, to simulate sandstone and carbonate rocks. Samples were characterized by pycnometry, XRCT scans, MPF determination and directional permeability measurements to obtain porosity, digital rock models, MPFs and permeability anisotropy. Porosity, permeability anisotropy, and MPFs were also computed based on digital rock models derived from XRCT data, and compared to direct measurements. Permeability anisotropy and MPF are both second-order tensors, representing the average property of the entire sample. To directly relate the XRCT-derived individual pore properties to these second-order tensor quantities, a total shape ellipsoid was computed by adding the second-order tensors reflecting the best-fit ellipsoids of single pores. Once all properties were described by second-order tensors, they were correlated in terms of fabric orientation, degree and shape of anisotropy. The MPF and total shape ellipsoids are coaxial when the samples have sufficiently large pores to be resolved, and good impregnation efficiency, and as expected, total shape ellipsoids have larger anisotropy degree. Preliminary results further indicate that the permeability anisotropy is partly consistent with total shape ellipsoids and MPFs. The defined quantitative relationships facilitate the interpretation of MPF data, thus making the method more applicable to geological and fluid migration studies.

How to cite: Zhou, Y., Pugnetti, M., Foubert, A., Lanari, P., Neururer, C., and Biedermann, A.: Correlations of magnetic pore fabrics with pore fabrics derived from high-resolution X-ray computed tomography and with permeability anisotropy in sedimentary rocks and synthetic samples, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13527, https://doi.org/10.5194/egusphere-egu21-13527, 2021.

The magnetic pore fabrics method is a useful technique to investigate the pore fabric of rocks. The method is based on impregnating porous samples with ferrofluid, a colloidal suspension of magnetic nanoparticles (particle size of 10 nm) in water or oil carrier fluid, and measuring the anisotropy of magnetic susceptibility. It succesfully provides the average pore shape and orientation. This information is important to determine the preferred direction of fluid flow. A crucial step in magnetic pore fabric studies is ferrofluid impregnation; several studies pointed out the importance of forced impregnation methods to enhance impregnation efficiency. This study compares directional impregnation techniques (applying external forces along the ferrofluid flow along the axis of the core sample) with standard vacuum- and immersion-based methods. The newly developed or adapted techniques include: (1) pressure experiment: a cylindrical sample is placed in a metal tube under confining pressure of 12 bar, and an external pump-syringe system injects ferrofluid at a constant rate of 100 ml/min that generates a differential pressure of 5 bar; (2) resin flowthrough: vacuum is applied at the bottom of the sample and a mixture of resin and oil-based ferrofluid supplied at the top, so that the resin drags the fluid into the pores, where it hardens; (3) magnetically assisted flowthrough: fluid flow is enforced by the combined action of a hydraulic pressure gradient in the ferrofluid reservoir (~10 kPa) and the magnetic force exerted by the field gradient of about 2 A/m2 in the vicinity of an electric coil. These impregnation methods were tested on natural and synthetic samples, for which previous experiments employing standard impregnation methods exist. The natural samples include calcarenite from Apulia, Italy (50% porosity) and sandstone from Schupfheim, Swiss molasse (20% porosity). Synthetic samples consist of calcite and quartz sand in different proportions, consolidated with liquid glass (sodium silicate) in a cubic consolidation cell (specifically designed for the experiment), applying uniaxial pressure along the z axis, to create uniaxial anisotropy. The cube was dried in the oven for three days and three cylindrical cores were drilled along the x, y and z axes. For each impregnation method, the magnetic anisotropy of the samples was measured before and after impregnation. Impregnation efficiency was tested  using bulk susceptibility measurements, visual microscopic investigations and susceptibility profiles along the flow direction. Initial results show that (1) directional forced impregnation is more efficient than traditional methods in impregnating smaller pores,  avoids particle aggregation, and allows viscous fluid such as resin to acess the sample’s pores; (2) directional impregnation methods require less  fluid;  (3) the distribution of the ferrofluid after impregnation is more uniform, overcoming the difficulty of impregnating the  centre of the sample;  and (4) the fluid flow rate must be faster than the particle aggregation rate. For future studies, directional forced impregnation systems are recommended over standard vacuum- and immersion-based impregnation methods.

 

How to cite: Pugnetti, M., Zhou, Y., and Biedermann, A.: Experimental improvements for ferrofluid impregnation of rocks using directional forced impregnation methods: results on natural and synthetic samples, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14695, https://doi.org/10.5194/egusphere-egu21-14695, 2021.

EMRP1.16 – Open Session in Rock Physics

EGU21-5053 | vPICO presentations | EMRP1.16

Seismic attenuation and rheology of crustal rocks: results from numerical tests

Maria Aurora Natale Castillo, Magdala Tesauro, and Mauro Cacace

Seismic attenuation of the rocks mainly depends on their intrinsic anelasticity, which is the dissipation of seismic energy as it propagates through the medium. Several studies have already demonstrated that seismic attenuation, described by the Q-factor, is intrinsically related to the rocks’ viscosity, considering their common dependency on composition, grain size, fluid content, and T-P conditions. However, viscous deformation of the rocks occurs through different mechanisms: diffusion creep, numerous mechanisms of the dislocation creep, pressure solution, which are expressed by several Arrhenius-type constitutive laws. This makes more complex the investigation of quantitative relationships between seismic attenuation and viscous rocks' rheology.

The main purpose of this study is to investigate the mutual dependence of the seismic attenuation and viscous deformation of the crustal rocks. To this aim, we performed several numerical tests to check the variability of some physical properties (e.g., elastic modulus, Poisson’s ratio) and improve the existing relationships between seismic attenuation and viscous deformation of several natural rock samples. The Burgers mechanical model and Arrhenius relation are included in these test series to achieve a closer approximation of the rocks’ viscous deformation.

In this way, it will be possible to predict the viscous rheology of rocks from the laws describing the seismic attenuation and viceversa. The obtained results will be used to (1) constrain the Q-factor and rheological (creep) parameters, which are still subjected to high uncertainties, (2) validate/modify the existing seismic attenuation and rheological laws, (3) increase the robustness of the geodynamic and rocks’ mechanics numerical codes and our understanding of the role that rocks’ rheology exerts on the tectonic processes.

How to cite: Natale Castillo, M. A., Tesauro, M., and Cacace, M.: Seismic attenuation and rheology of crustal rocks: results from numerical tests, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5053, https://doi.org/10.5194/egusphere-egu21-5053, 2021.

EGU21-5585 | vPICO presentations | EMRP1.16

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

Maya Kobchenko, Anne Pluymakers, Benoit Cordonnier, Nazmul Mondol, and Francois Renard

Shales are layered sedimentary rocks, which can be almost impermeable for fluids and act as seals and cap-rocks, or if a shale layer hosts a fracture network, it can act as a fluid reservoir and/or conduit. Organic-rich shales contain organic matter - kerogen, which can transform from solid-state to oil and gas during burial and exposure to a suitable temperature. When hydrocarbons are expelled from the organic matter due to maturation, pore-pressure increases, which drives the propagation of hydraulic fractures, a mechanism identified to explain oil and gas primary migration. 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. 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 shale deformation in-situ in 3D. We reproduced natural conditions of the shale deformation processes using a combination of axial load, confining pressure, and heating of the shale samples. Shales feature natural sedimentary laminations and hydraulic fractures 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 impermeable, could host transient episodes of micro-fracturing and high permeability during burial history.

How to cite: Kobchenko, M., Pluymakers, A., Cordonnier, B., Mondol, N., and Renard, F.: Time-lapse synchrotron X-ray imaging of deformation modes in organic-rich Green River Shale heated under confinement , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5585, https://doi.org/10.5194/egusphere-egu21-5585, 2021.

EGU21-11531 | vPICO presentations | EMRP1.16

Measurements of anisotropic poroelastic moduli in Westerly granite

Bobby Elsigood, Nicolas Brantut, Philip Meredith, Tom Mitchell, and David Healy

We measured both drained (dry and saturated) and undrained elastic and poro-elastic moduli in a sample of thermally cracked Westerly granite. Measurements were made at nominal effective confining pressures of 30 MPa and 100 MPa. Differential stress was also applied to the sample in order to induce increasing anisotropy (transverse isotropy). Radial and axial stress were independently cycled at increasing levels of differential stress to enable the measurement of anisotropic moduli in a sample with increasing anisotropy. Measurements were first conducted under dry conditions in order to obtain the drained dynamic moduli before the sample was saturated with water. We used newly developed, miniature differential pressure transducers were located directly around the sample surface, which allowed for direct measurement of Skempton's radial coefficient Bx and axial coefficient Bz. Wavespeeds measured on the dry sample were inverted to obtain the compliances Cijkl. The coefficients Bx and Bz were then calculated indirectly from the compliances using poro-elasticy theory for comparison with the direct measurements. Our results show that the values of Bz calculated from the compliances compare well with those measured directly, but that the calculated values of Bx significantly overestimate those measured directly, particularly at an effective pressure of 30 MPa. The discrepancy between the direct measurements and those obtained from drained moduli is likely due to nonlinear elastic effects, such as crack closure and opening, which occur during the small but finite pressure and stress steps.

How to cite: Elsigood, B., Brantut, N., Meredith, P., Mitchell, T., and Healy, D.: Measurements of anisotropic poroelastic moduli in Westerly granite, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11531, https://doi.org/10.5194/egusphere-egu21-11531, 2021.

EGU21-11824 | vPICO presentations | EMRP1.16

Prediction of Petrophysical Properties from Seismic Inversion and Neural Network: A case study

Siddharth Garia, Arnab Kumar Pal, Karangat Ravi, and Archana M Nair

Seismic inversion method is widely used to characterize reservoirs and detect zones of interest, i.e., hydrocarbon-bearing zone in the subsurface by transforming seismic reflection data into quantitative subsurface rock properties. The primary aim of seismic inversion is to transform the 3D seismic section/cube into an acoustic impedance (AI) cube. The integration of this elastic attribute, i.e., AI cube with well log data, can thereafter help to establish correlations between AI and different petrophysical properties. The seismic inversion algorithm interpolates and spatially populates data/parameters of wells to the entire seismic section/cube based on the well log information. The case study presented here uses machine learning-neural network based algorithm to extract the different petrophysical properties such as porosity and bulk density from the seismic data of the Upper Assam basin, India. We analyzed three different stratigraphic  units that are established to be producing zones in this basin.

 AI model is generated from the seismic reflection data with the help of colored inversion operator. Subsequently, low-frequency model is generated from the impedance data extracted from the well log information. To compensate for the band limited nature of the seismic data, this low-frequency model is added to the existing acoustic model. Thereafter, a feed-forward neural network (NN) is trained with AI as input and porosity/bulk density as target, validated with NN generated porosity/bulk density with actual porosity/bulk density from well log data. The trained network is thus tested over the entire region of interest to populate these petrophysical properties.

Three seismic zones were identified from the seismic section ranging from 681 to 1333 ms, 1528 to 1575 ms and 1771 to 1814 ms. The range of AI, porosity and bulk density were observed to be 1738 to 6000 (g/cc) * (m/s), 26 to 38% and 1.95 to 2.46 g/cc respectively. Studies conducted by researchers in the same basin yielded porosity results in the range of 10-36%. The changes in acoustic impedance, porosity and bulk density may be attributed to the changes in lithology. NN method was prioritized over other traditional statistical methods due to its ability to model any arbitrary dependency (non-linear relationships between input and target values) and also overfitting can be avoided. Hence, the workflow presented here provides an estimation of reservoir properties and is considered useful in predicting petrophysical properties for reservoir characterization, thus helping to estimate reservoir productivity.

How to cite: Garia, S., Pal, A. K., Ravi, K., and Nair, A. M.: Prediction of Petrophysical Properties from Seismic Inversion and Neural Network: A case study, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11824, https://doi.org/10.5194/egusphere-egu21-11824, 2021.

EGU21-12261 | vPICO presentations | EMRP1.16

Using AE based Machine Learning Approaches to Forecast Rupture during Rock Deformation Laboratory Experiments

Sergio Vinciguerra, Thomas King, and Philip Benson

The ability to detect precursors of dynamic failure in brittle rocks has key implications for hazard forecasting at the field scale. In recent years, laboratory scale rock deformation experiments are providing a wealth of information on the physics of the fracture process ranging from fracture nucleation, crack growth and damage accumulation, to crack coalescence and strain localization. Parametric analysis of laboratory Acoustic Emission (AE) data has revealed periodic trends and precursory behaviour of the rupture source mechanisms as a fault zone enucleates and develops, suggesting these processes are somehow repeatable and forecastable. However, due to the inherent anisotropy of rock media and the range of environmental conditions in which deformation occurs, finding full consistency between AE datasets and a prediction of rupture mechanisms from AE analysis is still an open goal. Here we apply a Time Delay Neural Network (TDNN) to Acoustic Emission (AE) sets recorded during conventional triaxial rock deformation tests. We forecast the Time-to-Failure using the discrete, non-continuous timeseries of AE rate, amplitude, focal mechanism and forward scattering properties. 4x10 cm samples of Alzo granite, a homogeneous medium-grained plutonic rock from NW Italy with an initial porosity as low as 0.72%, were triaxially deformed at strain rates of 3.6mm/hr under dry conditions until dynamic failure at confining pressures of 5, 10, 20 and 40 MPa respectively. Each sample was positioned inside an engineered rubber jacket fitted with ports where an array of twelve 1 MHz single-component Piezo-Electric Transducers were embedded, allowing to record AE during the experimentation. Several parameters were considered for the TDNN training: AE rate, deformation stages prior failure (elasticity, inelasticity and coalescence), AE amplitude, source mechanisms and scattering. All these parameters are key indicators of the evolving damage in the medium. Our training input consists of simplified timeseries of the previously discussed AE parameters from the experiments carried out at the lowest confining pressure (5 MPa). The inputs are classified as the stress-until failure and strain-until-failure for each AE. Once trained we then simulate the model on the untrained datasets to test it as a forecasting tool at higher confinements. At each step the model is simulated on AE data from the previous 0.2% of strain. At 10 MPa we observe a reliable forecast of failure that starts with the anelastic phase and becomes more accurate during strain-softening. At higher confining pressure, an increased limit of forecasting the solution is observed and interpreted with more complexity in the coalescence process. Despite these limitations, the model shows that when trained even on a limited input it is able to forecast dynamic failure in unseen data with surprising accuracy. Future studies should investigate AE spatial distribution for the TDNN training.

How to cite: Vinciguerra, S., King, T., and Benson, P.: Using AE based Machine Learning Approaches to Forecast Rupture during Rock Deformation Laboratory Experiments, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12261, https://doi.org/10.5194/egusphere-egu21-12261, 2021.

EGU21-12490 | vPICO presentations | EMRP1.16

Electrical-elastic modelling of rocks using cross-property DEM

Phillip Cilli and Mark Chapman

The combination of electrical and elastic measurements can be useful in lowering subsurface characterisation uncertainty. The majority of rock physics relations which relate a rock’s electrical and elastic properties, however, rely on the estimation of porosity as an intermediate step. By combining differential effective medium schemes which relate a rock’s electrical and elastic properties to porosity and pore shape, we obtain cross-property expressions which are independent of porosity, depending only on pore aspect ratio. Analysing published joint electrical-elastic measurements shows the cross-property model works well for clean sandstones, and models Vp/Vs ratios as a function of resistivity without porosity. Although clay-bearing sandstones are more complex, our model can still identify the correct trends. On theoretical grounds, it seems our approach has the potential to produce additional cross-property relations; a topic upon which we speculate.

How to cite: Cilli, P. and Chapman, M.: Electrical-elastic modelling of rocks using cross-property DEM, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12490, https://doi.org/10.5194/egusphere-egu21-12490, 2021.

EGU21-15915 | vPICO presentations | EMRP1.16

Is breaking through matter a hot matter? How to predict material failure by monitoring creep

Renaud Toussaint, Tom Vincent-Dospital, Alain Cochard, Eirik Grude Flekkøy, and Knut Jørgen Måløy

In any domain involving some stressed solids, that is, from seismology to rock physics or general engineering, the strength of matter is a paramount feature to understand.  The global failure of a mechanically loaded solid is usually dictated by the growth of its internal micro-cracks and dislocations. When this growth is rather smooth and distributed, the solid is considered to be in ductile condition. Alternatively, an abrupt propagation of localized defects leads to a brittle rupture of the full matrix.
It is then critical to understand what the physics and dynamics of isolated cracks are, when their tips are loaded at a given stress level. While the general elasticity theory predicts such stress to diverge, it is  acknowledged that some area around the crack fronts is rather plastic. In other words, some dissipation of mechanical energy, in a so-called process zone around a crack tip, prevents the - unphysical - stress divergence and shields the fronts from excessive load levels.

In this work, we focus on the local Joule heating, that significantly contributes to the energy dissipation. Analysing experimental data of the rupture of many materials, we indeed show that the scale for the thermal release around crack tips explains why the toughness of different media spans over orders of magnitude (we analysed materials spanning over 5 decades of energy release rate), whereas the covalent energy to separate two atoms does not.

We here discuss the ability of this simple thermally activated sub-critical model, which includes the auto-induced thermal evolution of crack stips [1], to predict the catastrophic failure of a vast range of materials [2]. It is in particular shown that the intrinsic surface energy barrier, for breaking the atomic bonds of many solids, can be easily deduced from the slow creeping dynamics of a crack. This intrinsic barrier is however higher than the macroscopic load threshold at which brittle matter brutally fails, possibly as a result of thermal activation and of a thermal weakening mechanism. We propose a novel method to compute the macroscopic critical energy release rate of rupture, Gc macroscopic, solely from monitoring slow creep, and show that this reproduces the experimental values within 50% accuracy over twenty different materials (such as glass, rocks, polymers, metals), and over more than four decades of fracture energy. We also infer the characteristic energy of rupturing bonds, and the size of an intense heat source zone around crack tips, and show that it scales as the classic process zone size, but is significantly (105 to 107 times) smaller.

References:

[1] Vincent-Dospital, T., Toussaint, R., Santucci, S., Vanel, L., Bonamy, D., Hattali, L., Cochard,  A, Flekkøy, E.G. and Måløy, K.J. (2020). How heat controls fracture: the thermodynamics of creeping and avalanching cracks. Soft Matter, 2020, 16, 9590-9602. DOI: 10.1039/D0SM01062F
 

[2] Vincent-Dospital, T., Toussaint, R., Cochard, A., Flekkøy, E. G., & Måløy, K. J. (2020). Is breaking through matter a hot matter? A material failure prediction by monitoring creep. arXiv preprint arXiv:2007.04866.  https://arxiv.org/abs/2007.04866

How to cite: Toussaint, R., Vincent-Dospital, T., Cochard, A., Flekkøy, E. G., and Måløy, K. J.: Is breaking through matter a hot matter? How to predict material failure by monitoring creep, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15915, https://doi.org/10.5194/egusphere-egu21-15915, 2021.

EGU21-15948 | vPICO presentations | EMRP1.16

High-speed tensile fractures in granite and gneiss: an experimental study

Beno J Jacob, Santanu Misra, Venkitanarayanan Parameswaran, and Nibir Mandal

Tensile fractures are ubiquitous in impact structures formed because of high strain rate deformations of the earth’s crust. At regions far from the point of meteorite impact, intense rupturing, fragmentation, and pulverisation are an implication of pressure waves limiting at the tensile strength of the host rock with little influence of shock deformation or shear failure. The branching and anastomosing of the fractures are controlled by the local stress state and anisotropy. Thus, a network of infilled fractures or impact breccia dikes is a common feature in the subsurface of impact sites.

We have investigated the failure processes under high strain rates responsible for the formation of Mode-I breccia dikes, at the laboratory scale. The control of planar fabric structures in the development of anastomosing tensile fracture networks was studied through high-strain-rate Brazilian disc tests on gneiss (foliated) and granite (isotropic) samples. A Split Hopkinson Pressure Bar, equipped with high-speed photography (~105 fps), was employed in the study. The gneissic foliation in the gneiss samples were oriented at θ = 0, 45 and 90° to the compression direction. The strength of granite lies between 24 and 26 MPa, and the gneisses failed in the range of 29-37MPa at about 70-90 μs. The fracture network formation was seen in the time series images. There is a stark disparity in the nature of failure of granite from gneiss and the geometry of clasts formed in each rock type. While granite samples fail with pulverised clasts localised along a single fracture spanning the diameter of the sample along the compression direction, the gneisses further developed a network of secondary fractures forming large elongate clasts. Preferential orientation of secondary crack growth in relation to the foliation is strongly influenced by θ in gneiss samples. The aspect ratio of the pulverised clasts (size < 10mm) formed in granite was about 1:2, whereas the gneisses produced larger clasts. The clasts in gneisses had an aspect ratio of 1:4 for θ = 45 and 90º, and 1:5 for θ = 0º.

The branching and anastomosing nature of fractures is similar in fracture networks observed from the field and in the experiments, thus providing an insight into the formation of high-speed impact breccia dikes in isotropic and foliated rocks. Our experiments demonstrate that monomict breccia dikes may by formed in situ inclusive of clasts, rather than by infilling in previously formed tensile fractures.

How to cite: Jacob, B. J., Misra, S., Parameswaran, V., and Mandal, N.: High-speed tensile fractures in granite and gneiss: an experimental study, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15948, https://doi.org/10.5194/egusphere-egu21-15948, 2021.

EMRP1.18 – Coupled thermo-hydro-mechanical-chemical (THMC) processes in geological media

EGU21-14418 | vPICO presentations | EMRP1.18

Thermo-mechanical Effects on Fracture Growth and Apertures in Three-Dimensional Subsurface Fractured Rocks 

Adriana Paluszny, Robin N. Thomas, M. Cristina Saceanu, and Robert W. Zimmerman

A finite-element based, quasi-static growth algorithm models mixed mode concurrent fracture growth in three dimensions, leading to the formation of non-planar arrays and networks. To model the fully coupled THM model, equations describing mechanical deformation as well as heat transfer in the matrix and in the fractures are introduced in the formulation, simultaneously accounting for the effect of fluid flow and stress-strain response. This results in five separate, but two-way coupled model equations: a thermoporoelastic mechanical model; two fluid flow equations, one for the rock matrix and one for the fractures; two heat transfer equations, similarly for both the matrix and fractures. Fractures are represented explicitly as discrete surfaces embedded within a volumetric domain [1]. Growth is computed as a set of vectors that modify the geometry of a fracture by accruing new fracture surfaces in response to brittle deformation. Fracture tip stress intensity factors drive fracture growth. This growth methodology is validated against analytical solutions for fractures under compression and tension [2]. Thermal effects on the apertures and growth patterns will be presented. Isolated fracture geometries are compared with selected experimental results on brittle media. Accurate growth is demonstrated for domains discretised by refined and coarse volumetric meshes. Fracture and volume-based growth rates are shown to modify fracture interaction patterns. Two-dimensional cut-plane views of fracture networks show how fractures would appear on the surface of the studied volume.

REFERENCES

[1] N. Thomas, A. Paluszny and R. W. Zimmerman. Growth of three-dimensional fractures, arrays, and networks in brittle rocks under tension and compression. Computers and Geotechnics, 2020. doi: 10.1016/j.compgeo.2020.103447

[2] Paluszny and R. W. Zimmerman. Numerical fracture growth modeling using smooth surface geometric deformation. Eng. Fract. Mech., 108, 19-36, 2013. doi: 10.1016/j.engfracmech.2013.04.012

How to cite: Paluszny, A., Thomas, R. N., Saceanu, M. C., and Zimmerman, R. W.: Thermo-mechanical Effects on Fracture Growth and Apertures in Three-Dimensional Subsurface Fractured Rocks , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14418, https://doi.org/10.5194/egusphere-egu21-14418, 2021.

In this talk, I give an overview of our software ResFrac, which fully integrates a ‘true’ hydraulic fracturing simulator and a multiphase reservoir simulator (McClure et al., 2020a). Conventionally, these processes have been described with separate codes, using separate meshes, and with different physics. Integrating these two categories of software is advantageous because it enables seamless description of the entire lifecycle of a well. It is possible to seamlessly integrate wells with complex histories such as frac hits from offset wells, refracs, and huff and puff EOR injection.

ResFrac has been applied on 25+ studies for operators optimizing development of oil and gas resources in shale and has been commercially licensed by 15+ companies (https://www.resfrac.com/case-studies; https://www.resfrac.com/publications; https://www.resfrac.com/about-us/our-team). The simulator has a modern user-interface with embedded help-documentation, wizards to help set up simulations, automated validators to identify issues with the setup prior to submitting, and plotting capabilities to preview 3D and tabular inputs. Simulations are run on the cloud and results are continuously downloaded to the user’s computer. This allows a user to easily run a large number of simultaneous simulations from their personal computer. The user-interface includes a custom-built and fully-featured visualization tool for 3D visualization and 2D plotting.

Hydraulic fracturing simulators must handle a diverse set of coupled physics: mechanics of crack propagation and stress shadowing, fluid flow in the fractures, leakoff, transport of fluid additives that impart non-Newtonian flow characteristics, and proppant transport. Proppant transport is particularly complex because proppant settles out into an immobile bed and may screen out at the tip. Many fracturing simulators approximate wellbore flow effects. However, because these effects are closely coupled to fracturing processes (especially in horizontal wells that have multiple simultaneously propagating fractures), we include a fully meshed, detailed wellbore model in the code, along with treatment of perforation pressure drop and near-wellbore tortuosity.

In the literature, separate constitutive relations are available to describe transport in open cracks, closed unpropped cracks, and closed propped cracks. However, there were not relations in the literature designed to describe transport under conditions transitional between these end-member states. A general numerical simulator must be able to describe all conditions (and avoid discontinuous changes between equations). To address this limitation, we developed a new set of constitutive equations that can smoothly transition between these end-member states – smoothly handling any general combination of aperture, effective normal stress, saturation, proppant volume fraction, and non-Newtonian fluid rheology (McClure et al., 2020).

The code solves all equations in a fully coupled way, using an adaptive implicit method. The fully coupled approach is chosen because of the tight coupling between many of the key physical processes. Iterative coupling converges very slowly and/or forces excessively small timesteps when tightly coupled processes are handled with iterative or explicit coupling.

McClure, Kang, Hewson, and Medam. 2020. ResFrac Technical Writeup (v5). arXiv.

How to cite: McClure, M.: Fully Integrating a Hydraulic Fracturing, Reservoir, and Wellbore Simulator into a Practical Engineering Tool, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9770, https://doi.org/10.5194/egusphere-egu21-9770, 2021.

EGU21-7928 | vPICO presentations | EMRP1.18

A Geothermal Energy Concept based on Heat Storage in Geological Media

Rubén Vidal, Maarten W. Saaltink, and Sebastià Olivella

Aquifer Thermal Energy Storage (ATES) can help to balance energy demand and supply to make better use of infrastructures and resources. ATES consists of a pair or more wells that simultaneously inject or extract thermal energy into aquifers. The aim of ATES is to store the excess of energy during summer and to reuse it during winter, when there is an energy deficit. High-temperature Aquifer Thermal Energy Storage (HT-ATES) provides a good option to store water over 50°C, but it requires facing some problems, such as low efficiency recoveries and the uplift of the surface. Coupled thermo-hydro-mechanical (THM) modelling is a good tool to analyze the viability and cost effectiveness of the HT-ATES systems and understand the interaction of processes, such as heat flux, groundwater flow and ground deformation. We present the 3D THM modelling of a pilot HT-ATES system, inspired by one of the projects of HEATSTORE, which is a GEOTHERMICA ERA-NET co-funded project. The model aims to simulate the injection of hot water of 90°C in a central well and the extraction of water in four auxiliary wells during summer. In winter, the auxiliary wells inject water of 50°C and the central well extract water. The loading lasts longer than the unloading (8 months versus 4 months) and overall more heat is injected than extracted. We found that the system is more efficient in terms of energy recovery, the more years the system is operating. In the aquifer, both thermal loads and hydraulic loads have an important role in terms of displacements. At the surface, the vertical displacements are only a consequence of the hydraulic strains generated by the injection of water in the aquifer.

How to cite: Vidal, R., Saaltink, M. W., and Olivella, S.: A Geothermal Energy Concept based on Heat Storage in Geological Media, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7928, https://doi.org/10.5194/egusphere-egu21-7928, 2021.

EGU21-8561 | vPICO presentations | EMRP1.18

Flow-induced microfracturing of granite in superhot geothermal environments

Ryota Goto, Noriaki Watanabe, Kiyotoshi Sakaguchi, Youqing Chen, Takuya Ishibashi, Eko Pramudyo, Francesco Parisio, Keita Yoshioka, Kengo Nakamura, Takeshi Komai, and Noriyoshi Tsuchiya

Superhot geothermal environments with temperatures of approximately 400-500C at depth of approximately 2-4 km are expected as a new geothermal energy frontier. In order to efficiently exploit the superhot geothermal resources, fracture systems are necessary as flow path of working fluid. Hydraulic fracturing is a promising technique because it is able to create a new fracture system or enhance the permeability of preexisting fracture system. Laboratory-scale hydraulic fracturing experiments of granite have demonstrated the formation of densely distributed network of permeable fractures throughout the entire rock body at or near the supercritical temperature for water. Though the process has been presumed to involve continuous infiltration of low-viscosity water into preexisting microfractures followed by creation and merger of the subsequent fractures, plausible criterion for the fracturing is yet to be clarified. The possibility that the Griffith failure criterion is available to predict the occurrence of fracturing was shown by hydraulic fracturing experiments with acoustic emission measurements of granite at 400C under true triaxial stress. The present study provides a theoretical basis required to establish the procedure for hydraulic fracturing in superhot geothermal environment.

How to cite: Goto, R., Watanabe, N., Sakaguchi, K., Chen, Y., Ishibashi, T., Pramudyo, E., Parisio, F., Yoshioka, K., Nakamura, K., Komai, T., and Tsuchiya, N.: Flow-induced microfracturing of granite in superhot geothermal environments, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8561, https://doi.org/10.5194/egusphere-egu21-8561, 2021.

Wellbore instability is one of the most serious drilling problems increasing well cost in well construction processes. It is widely known that many wellbore instability problems are reported in shale formations where water sensitive clay mineral exist. The problems become further complicated when the shale exhibits variation in strength properties along and across bedding planes. In this study, a coupled thermal-hydro-mechanical-chemical (THMC) model was developed for time-dependent anisotropic wellbore stability analysis considering chemical interactions between swelling shale and drilling fluids, thermal effects, and poro-elastoplastic stress-strain behaviors.

The THMC simulator developed in this work assumes that the shale formation behaves as an ion exchange membrane where swelling depends on chemical potential of drilling fluids invading from the wellbore to the pore spaces. The time-dependent chemical potential changes of water within the shale are evaluated using an analytical diffusion equation resulting in the evolution of swelling strain around the wellbore. On the other hand, the thermal and pressure diffusion equations are evaluated numerically by finite differences. The stress changes associated with thermal, hydro, and chemical effects are coupled to the 3D poroelastoplastic finite element model. The effects of bedding planes are also taken into account in the FEM model through the crack tensor method in which the normal and tangential stiffnesses of the bedding planes have stress dependency. The failure of the formation rock is judged based on the critical plastic strain limit.

The numerical analysis results indicate that the rock strength anisotropy induced by the existence of bedding planes is the most important factor influencing the stability of the wellbore among various THMC process parameters investigated in this work. The numerical results also reveal that an established theory to orient the wellbore in the direction of the minimum principal stress is not always a favorable option when the effect of the anisotropy of in-situ stresses and the distribution angle of bedding planes cancel out each other. Depending on both the distribution angle of bedding plane and ratio of the vertical to the horizontal stress, the trend of minimum mud pressure showed a great variation as predicted by the yield and failure criterion implemented in the model. Furthermore, the analysis results reveal that the distribution and evolution of plastic strains caused by the THMC processes have the time dependency, which can be controlled by the temperature and salinity of the drilling fluids.

The numerical wellbore stability analysis model considering shale swelling and bedding plane effects provides an effective tool for designing optimum well trajectories and determining safe mud weight windows for drilling complex shale formations. The time-dependent margins of safe mud weight window of drilling can be fine-tuned when the interaction among various parameters is fully considered as the THMC processes.

How to cite: Xue, J. and Furui, K.: Coupled Thermal-Hydro-Mechanical-Chemical Modeling for Time-Dependent Anisotropic Wellbore Stability Analysis, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9138, https://doi.org/10.5194/egusphere-egu21-9138, 2021.

Numerous intersected rock fractures constitute the fracture network in enhanced geothermal systems. The complicated convective heat transfer behavior in intersected fractures is critical to the heat recovery in fractured geothermal reservoirs. A series of three-dimensional intersected fracture models are constructed to perform the flow-through heat transfer simulations. The geometries effects of dead-end fractures on the heat transfer are evaluated in terms of intersected angles, apertures, lengths, and the connectivity. The results indicate that annular streamlines appear in the rough dead-end fracture and cause an ellipsoidal distribution of the cold front. Compared to the steady flow in plate dead-end fractures, the fluid flow formed in the rough dead-end fracture enhances the heat transfer. Both the outlet water temperature Tout and heat production Q present the largest when the intersected angle is 90°. A larger intersected angle and longer length extension of the intersected dead-end fracture, raising Tout and Q, are beneficial to the heat production, while increasing the aperture is ineffective. Solely increasing numbers of dead-end fractures poses a little increase on Tout and Q. More significant heat extraction is obtained through connecting these dead-end fractures with the main flow fracture forming the flow network.

How to cite: chen, Y.: Convective heat transfer of water flowing in intersected rock fractures for enhanced geothermal extraction, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9347, https://doi.org/10.5194/egusphere-egu21-9347, 2021.

EGU21-10538 | vPICO presentations | EMRP1.18

Characteristics and mechanism of CO2-based fracturing of granite in conventional and superhot geothermal environments

Eko Pramudyo, Noriaki Watanabe, Ryota Goto, Kiyotoshi Sakaguchi, Kengo Nakamura, and Takeshi Komai

Creation of fractured reservoir for enhanced geothermal system (EGS) in granitic rock at unconventional superhot geothermal environments (>400°C) has been found possible by injection of low viscosity supercritical water (Watanabe et al., 2017, Geoph. Res. Lett.; Watanabe et al., 2019, Sci. Rep.). Accordingly, the complex cloud-fracture network is formed through stimulation of pre-existing microfractures by the low viscosity (<50 μPa∙s) water. Nonetheless, water reactivity to rock forming minerals (Brown, 2000, Proc. 25th wksh. Resv. Engr.) and its high water footprint (Wilkins et al., 2016, Environ. Sci. Technol.) hinder water application. Therefore, CO2 is proposed to replace water, since it less reactive to rock forming minerals (Brown, 2000), and can reduce water footprint (Wilkins et al., 2016). CO2 has also low viscosity at conventional (c.a. 150 – 300°C) and superhot geothermal temperatures (based on Heidaryan et al., 2011, J. Supercrit. Fluid), which motivates to create similarly complex fracture pattern in those geothermal environments. A set of traditional-triaxial stress fracturing experiments was performed on cylindrical granite samples subjected to 200 – 450-°C temperature and varying differential stress to determine characteristics and mechanism of CO2-based fracturing in conventional and superhot geothermal environments. The experiments demonstrated that complex fracture pattern was formed at low pressure in all experimental-stress state and temperature. Breakdown pressure was also found to decrease with increasing differential stress. Hence, it was hypothesized that fracturing mechanism by injection of CO2 is governed by Griffith fracture theory. To unveil the fracturing process in detail, a CO2-based fracturing experiment was conducted on cubical granite sample with a realistic true-triaxial stress state and 300-°C temperature. The later experiment confirmed the above-mentioned hypothesis and showed that the fracturing occurs gradually.

How to cite: Pramudyo, E., Watanabe, N., Goto, R., Sakaguchi, K., Nakamura, K., and Komai, T.: Characteristics and mechanism of CO2-based fracturing of granite in conventional and superhot geothermal environments, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10538, https://doi.org/10.5194/egusphere-egu21-10538, 2021.

EGU21-9616 | vPICO presentations | EMRP1.18

On the consolidation behaviour of double-porosity geomaterials

Jinhyun Choo

Many natural and engineered geomaterials have double-porosity structure where two dominant pore systems coexist. Examples include structured soils where the two pore systems are inter-aggregate pores and intra-aggregate pores, and fissured rocks where the two pore systems are fissures and matrix pores. Although such double-porosity materials are frequently observed in geosciences and geoengineering applications, it remains mostly unclear how fluid flow and solid deformation interact differently in single- and double-porosity materials. The presentation explores this question through numerical simulation of consolidation – a paradigmatic problem in poromechanics – based on a recently developed modelling framework for fluid-infiltrated, inelastic materials with double porosity. Built on a combination of continuum principles of thermodynamics and standard plasticity theory, the framework can capture deformation, flow, and their coupling that occur individually in each pore system. Simulation results using this framework suggest that double-porosity structure gives rise to a two-staged consolidation behaviour, where the second stage appears similar to secondary compression in clays. It is also found that the simulated two-staged behaviour bears a striking semblance to experimentally observed consolidation processes in shales. These findings suggest that double-porosity structure may exert dominant control over the long-term hydro-mechanical behaviour of geomaterials.

How to cite: Choo, J.: On the consolidation behaviour of double-porosity geomaterials, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9616, https://doi.org/10.5194/egusphere-egu21-9616, 2021.

This work presents the advancement of a self-developed Cellular Automata Software for engineering Rockmass fracturing processes (CASRock, http://en.casrock.cn/) in the applications of thermo-hydro-mechanical-chemical (THMC) processes in fractured geological media. It contains a series of previous developed numerical systems, namely EPCA for simulation of heterogeneous rock failure process, VEPCA for visco elastoplastic analysis, D-EPCA for rock dynamic response simulation, THMC-EPCA for coupled THMC processes in geological media and RDCA for simulation of rock cracking process from continuity to discontinuity. In CASRock, the non-isothermal, unsaturated fluid flow, mechanical process and chemical reaction are sequentially coupled by updating all the state variables using cellular automaton technique and finite difference method on spatial and temporal scale, respectively. The Lagrangian method is used to simulate the particle transport. The control equations, coupling scheme and numerical implementation are briefly introduced. Several applications, including  in the background of high level nuclear waste disposal are provided to show the abilities of CASRock in the simulation of coupling processes between physical fields. These applications include, (1) stability analysis of engineering rockmass under mechanical loading, (2) numerical study on coupled TM processes in hard rock pillar, (3) study on coupled THM processes in engineering barrier, (4) simulation of the THMC process in fractured rock.

How to cite: Pan, P.: CASRock—a code to simulate THMC processes in fractured geological media, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12203, https://doi.org/10.5194/egusphere-egu21-12203, 2021.

EGU21-16373 | vPICO presentations | EMRP1.18

Thermodynamic-informed machine learning for pressure-sensitive plasticity model

WaiChing Sun and Nikolas Vlassis

This talk will present a machine learning framework that builds interpretable macroscopic surrogate elasto-plasticity models inferred from sub-scale direction numerical simulations (DNS) or experiments with limited data. To circumvent the lack of interpretability of the classical black-box neural network, we introduce a higher-order supervised machine learning technique that generates components of elasto-plastic models such as elasticity functional, yield function, hardening mechanisms, and plastic flow. The geometrical interpretation in the principal stress space allows us to use convexity and smoothness to ensure thermodynamic consistency. The speed function from the Hamilton-Jacobi equation is deduced from the DNS data to formulate hardening and non-associative plastic flow rules governed by the evolution of the low-dimensional descriptors. By incorporating a non-cooperative game that determines the necessary data to calibrate material models, the machine learning generated model is continuously tested, calibrated, and improved as new data guided by the adversarial agents are generated. A graph convolutional neural network is used to deduce low-dimensional descriptors that encodes the evolutional of particle topology under path-dependent deformation and are used to replace internal variables. The resultant constitutive laws can be used in a finite element solver or incorporated as a loss function for the physical-informed neural network run physical simulations.

How to cite: Sun, W. and Vlassis, N.: Thermodynamic-informed machine learning for pressure-sensitive plasticity model, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16373, https://doi.org/10.5194/egusphere-egu21-16373, 2021.

EGU21-13883 | vPICO presentations | EMRP1.18

Wormhole Growth in Dissolving Limestones: Insights from 4D Tomography

Piotr Szymczak, Max P Cooper, Silvana Magni, Rishabh P Sharma, Tomasz P Blach, Andrzej P Radlinski, Marek Dohnalik, and Alessandro Tengattini

Dissolution of porous media introduces a positive feedback between fluid transport and chemical reactions at mineral surfaces leading to the formation of pronounced wormhole-like channels. While the impact of flow rate and reaction rate on the shapes of the wormholes is now well understood, much less is known about the dynamics of their propagation. In this study we capture the evolution of wormholes and their effects on flow patterns by in-situ X-ray microCT imaging of dissolving limestone cores. 4D tomography allows us in particular to correlate the permeability changes in a dissolving core with the advancement of the tip position of the wormhole. Surprisingly, we find that the relation between the two is highly nonlinear, with extensive periods of relatively fast growth of the wormhole which is nevertheless not reflected in any significant change in the overall permeability.  We hypothesize that this is caused by the presence of highly cemented regions in the core which act as permeability barriers for the flow. The presence of such regions is confirmed by a detailed analysis of the pore geometry based on the tomographic data. The results demonstrate that the analysis of the wormhole dynamics in 4d tomography can be used to probe the internal structure of the rock. 

How to cite: Szymczak, P., Cooper, M. P., Magni, S., Sharma, R. P., Blach, T. P., Radlinski, A. P., Dohnalik, M., and Tengattini, A.: Wormhole Growth in Dissolving Limestones: Insights from 4D Tomography, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13883, https://doi.org/10.5194/egusphere-egu21-13883, 2021.

EGU21-12226 | vPICO presentations | EMRP1.18

Hydro-geochemical processes in the emplacement cavern of a low and intermediate-level waste repository in an indurated clay-rock

Vanessa Montoya, Jaime Garibay-Rodriguez, and Olaf Kolditz

By 2080, Germany will have to store around 600 000 m3 of low and intermediate-level nuclear waste (L-ILW) with negligible heat generation. This kind of waste is largely made up of used parts of nuclear power stations such as pumps, pipelines, filters, etc. placed in various types of waste containers made from either steel, cast iron, or reinforced concrete in different designs and sizes (i.e. cylindrical or box shaped). It is already decided that a total of 303 000 of the 600 000 m3 L-ILW will be disposed in a final storage facility in the former iron ore mine Schacht Konrad which is under construction. However, it is still not clear where the L-ILW emplaced in in the old salt mine Asse (200 000 m3) will be stored in the future. The situation is particularly critical, as the waste have to be retrieved from the instable mine shafts partially flooded with groundwater, causing strong socio-political concerns as radioactive waste could contaminate the water nearby. For this reason, the new search for a nuclear waste repository for high-level waste (HLW), started in 2017, should also consider the possibility to accommodate the waste from Asse. Obviously, this is still subject to critics as this will make finding a final repository more difficult as storing HLW and L-ILW together requires different concepts and designs for each other and, above all, much more space.

In this context, in this contribution we have defined conceptual and numerical models to assess the hydro-chemical evolution of a L-ILW disposal cell in indurated clay rocks, involving the interaction of different components/materials and the expected hydraulic and/or chemical gradients over 100 000 years. The L-ILW disposal cell leverages a multi-barrier concept buried 400 m below the surface. The multi-barrier system is comprised of the waste matrix (i.e. backfilling the waste drums), the disposal container, the mortar backfill in the emplacement tunnel (where the disposal containers are located) and the clay host rock. The dimensions and design of the emplacement tunnel (e.g. 11 × 13 m) and disposal cells represent and consider some aspects taken into account in the designs of some European countries. In addition, tunnel walls reinforced with a shotcrete liner and the Excavation Damaged Zone is considered in the concept. The model is implemented in OpenGeoSys-6, an open-source version-controlled scientific software based on Finite Element Method which is capable of handling fully coupled hydro-chemical models by coupling OpenGeoSys to iPHREEQC. First calculation results, demonstrate that the most important processes affecting the near-field chemical evolution are i) the degradation of the concrete and cementitious grouts with porewater migrating inwards from the host rock and ii) the significant quantities of reactive and non-reactive gases (i.e. hydrogen, carbon dioxide and methane) that are generated as a result of: i) the anaerobic corrosion of metals present in the waste and containers and ii) the degradation of organic compounds by microbial and chemical processes. As a first approximation, some assumptions and simplifications have been considered, probably resulting in a wort case scenario.

How to cite: Montoya, V., Garibay-Rodriguez, J., and Kolditz, O.: Hydro-geochemical processes in the emplacement cavern of a low and intermediate-level waste repository in an indurated clay-rock, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12226, https://doi.org/10.5194/egusphere-egu21-12226, 2021.

EGU21-13313 | vPICO presentations | EMRP1.18

Impact of dilatant strengthening and energy dissipation on fault slip stability

Antoine Jacquey, Manolis Veveakis, and Ruben Juanes

The temporal and spatial distribution of fluid pressure and temperature within a fault core are key determinants of the onset and nature (seismic or aseismic) of fault slip. Laboratory and field observations indicate that transient localization of fluid pressure and temperature often go hand in hand with strain localization upon seismic rupture: as slip occurs on a fault plane, temperature increases due to dissipated energy and fluid pressure decreases due to dilatant strengthening. An accurate description of this thermo-hydro-mechanical multiphysics coupling controlling slip mechanisms is therefore essential to characterize the stability of fault slip.

Here, we present results from analytical and numerical analyses of the stability of fault slip adopting a thermo-hydro-mechanical coupling scheme together with a rate-dependent plasticity formulation. In particular, we focus on the relevance of dilatant strengthening competing with energy dissipation as driving processes for stick-slip events and aseismic slip. We analyze the multiple steady states of the system and their respective stability by means of a numerical continuation technique, and we describe the dynamic evolution of deformation, fluid pressure and temperature fields by considering an associated transient problem.

The results presented here provide insights into the stability criterion for aseismic slip and the dynamic evolution of slip instability as a function of the physical (thermal and hydraulic) properties of the fault material and the boundary conditions (tectonic stresses and off-fault fluid pressure and temperature conditions). We identify two mechanisms for periodic slip, one driven by elastic loading and the other by multiphysics oscillations. We discuss the implications of these results for characterizing the transition from stable aseismic slip to unstable seismic slip in the context of natural and induced seismicity.

How to cite: Jacquey, A., Veveakis, M., and Juanes, R.: Impact of dilatant strengthening and energy dissipation on fault slip stability, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13313, https://doi.org/10.5194/egusphere-egu21-13313, 2021.

EGU21-12893 | vPICO presentations | EMRP1.18

CO2 fracturing using the phase field approach for the brittle fracture

Mostafa Mollaali, Vahid Ziaei-Rad, and Yongxing Shen

To simulate CO2 fracturing under an isothermal condition, we propose a phase field model. We take advantage of the ability of the phase field approach to predict fracture initiation and branching, as well as to avoid tracking the fracture path. We model the CO2 as a compressible fluid by modifying Darcy's law. In particular, we assume that the permeability is correlated to the value of the phase field by the exponential function. The dependence of the CO2 density as a function of the pressure is captured by the Span-Wagner state equation. The computed pressure breakdown values show good agreement with analytical solutions and experimental results.

How to cite: Mollaali, M., Ziaei-Rad, V., and Shen, Y.: CO2 fracturing using the phase field approach for the brittle fracture, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12893, https://doi.org/10.5194/egusphere-egu21-12893, 2021.

EGU21-15088 | vPICO presentations | EMRP1.18

Hydro-Mechanical Modeling of the Year 2000 Hydraulic Stimulation of GPK2 Well, Soultz-sous-Forêts, France

Dariush Javani, Jean Schmittbuhl, and Francois Cornet

 Hydraulic stimulation of pre-existing fractures and faults plays a significant role in improving hydraulic conductivity of the fracture network around injection and production wells in geothermal reservoirs. It is therefore important to characterize the hydro-mechanical behavior of the faults against fluid injection. The Soultz-sous-Forêts site (France) has been an EGS pilot site where several major hydraulic stimulations have been performed and are well documented (https://cdgp.u-strasbg.fr/ and https://tcs.ah-epos.eu/).

Here we use the 3DEC numerical modeling tool (Itasca) to analyze the year 2000 stimulation of GPK2 well where large scale seismic anomalies have been evidenced during the different stages of the stimulation using 4D-P-wave tomography (Calo et al, 2011). The specificity of the approach is to combine two modeling at different scales. First, a small-scale model (100x100x100 m3) is built to analyze the effective mechanical response of a stochastic discrete fracture network (DFN) following the statistical features of the observed fracture network (Massart et al, 2010). Second, a large-scale numerical model of the Soultz-sous-Forêts reservoir (5000x5000x5000 m3) containing the largest faults of the reservoir defined by Sausse et al., 2010, is developed including regional stresses. The objective is to constrain the large-scale mechanical properties of the surrounding matrix around the fault from the small-scale model, in particular, its hydro-mechanical behavior in terms of non-linear elastic response related to the stochastic DFN. As a first step only the largest fault (GPK3-FZ4770) is considered. The first stage of the stimulation is modelled as a constant flow rate of 30 ls-1 of water injected into the fault at the depth of approximately 4.7 km. We explored the effect of the normal and shear stiffness of the fault on the dynamical response of pore pressure along the fracture and the onset of slip. It is found that the increase of the aperture of the fault during the injection shows a slow migration (~2 cm/s) owing to poro-elastic effects. Also generated fluid pressure throughout the fault shows a long period oscillating behavior (~5 hr) sensitive to the magnitude of the fracture normal stiffness.

How to cite: Javani, D., Schmittbuhl, J., and Cornet, F.: Hydro-Mechanical Modeling of the Year 2000 Hydraulic Stimulation of GPK2 Well, Soultz-sous-Forêts, France, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15088, https://doi.org/10.5194/egusphere-egu21-15088, 2021.

Understanding fluid flow in rough fractures is of high importance to large scale geologic processes and to most anthropogenic geo-energy activities. Here, we conducted fluid transport experiments on Carrara marble fractures with a novel customized surface topography. Transmissivity measurements were conducted under normal stresses from 20 to 50 MPa and shear stresses from 0 to 30 MPa. An open-source numerical procedure was developed to simulate normal contact and fluid flow through fractures with complex geometries. It was validated towards experiments. Using it, we isolated the effects of roughness parameters on fracture fluid flow. Under normal loading, we find that i) the transmissivity decreases with normal loading and is strongly dependent on fault surface geometry ii) the standard deviation of heights (hrms) and macroscopic wavelength of the surface asperities control fracture transmissivity. Transmissivity evolution is non-monotonic, with more than 4 orders of magnitude difference for small variations of macroscopic wavelength and roughness. Reversible elastic shear loading has little effect on transmissivity, it can increase or decrease depending on contact geometry and overall stress state on the fault. Irreversible shear displacement (up to 1 mm offset) slightly decreases transmissivity and its variation with irreversible shear displacements can be predicted numerically and geometrically at low normal stress only. Finally, irreversible changes in surface roughness (plasticity and wear) due to shear displacement result in a permanent decrease of transmissivity when decreasing differential stress. Generally, reduction of a carbonate fault’s effective stress increases its transmissivity while inducing small shear displacements doesn’t.

How to cite: Acosta, M., Maye, R., and Violay, M.: Effect of normal and shear loading on the hydraulic transport properties of calcite bearing faults with customized roughness., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16294, https://doi.org/10.5194/egusphere-egu21-16294, 2021.

EGU21-13536 | vPICO presentations | EMRP1.18

Selective mineral dissolution and permeability enhancement of fractured volcanic rocks by chelating agent flooding in geothermal environments 

Luis Salala, Noriaki Watanabe, Kaori Takahashi, Jose Erazo, and Noriyoshi Tsuchiya

Chemical stimulation using high-concentration hydrofluoric and hydrochloric acids has been a classic method to enhance the permeability of a geothermal reservoir. Our research group has recently proposed a new chemical stimulation using a weakly acidic (moderate-reactivity) aqueous solution containing an environmentally friendly chelating agent to create voids, which are sustained under crustal stress, by selective mineral dissolution with preventing precipitation by chelation of metal ions. In the present study, we have conducted chelating agent flooding experiments using an aqueous solution of pH 4 containing readily biodegradable chelating agent (GLDA) on various types of fractured volcanic rocks at 200 oC and effective confining stress of 15 MPa. The experiments have revealed fast permeability enhancement of up to approximately four times, from the initial value, in two hours. Further analyses have revealed phenocrysts of Fe-bearing minerals (ex. Hematite) dissolved faster than the groundmass of the rocks to create the voids. These results show the possibility of the new chemical stimulation.

Keywords: Chemical stimulation, Chelating agents, Geothermal energy, EGS

How to cite: Salala, L., Watanabe, N., Takahashi, K., Erazo, J., and Tsuchiya, N.: Selective mineral dissolution and permeability enhancement of fractured volcanic rocks by chelating agent flooding in geothermal environments , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13536, https://doi.org/10.5194/egusphere-egu21-13536, 2021.

EGU21-16079 | vPICO presentations | EMRP1.18

Dynamic permeability evolution during fluid injection and production in ultra-deep geothermal reservoirs

Samuel Scott, Alina Yapparova, Philipp Weis, and Matthew Houde

A geothermal well drilled into a reservoir at temperatures exceeding the critical point of pure water (>374 °C) could generate substantially greater quantities of energy than conventional geothermal wells. Although these temperatures can be found at shallow depths (<2-3 km) in high-grade geothermal resources located in volcanically active areas, similar temperatures are only found at depths >10 km beneath vast areas of continental crust with lower heat fluxes. Permeability decreases markedly with increasing depth below 2-3 km, so exploiting the tremendous heat resources of high temperature rock at such great depths will require permeability stimulation by the injection of high-pressure fluids. In this study, we use the CSMP++ platform to perform 3D simulations of transient permeability evolution around a geothermal doublet drilled to depths between 10-16 km. The simulations incorporate a well model initially devised by Peaceman (1978) to calculate well pressures and rates of fluid production/injection. The dynamic permeability model is based on Weis et al. (2012), initially developed to simulate the evolution of ore-forming magmatic-hydrothermal systems, and links a failure criterion for critically-stressed crust with depth-dependent permeability profiles characteristic for tectonically active crust as well as pressure- and temperature-dependent relationships describing hydraulic fracturing and the transition from brittle to ductile rock behavior. We investigate the permeability changes in response to high-pressure fluid injection in brittle and ductile rock, the timescales over which the zone of permeability stimulation migrates towards production wells, and dynamic permeability evolution in response to changes in injection and production parameters. These simulations aim to mitigate resource risks that could limit the ability to extract heat from geothermal resources in ductile upper crust and to help anticipate the conditions that would be required to make the exploitation of ultra-deep supercritical geothermal resources a reality. 

References

Peaceman, D. W. (1978) Interpretation of Well-Block Pressures in Numerical Reservoir Simulation. SPE 6893, 183–194.

Weis, P., Driesner, T., & Heinrich, C. A. (2012). Porphyry-copper ore shells form at stable pressure-temperature fronts within dynamic fluid plumes. Science, 338(6114), 1613–1616.

How to cite: Scott, S., Yapparova, A., Weis, P., and Houde, M.: Dynamic permeability evolution during fluid injection and production in ultra-deep geothermal reservoirs, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16079, https://doi.org/10.5194/egusphere-egu21-16079, 2021.

EMRP2.10 – Space weather through ground and space magnetic data

EGU21-812 | vPICO presentations | EMRP2.10 | Highlight

Dynamics of ionospheric and telluric currents during large events of geomagnetically induced currents

Mirjam Kellinsalmi, Ari Viljanen, Liisa Juusola, and Sebastian Käki

Geomagnetic variations are mainly produced by external currents in the ionosphere and magnetosphere, and secondarily by induced (internal/telluric) currents in the conducting Earth. Large geomagnetically induced currents (GIC) are associated with large time derivatives of the horizontal magnetic field. Recent results show that the time derivative is typically dominated by the contribution from the telluric currents. Our study aims to find measures to quantify the behaviour of external and internal currents and their time derivatives during large GIC events. Results of this study show that strong external currents have quite narrow directional distributions. Angular variation is larger for internal currents, and especially for their time derivatives. For external currents angular variation is larger at higher latitudes. Similar behaviour is not seen with internal currents.

How to cite: Kellinsalmi, M., Viljanen, A., Juusola, L., and Käki, S.: Dynamics of ionospheric and telluric currents during large events of geomagnetically induced currents, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-812, https://doi.org/10.5194/egusphere-egu21-812, 2021.

EGU21-6934 | vPICO presentations | EMRP2.10 | Highlight

Sources of hazardous geoelectric fields in the United States during magnetic storms

Xueling Shi, Michael Hartinger, Joseph Baker, Paul Bedrosian, Benjamin Murphy, Anna Kelbert, Joshua Rigler, and Michael Ruohoniemi

Geomagnetic perturbations related to various phenomena in the near-Earth space environment can induce electric fields within the electrically conducting Earth. The geoelectric field is an important link between magnetospheric/ionospheric phenomena and geomagnetically induced currents in grounded electricity transmission networks. In evaluations of contiguous United Sates hazards, most previous studies have been focused on either 1-minute resolution geoelectric field measurements or geoelectric field time series derived from convoluting 1-minute geomagnetic field data with surface impedance tensors. To investigate sources of hazardous geoelectric fields during magnetic storms, including geoelectric fields induced by ultra-low frequency (ULF: 1 mHz to 1 Hz) waves, we use directly measured 1-second geoelectric field data from magnetotelluric survey stations that are distributed across the contiguous United States. Temporally-localized perturbations in measured geoelectric fields with a prominence of at least 0.5 V/km are detected during magnetic storms with a Dst minimum of at least -100 nT from 2008 to 2019. Most of these perturbations cannot have been resolved with 1-minute data since they correspond to phenomena that vary on smaller timescales and higher frequencies. The sources of geomagnetic perturbations inducing these extreme geoelectric fields can be categorized as interplanetary shocks, substorms, and ULF waves. We compare the geoelectric fields associated with the three sources and characterize their features. Extreme geoelectric fields related to these sources can have amplitudes of 1-2 V/km, comparable to the thresholds commonly used to identify hazardous events.

How to cite: Shi, X., Hartinger, M., Baker, J., Bedrosian, P., Murphy, B., Kelbert, A., Rigler, J., and Ruohoniemi, M.: Sources of hazardous geoelectric fields in the United States during magnetic storms, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6934, https://doi.org/10.5194/egusphere-egu21-6934, 2021.

EGU21-8410 | vPICO presentations | EMRP2.10

Investigation of the possibility of GIC development in Greece during the strongest magnetic storms of solar cycle 24 

Adamantia Zoe Boutsi, Georgios Balasis, Ioannis A. Daglis, Kanaris Tsinganos, and Omiros Giannakis

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. Here, we exploit data from the HellENIc GeoMagnetic Array (ENIGMA), which is located in Greece, complemented by magnetic observatories in Italy, to calculate corresponding values of the GIC index, i.e., a proxy of the geoelectric field calculated entirely from geomagnetic field variations. We perform our analysis for the most intense magnetic storms (Dst<-150 nT) of solar cycle 24. Our results show a good correlation between the storm sudden commencement (SSC) and an increase of the GIC index value. These investigations indicate that despite the elevated amplitude of the GIC index the associated risk remains at low level for the power networks in Greece and Italy during the considered storm events.

How to cite: Boutsi, A. Z., Balasis, G., Daglis, I. A., Tsinganos, K., and Giannakis, O.: Investigation of the possibility of GIC development in Greece during the strongest magnetic storms of solar cycle 24 , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8410, https://doi.org/10.5194/egusphere-egu21-8410, 2021.

EGU21-1083 | vPICO presentations | EMRP2.10

Spatio-temporal development of large scale auroral electrojet currents relative to substorm onsets

Sebastian Käki, Ari Viljanen, Liisa Juusola, and Kirsti Kauristie

The electric currents flowing in the ionosphere change rapidly and a large amount of energy is dissipated in the auroral ionosphere during auroral substorms. An important part of the auroral current system are the auroral electrojets whose profiles can be estimated from magnetic field measurements from low Earth orbit satellites. We have combined electrojet data derived from the Swarm satellite mission of ESA with the substorm database derived from the SuperMAG ground network data. We organize the electrojet data in relation to the location of the onset and obtain statistics for the development of the integrated current and latitudinal location for the auroral electrojets relative to the onset. Especially we show that just after the onset the latitudinal location of the maximum of the westward electrojet determined from Swarm satellite data is mostly located close to the onset latitude in the local time sector of the onset regardless of where the onset happens.

How to cite: Käki, S., Viljanen, A., Juusola, L., and Kauristie, K.: Spatio-temporal development of large scale auroral electrojet currents relative to substorm onsets, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1083, https://doi.org/10.5194/egusphere-egu21-1083, 2021.

EGU21-1263 | vPICO presentations | EMRP2.10

Introducing the “VALOR” analysis framework for auroral field-aligned current sheets using observations from Swarm Alpha and Charlie

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

The investigation of auroral field-aligned current (FAC) sheets is crucial in the context of space weather research since they serve as main transmitters of energy and momentum across geospace domains. Different magnetosphere-ionosphere coupling modes are reflected by the FACs’ multiscale nature with spatial scales, i.e., latitudinal extensions, ranging from below 1 km to hundreds of kilometers. The multiscale property can be addressed conveniently using ESA’s three-spacecraft mission Swarm. According to common practice a linear correlation analysis is performed on lagged and band-pass filtered scalar FAC density estimates from two nearby spacecraft.

We introduce the framework VALOR (Vectorial Association of Linearly Oriented Residua) which generalizes the common approach in two ways. First, VALOR utilizes the full magnetic field vector primarily observed at both spacecraft without filtering. Second, VALOR allows to test statistical association measures other than linear correlation in dependence of both time and along-track spacecraft lag. The method is further refined by considering the current sheet’s polarization, i.e., the directional preference of the associated magnetic field perturbation, which additionally constrains the sheet’s orientation.

Here, we apply VALOR to 1 Hz magnetic field observations from Swarm Alpha and Charlie and base the association measure on a vectorial version of the mean squared deviation. By means of a sample auroral oval crossing event we demonstrate that the incorporation of vectorial and polarization information helps to focus the association measure in the time-lag parameter plane leading to a smaller FAC spatial scale estimate. This result seems to hold in a statistical context including over 9000 quasi-perpendicular auroral oval crossings from 2014 to 2020. The fact that the VALOR derived FAC locations reflect the known ellipsoidal shapes of the auroral ovals speaks to the overall plausibility of the method as well as the independently supported finding that large-scale FACs (>300 km) dominate the dawn and dusk sectors while smaller scale FACs gain importance at noon and midnight. Among the various opportunities for future work are an application to 50 Hz high-resolution Swarm data as well as the investigation of the solar controlling parameters.

How to cite: Pick, L., Vogt, J., Blagau, A., and Stachlys, N.: Introducing the “VALOR” analysis framework for auroral field-aligned current sheets using observations from Swarm Alpha and Charlie, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1263, https://doi.org/10.5194/egusphere-egu21-1263, 2021.

EGU21-12667 | vPICO presentations | EMRP2.10

On Magnetic Helicity and Field-Aligned Currents in the Polar Ionosphere

Paola De Michelis, Giuseppe Consolini, Tommaso Alberti, Vincenzo Carbone, Roberta Tozzi, Igino Coco, and Fabio Giannattasio

Magnetic helicity, which is a measure of twist and linkage of magnetic field lines, is a useful quantity to investigate some processes occurring in space plasmas. In particular, there is a strong link between magnetic helicity, magnetic flux structures, turbulence and dissipation. We investigate the connection between the reduced magnetic helicity and the structure of field-aligned currents in the high-latitude ionosphere using high resolution (50 Hz) magnetic data collected on board the ESA Swarm constellation. We show the existence of a clear link between the multiscale coarse-grained structure of reduced magnetic helicity and the field-aligned currents. This finding strongly supports the idea that turbulence processes might be at the origin of the observed small-scale current structures. A discussion of the relevance of our results in the framework of the filamentary nature of the field-aligned current is also presented.

This work is supported by Italian PNRA under contract PNRA18_00289-A “Space weather in Polar Ionosphere: the Role of Turbulence ".

How to cite: De Michelis, P., Consolini, G., Alberti, T., Carbone, V., Tozzi, R., Coco, I., and Giannattasio, F.: On Magnetic Helicity and Field-Aligned Currents in the Polar Ionosphere, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12667, https://doi.org/10.5194/egusphere-egu21-12667, 2021.

EGU21-2741 | vPICO presentations | EMRP2.10

High-latitude crochet (Solar flare effect) as a trigger of pseud-substorm onset

Masatoshi Yamauchi, Magnar Johnsen, Shin-Ichi Othani, and Dmitry Sormakov

Solar flares are known to enhance the ionospheric electron density and thus influence the electric currents in the D- and E-region.  The geomagnetic disturbance caused by this current system is called a "crochet" or "SFE (solar flare effect)".  Crochets are observed at dayside low-latitudes with a peak near the subsolar region ("subsolar crochet"), in the nightside high-latitude auroral region with a peak where the geomagnetic disturbance pre-exists during solar illumination ("auroral crochet"), and in the cusp ("cusp crochet").  In addition, we recently found a new type of crochet on the dayside ionospheric current at high latitudes (European sector 70-75 geographic latitude/67-72 geomagnetic latitude) independent from the other crochets.  The new crochet is much more intense and longer in duration than the subsolar crochet and is detected even in AU index for about half the >X2 flares despite the unfavorable latitudinal coverage of the AE stations (~65 geomagnetic latitude) to detect this new crochet (Yamauchi et al., 2020).  

The signature is sometime s seen in AL, causing the crochet signature convoluting with substorms.  From a theoretical viewpoint, X-flares that enhances the ionospheric conductivity may influence the substorm activity, like the auroral crochet.  To understand the substorm-crochet relation in the dayside, we examined SuperMAG data for cases when the onset of the substorm-like AL (SML) behavior coincides with the crochet.  We commonly found a large counter-clockwise ∆B vortex centered at 13-15 LT, causing an AU peak during late afternoon and an AL peak near noon at higher latitudes than the high-latitude crochet.  In addition, we could recognize a clockwise ∆B vortex in the prenoon sector, causing another poleward ∆B, but this signature is not as clear as the afternoon vortex.  With such strong vortex features, it becomes similar to substorms except for its local time.  In some cases, the vortex expends to the nightside sector, where and when nightside onset starts, suggesting triggering of onset.  Thus, the crochet may behave like pseudo-onset at different latitude than midnight substorms, and may even trigger substorm onset.

How to cite: Yamauchi, M., Johnsen, M., Othani, S.-I., and Sormakov, D.: High-latitude crochet (Solar flare effect) as a trigger of pseud-substorm onset, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2741, https://doi.org/10.5194/egusphere-egu21-2741, 2021.

EGU21-15692 | vPICO presentations | EMRP2.10

K indices and K-derived magnetic activity indices:  context’s reminder

Aude Chambodut

The K index was devised by Bartels et al. (1939) to provide an objective monitoring of irregular geomagnetic activity at subauroral latitudes. K indices are based upon geomagnetic disturbances, measured in horizontal geomagnetic components at magnetic observatories, after « eliminating » the regular daily variation. An individual K index is an integer in the range 0 to 9 corresponding to a class that contains the largest range of geomagnetic disturbances (in either of the two horizontal components) during a 3-hour UT interval. Limits of range vary from one observatory to another since they depend on the corrected geomagnetic latitude of the observatory.

A great number of Space Weather applications rely on K-derived magnetic activity indices at subauroral latitudes. These historical indices; endorsed by IAGA such as Kp, aa and am; represent unprecedented homogeneous time series, up to more than 150 years, highly valuable for all studies related to long-term geomagnetic activity.

However, one has to keep in mind that local K indices and subauroral related ones (K-derived) were developed during other time, under specific societal and technological conditions.

We recall the local K indices derivation processes and characteristics to enlight possible nowadays drawbacks and their simple mitigations.

How to cite: Chambodut, A.: K indices and K-derived magnetic activity indices:  context’s reminder, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15692, https://doi.org/10.5194/egusphere-egu21-15692, 2021.

EGU21-2169 | vPICO presentations | EMRP2.10

Local diurnal variations of the geomagnetic field during magnetically quiet conditions

Veronika Haberle, Aurélie Marchaudon, Pierre-Louis Blelly, and Aude Chambodut

The Earth’s magnetic field as measured from ground-based magnetometers is composed of a variety of fields generated by diverse sources, spanning a broad amplitude and frequency spectrum. Long-term variable sources induce smooth changes, whereas short-term variable sources are able to induce rapid spikes in the geomagnetic field. An important aspect of Space Weather research is to understand the contribution and impact of each of these sources. In particular, knowing the amplitude and frequency of steady-like sources, like diurnal variations, enables us to determine the impact of sudden and hazardous events such as solar storms. The basic approach to this challenge is to identify the quiet magnetic field information within the recorded time-varying signal.
In this work, we examine the variance of the magnetically quiet diurnal and semi-diurnal components of the geomagnetic field, as recorded by ground-based magnetic observatories of the INTERMAGNET network. These variations are extracted by applying appropriately designed digital filters on the geomagnetic field time series. The residual signal is analysed in terms of local time and seasonal variations for selected locations under quiet magnetic conditions. This approach allows us to evaluate the applicability of the introduced filtering method. The obtained results improve our understanding of the driving sources of quiet currents such as the Sq current and the variations of their distributions with respect to regular solar irradiance variations. They will also contribute to a better extraction and description of the remaining/residual signal related to solar wind stimuli (e.g. ICMEs, CIRs) causing magnetic storms.

How to cite: Haberle, V., Marchaudon, A., Blelly, P.-L., and Chambodut, A.: Local diurnal variations of the geomagnetic field during magnetically quiet conditions, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2169, https://doi.org/10.5194/egusphere-egu21-2169, 2021.

EGU21-7756 | vPICO presentations | EMRP2.10

Seasonal behavior of H component during solar cycle 24

Yasmina Bouderba, Ener Aganou, and Abdenaceur Lemgharbi

In this work we will show the behavior of the horizontal component H of the Earth Magnetic Field (EMF) along the seasons during the period of solar cycle 24 lasting from 2009 to 2019. By means of  continuous measurements of geomagnetic components (X, Y) of the EMF, we compute the horizontal component H at the Earth’s surface. The data are recorded with a time resolution of one minute at Tamanrasset observatory in Algeria at the geographical coordinates of 22.79° North and 5.53° East. These data are available from the INTERMAGNET network. We find that the variation in amplitude of the hourly average of H component at low latitude changes from a season to another and it is greater at the maximum solar activity than at the minimum solar activity.

Keywords: Solar cycle 24, Season, Horizontal component H. 

How to cite: Bouderba, Y., Aganou, E., and Lemgharbi, A.: Seasonal behavior of H component during solar cycle 24, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7756, https://doi.org/10.5194/egusphere-egu21-7756, 2021.

EGU21-12047 | vPICO presentations | EMRP2.10

Algorithm for interpolation of magnetic field variations

Dmitrii Vishniakov, Ivan Lygin, and David Arutyunyan

To solve many geological and geophysical problems, it is very important to study variations of the Earth's magnetic field. The observed variations are usually obtained from data from observatories or temporary variation stations. However, while performing various regional magnetic prospecting works, the network of observatories is not complete enough to account for the variation field correctly.

In this regard, it is becoming necessary to interpolate the data on variations from the points of irregular network. At the same time, obtaining the optimal algorithm is an ambiguous task, its solution requires taking a whole list of factors into account that determine regularity of distribution of physical parameters over the area.

This project represents an interpolation algorithm using method of complex weighting coefficients. The technique was tested on data from the Intermagnet observatories for central Europe, and the obtained accuracy was ± 2 nT. Comparative analysis with known interpolation methods by interpolation methods was carried out.

How to cite: Vishniakov, D., Lygin, I., and Arutyunyan, D.: Algorithm for interpolation of magnetic field variations, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12047, https://doi.org/10.5194/egusphere-egu21-12047, 2021.

EGU21-1436 | vPICO presentations | EMRP2.10

Identification of the ground signatures of magnetospheric current systems as a function of latitude during intense magnetic storms

Vasilis Pitsis, Georgios Balasis, Ioannis Daglis, and Dimitris Vassiliadis

We show that changes in the magnetospheric ring current and auroral currents during the magnetic storms of March 2015 and June 2015, are recorded in several specific ways by ground magnetometers. The ring current changes are detected in geomagnetic field measurements of ground stations at magnetic mid-latitudes from -50 to +50 degrees. The auroral currents changes are detected at high magnetic latitudes from 50 to about 73 degrees. Finally, for stations between 73 and about 85 degrees the measurements of the ground magnetometers seem to be directly correlated with the convection electric field VBSouth of the solar wind. Using the correlations among magnetic fields measured at stations ordered by latitude, a correlation diagram is obtained where the maximum correlation values for fields determined by the ring current form a distinct block. High-latitude magnetic fields from stations at higher latitudes, which are mainly determined by auroral currents, form a different block in the same diagram. This is in agreement with our earlier work using wavelet transforms on ground magnetic-field time series, where mid-latitude fields stations that are influenced mainly by the ring current, give a critical exponent greater than 2 while higher-latitude fields show a more complex dependence with two exponents. The maximum correlation values for mid-latitude fields correlated with the SYM-H index vary from 0.8 to 0.9, and, thus, we infer that those geomagnetic disturbances are mainly due to the ring current. The maximum correlations between the same fields and the solar wind VBSouth vary from 0.5 to 0.7. Fields at magnetic latitudes between 50 and 73 degrees exhibit greater correlation values for the AL index rather than the SYM-H index. This is expected since in the auroral zone, the convection- and substorm-associated auroral electrojets contribute significantly to the deviation of the geomagnetic field from its quiet-time value. In this case, maximum correlations vary between 0.6 and 0.7 for auroral latitude stations when compared with AL, as opposed to 0.4–0.5 when compared with SYM-H. Our results show how different measures of ground geomagnetic variations reflect the time evolution of several magnetospheric current systems and of the solar wind – magnetosphere coupling.

How to cite: Pitsis, V., Balasis, G., Daglis, I., and Vassiliadis, D.: Identification of the ground signatures of magnetospheric current systems as a function of latitude during intense magnetic storms, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1436, https://doi.org/10.5194/egusphere-egu21-1436, 2021.

EGU21-13239 | vPICO presentations | EMRP2.10

Altitude dependence of geomagnetic external fields using Swarm and ground observatories data

Diana Saturnino, M. Alexandra Pais, João Domingos, and Fernando J. G. Pinheiro

The separation of different sources  in the geomagnetic field signal measured at satellite altitude is still an open issue. One approach to tackle this problem may be non-parametric statistical methods, such as the Principal Component Analysis (PCA). Here, PCA is applied to Virtual Observatories (VO) geomagnetic time series, computed from an enlarged Swarm dataset  covering all local times and geomagnetic activity levels, from January 2014 to December 2019. For each 30-days time window, an Equivalent Source Dipole mathematical model is fitted to the data to reduce a cloud of satellite data points inside a cylinder to one single 'observation' at its axis and 500 km altitude. A VO mesh is constructed with 3394 VOs, with 2 degrees radius each and 3.5 degrees apart in latitude. We study the distribution of satellite data among the cylinders to test if any spatial or temporal sampling asymmetries can be present in the VO dataset and propagate to the PCA results. We also compare observed time series at ground level with VO time series at satellite altitude for the same latitude and longitude. After subtracting a main field model to both series, comparison of the residuals can give further insight on the dependence of external fields with altitude, with a 30-day time resolution.

How to cite: Saturnino, D., Pais, M. A., Domingos, J., and Pinheiro, F. J. G.: Altitude dependence of geomagnetic external fields using Swarm and ground observatories data, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13239, https://doi.org/10.5194/egusphere-egu21-13239, 2021.

EGU21-15343 | vPICO presentations | EMRP2.10

New Earth-space infrastructures enable full-scale monitoring capability

Eija Tanskanen, Tero Raita, Joni Tammi, Jouni Pulliainen, Hannu Koivula, Thomas Ulich, Reko Hynönen, Ilmo Kukkonen, and Annakaisa Korja

The near-Earth environment is continuously changing by disturbances from external and internal sources. A combined research ecosystem is needed to be able to monitor short- and long-term changes and mitigate their societal effects. Observatories and large-scale infrastructures are the best way to guarantee continuous 24/7 observations and full-scale monitoring capability. Sodankylä Geophysical Observatory takes care of continuous geoenvironmental monitoring in Finland and together with national infrastructures such as FIN-EPOS and E2S enable extending and expanding the monitoring capability. European Plate Observing System of Finland (FIN-EPOS) and flexible instrument network of FIN-EPOS (FLEX-EPOS) will create a national pool of instruments including geophysical instruments targeted for solving topical questions of solid Earth physics. Scientific and new hardware building by FLEX-EPOS is essential in order to identify and reduce the impact of seismic, magnetic and geodetic hazards and understand the underlying processes.

 

New national infrastructure Earth-Space Research Ecosystem (E2S) will combine measurements from atmosphere to near-Earth and distant space. This combined infrastructure will enable resolving how the Arctic environment change over the seasons, years, decades and centuries. We target our joint efforts to improve the situational awareness in the near-Earth and space environments, and in the Arctic for enhancing safety on ground and in space. This presentation will give details on the large-scale Earth-space infrastructures and research ecosystems and will give examples on how they can improve the safety of society.

How to cite: Tanskanen, E., Raita, T., Tammi, J., Pulliainen, J., Koivula, H., Ulich, T., Hynönen, R., Kukkonen, I., and Korja, A.: New Earth-space infrastructures enable full-scale monitoring capability, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15343, https://doi.org/10.5194/egusphere-egu21-15343, 2021.

EGU21-10693 | vPICO presentations | EMRP2.10

XGBoost Algorithm for Estimating Equatorial Plasmaspheric Mass Density Using ULF Wave Measurements

Raffaello Foldes, Alfredo Del Corpo, Ermanno Pietropaolo, and Massimo Vellante

Monitoring the plasmasphere is an important task to achieve in the Space Weather context. A consolidated technique consists of remotely inferring the equatorial plasma mass density in the inner magnetosphere using Field Line Resonance (FLR) frequency estimates derived from Ultra-Low Frequency (ULF) measurements. FLR frequencies can be obtained via cross-phase analysis of magnetic signals recorded from pairs of latitude separated stations. In the last years, machine learning (ML) has been successfully applied in Space Weather, but this is the first attempt to estimate FLR frequencies with these techniques. EXtreme Gradient Boosting (XGB) is a recent ensemble-based algorithm that is resulted in being highly efficient in regression/classification competitions and many real-world applications. Here we employ XGB for identifying FLR frequencies by using measurements of the European quasi-Meridional Magnetometer Array (EMMA). Our algorithm takes as input the 30-min cross-phase spectra of magnetic signals and returns the FLR frequency as output; we evaluated the algorithm performance on four different station pairs from L=2.4 to L=5.5. Results show that XGB algorithm can be a robust and accurate method to achieve this goal. Its performances slightly decrease with increasing latitude and tend to deteriorate during nighttime. However, at high latitudes, the error increases during highly disturbed geomagnetic conditions such as the storm's main phase. Finally, we compare the equatorial plasmaspheric mass density obtained by XGB estimates with the density profiles by Del Corpo et al. (2019) for a case study, the geomagnetic storm of the 1st June 2013. Our approach may represent a prominent space weather tool included in an automatic monitoring system of the plasmasphere. This work represents only a preliminary step in this direction; applying this technique on a more extensive data set and on more pairs of stations is straightforward and necessary to create more robust and accurate models.

How to cite: Foldes, R., Del Corpo, A., Pietropaolo, E., and Vellante, M.: XGBoost Algorithm for Estimating Equatorial Plasmaspheric Mass Density Using ULF Wave Measurements, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10693, https://doi.org/10.5194/egusphere-egu21-10693, 2021.

EGU21-12740 | vPICO presentations | EMRP2.10

Observation and modelling of whistlers in the ELF as observed by Swarm satellites during regular ASM burst sessions

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

The magnetic component of electromagnetic signals in the Extremely Low Frequencies (ELF) has been rarely observed from space. The Swarm satellites have the capability of observing part of this spectral band during burst sessions of the Absolute Scalar Magnetometer (ASM), when the sampling frequency of the instrument is raised to 250 Hz. Burst sessions of one week duration have been acquired regularly since 2019. Swarm satellites drift slowly in local time, therefore it has been possible to progressively acquire burst data to cover all hours at all latitudes. This is a unique opportunity at Low Earth Orbits (LEO) in recent years.

This study focuses on whistlers excited by lightning strikes generated by strong storm systems in the troposphere. The ELF component of the lightning signal propagates in the neutral atmosphere at very long distances. We used data from the ground stations of the World ELF Radiolocation Array (WERA) in order to estimate lightning locations and intensity for remarkable events. Part of the lightning signal penetrates into the ionosphere, where the ionospheric plasma produces its dispersion, depending on the spatial distribution of the plasma and the direction of the magnetic field.

We selected events to simulate their propagation through the ionosphere, using ionosonde data, IRI Real-Time Assimilative Mapping (IRTAM) and International Reference Ionosphere (IRI) model as backgrounds, along with the latest version of the International Geomagnetic Reference Field (IGRF). This technique allows to use these signals to sound the ionosphere and validate ionospheric models.

A database of whistler occurrences and parameters has been constructed and a new Swarm L2 product has been defined to make this data accessible to the scientific community.

How to cite: Coïsson, P., Truhlik, V., Mlynarczyk, J., Hulot, G., Brocco, L., Bonnot, O., Vigneron, P., Burešová, D., Chum, J., Rzonca, P., and Kulak, A.: Observation and modelling of whistlers in the ELF as observed by Swarm satellites during regular ASM burst sessions, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12740, https://doi.org/10.5194/egusphere-egu21-12740, 2021.

EGU21-1781 | vPICO presentations | EMRP2.10

Tracking the split: a non-linear iterative approach to the monitoring of recent SAA evolution

Alexandra Parmentier, Antonio Cicone, Mirko Piersanti, Roberta Tozzi, Matteo Martucci, Alessandro Sotgiu, and Paola De Michelis

Still today vaguely defined, the South Atlantic Anomaly (SAA) is the vast
geographic region where the Earth’s magnetic field is weakest relative to an
ideal Earth-centered dipole field, and the inner radiation belt comes closest
to the planet. Nonetheless it represents a major concern to the space science
community, since the local reduced magnetic intensity often results in satellite
outages and radiation hazard to humans, especially in geomagnetically disturbed
periods.
Since 1958, relentless investigation of the various morphological and dynamic
features of the SAA has been taking place, robustly relying on field, plasma and
particle measurements from Low-Earth-Orbit (LEO) satellites since the late
1970s.
New readings provided by magnetometers operating at LEO altitudes show that,
within the past decade, an apparent second center of minimum field intensity
has begun to be clearly resolved southwest of Africa, suggesting a possible rapid
splitting of the SAA into two cells. In addition to magnetic determinations, the
tracking of fluxes of sub-MeV electrons that are lost to the atmosphere when
drifting into the SAA due to its increased bounce loss cone, offers a specular
view of the same phenomenon. This multi-messenger approach from different
platforms is best suited to catch fine details of the splitting.
Directly stemming from the data-adaptive Empirical Mode Decomposition (EMD)
developed at NASA in the 1990s for the analysis of non-stationary signals, the
Fast Iterative Filtering (FIF) class of signal mode decompositions is recently
taking center stage due to enhanced rigorous formalization in terms of con-
vergence and stability. Multidimensional and Multivariate FIF (MMFIF) is a
brand-new extension that handles multidimensional and multichannel datasets.
The application of MMFIF techniques to magnetic-field and particle data from
an ensemble of LEO satellites has allowed us to best characterize the dynamic
evolution of the SAA lobes in the 2010s, and compare it to analogous data in
the literature from the previous decades.

How to cite: Parmentier, A., Cicone, A., Piersanti, M., Tozzi, R., Martucci, M., Sotgiu, A., and De Michelis, P.: Tracking the split: a non-linear iterative approach to the monitoring of recent SAA evolution, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1781, https://doi.org/10.5194/egusphere-egu21-1781, 2021.

EMRP2.11 – Electromagnetic induction in Geophysics: Methodology, Data, Modelling and Inversion

EGU21-14800 | vPICO presentations | EMRP2.11

Multiple-channel Singular Spectrum Analysis-based noise reduction for MT data observed in Boso Peninsula, Japan

Shu Kaneko, Katsumi Hattori, Toru Mogi, and Chie Yoshino

Off the coast of the Boso Peninsula, there is a triple junction of the Pacific Plate, the Philippine Sea Plate, and the North American Plate and the Boso Peninsula is one of the seismically active areas in Japan. There are also epicenter areas such as the 1703 Genroku Kanto Earthquake (M8.2), the 1923 Taisho Kanto Earthquake (M7.9), and the Boso Slow Slip which occurs every 6 years, which are geologically interesting places. To estimate the subsurface resistivity structure of the whole Boso area, Magnetotelluric (MT) survey with 41 sites (inter-sites distance of 7 km) has been conducted in 2014-2016, using U43 (12 sites, 1 Hz sampling ; Tierra Technica) and MTU-5, 5A, net (41 sites, 15, 150, and 2400 Hz sampling; Phoenix Geophysics). However, the Boso area is greatly affected by leak current from DC-driven trains, factories, and power lines, so the observed data are contaminated by artificial noises. When we tried to apply the conventional noise reduction method (e.g., remote reference (Gamble et al., 1979) and BIRRP (Chave and Thomson, 2004)) in frequency domain, the obtained MT sounding curve was not ideal. In particular, the phase between the periods of 20 and 400 sec was close to 0 degrees. It suggests that the method used is insufficient to reduce the near-field effect for the Boso data. Thus, we developed a new noise reduction method using MSSA (Multi-channel Singular Spectrum Analysis) as a pre-processing method in time domain.

The procedure is as follows;

(1) Decompose 6 component data (Hx, Hy, Ex, Ey, Hxr and Hyr: H and E means magnetic and electric field, respectively, x and y indicates NS and EW component, and r denotes the reference field observed at a quiet station) using MSSA into 6×M principal components (PCs).  Here, M shows the window length of MSSA.

(2) Check contribution and periods of each PC and eliminate the PCs which are corresponding to the longer periods of variation. That is “detrend” of the original data.

(3) Apply the second MSSA to the detrended time series data to separate signals and noises shorter than 400 sec.

(4) Calculating correlation coefficients between H and Hr and between E and Hr for each PC and select the PCs with higher correlation to reconstruct time series data to make MT analysis.

Then, we perform MT analysis by BIRRP to estimate apparent resistivity,

As a result, the coherences of H-Hr, and E-Hr were improved and the MT sounding curve became smoother than those results by the conventional noise reduction methods. This indicated that the effectiveness of the proposed noise reduction. However, further investigation in different periods and sites will be required.

How to cite: Kaneko, S., Hattori, K., Mogi, T., and Yoshino, C.: Multiple-channel Singular Spectrum Analysis-based noise reduction for MT data observed in Boso Peninsula, Japan, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14800, https://doi.org/10.5194/egusphere-egu21-14800, 2021.

EGU21-12448 | vPICO presentations | EMRP2.11

FFproc - an improved multivariate robust statistical data processing software for the estimation of MT transfer functions

César Daniel Castro, Philip Hering, Colin Hogg, and Andreas Junge

The presence of cultural electromagnetic (EM) noise hampers the estimation of Magnetotelluric (MT) transfer functions and therefore distorts the imaging of subsurface resistivity structures. Many advanced processing approaches are available to improve the quality of EM signals. However, very few of them are capable to identify and remove time segments contaminated by correlated (anthropogenic) electromagnetic noise.

We present FFproc, an MT multi-site/multi-device robust remote reference processing code, that is implemented as a MATLAB© Graphical User Interface (GUI). The presented processing software builds on the eigenvalue decomposition method by Egbert (1997) and includes a robust estimation of the spectral density and the noise covariance matrices. The multivariate approach accounts for the statistical dependence between regression residuals related to the data channels at a single site (Advanced Noise Model, ANM) and therefore significantly reduces the impact from locally coherent noise signals.

The code also provides a semi-automatic algorithm, that analyzes the number of source-field processes in the multivariate data space. Here, an eigenvalue criterion, calculated for each single time window, favors time segments with high MT signal-to-noise ratio and simultaneously reduces the influence of regional coherent noise signals.

The modular software package allows the user to apply uni- or multi-variate processing routines to data collected with most of the common commercial data loggers (e.g. Metronix, Phoenix). The results can be exported to different data formats and can be analyzed and manipulated within the visualization environment of our MT software package - FFMT.

The processing algorithm is validated using simulated MT signals related to a 3-D resistivity benchmark model. Additionally, MT data from the Azores are used to compare the code to well established processing codes and to highlight its efficiency. See presentation “Three-Dimensional Interpretation of Broadband Magnetotelluric data at Fogo Volcano, Azores Islands” (Hogg et al.).

How to cite: Castro, C. D., Hering, P., Hogg, C., and Junge, A.: FFproc - an improved multivariate robust statistical data processing software for the estimation of MT transfer functions, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12448, https://doi.org/10.5194/egusphere-egu21-12448, 2021.

EGU21-10316 | vPICO presentations | EMRP2.11

Modeling tippers on a sphere

Alexey Kuvshinov and Mikhail Kruglyakov

In the past three decades, a huge amount of long-period magnetic data (from weeks to years of measurements) has been collected around the world either inland or at sea bottom. It makes tempting to estimate from these data magnetotelluric (MT) vertical transfer functions - tippers – in as wide  period range as practicable and further probe with them the three-dimensional (3-D) distribution of electrical conductivity in the upper mantle on a global or semi-global/continental scale. Such problem setup requires modelling MT responses in spherical geometry. It is known that MT impedances in spherical coordinates can be modelled using different polarizations of a uniform external magnetic field. As for tippers, one cannot compute them on a sphere using this type of excitation, because the uniform external magnetic field of any polarization contains a non-zero vertical component. To overcome the problem, we elaborate a model of the source, which leads to valid MT tippers on a sphere or a part thereof. To compute tippers in spherical Earth models with 3-D conductivity distribution we use a novel accurate and computationally efficient solver called GEMMIE. The solver is based on nested integral equations, allowing researchers to calculate high-resolution MT tippers globally and regionally taking into account realistic oceans, sediments, and Earth’s conductivity.

How to cite: Kuvshinov, A. and Kruglyakov, M.: Modeling tippers on a sphere, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10316, https://doi.org/10.5194/egusphere-egu21-10316, 2021.

This work presents global 3-D electrical conductivity atlas of the world ocean and seabed sediments. Ocean salinity and temperature data were converted to electrical conductivity by solving a thermodynamic equation of state of seawater. A sediment compaction model was used to estimate the depth-dependent electrical conductivity of the seabed sediments. Electromagnetic responses in a wide period band, including typical ranges used in CSEM, MT and Global studies, are significantly affected by varying ocean and sediments conductivity. Incorporating this information in a model prior to inversion helps avoid artifacts and improve data fit. The atlas is openly available along with a concise and easy to use Python toolbox.

How to cite: Grayver, A.: Three-dimensional electrical conductivity of the world ocean and seabed sediments: effect on EM induction responses, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13412, https://doi.org/10.5194/egusphere-egu21-13412, 2021.

EGU21-7555 | vPICO presentations | EMRP2.11

Joint inversion of magnetotelluric tippers and geomagnetic depth sounding transfer functions constrains electrical conductivity beneath islands

Chaojian Chen, Mikhail Kruglyakov, Rafael Rigaud, and Alexey Kuvshinov

Geomagnetic field variations recorded at island geomagnetic observatories are one of the data sources that can be used to constrain the electrical conductivity beneath oceans. Hitherto, magnetotelluric (MT) tippers (period range from a few minutes to 3 hours) and geomagnetic depth sounding (GDS) transfer functions (TFs; period range from 6 hours to a few months) were inverted separately to reveal the electrical conductivity structure underneath island observatories.  In this study, we develop a quasi 1-D tool to simultaneously invert MT tippers and GDS TFs. To account for source complexity, we resort to GDS TFs that relates a set of spherical harmonics coefficients describing the source (of ionospheric or magnetospheric origin) to a locally measured vertical magnetic field component. Joint inversion of multiple data sets from different sources helps to improve vertical resolution and reduce uncertainties in the recovered conductivity models. The stochastic optimization method, known as Covariance Matrix Adaptation Evolution Strategy, is applied to solve the inverse problem. The term “quasi” is used here to stress the fact that during 1-D inversion the 3-D forward modeling operator is exploited to account for the ocean induction effect (OIE), which is known to strongly influence the island electromagnetic (EM) responses. To efficiently model MT tippers and GDS TFs, the Cartesian-to-Cartesian and spherical-to-Cartesian 3-D EM modeling engines, based on a nested integral equation approach, are adopted. We apply the developed tool to jointly invert MT tippers and GDS TFs estimated at Honolulu geomagnetic observatory, located at Oahu island (Hawaii) in Pacific Ocean, and discuss the recovered conductivity structure.

How to cite: Chen, C., Kruglyakov, M., Rigaud, R., and Kuvshinov, A.: Joint inversion of magnetotelluric tippers and geomagnetic depth sounding transfer functions constrains electrical conductivity beneath islands, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7555, https://doi.org/10.5194/egusphere-egu21-7555, 2021.

EGU21-11123 | vPICO presentations | EMRP2.11

3D Finite-Element Modeling of Electromagnetic Data with the Open-Source Toolbox custEM 1.0

Raphael Rochlitz and Thomas Guenther

We present enhancements and simulation capabilities of the open-source Python toolbox custEM, which was primarily designed for the 3D finite-element (FE) modeling of controlled-source electromagnetic (CSEM) surveys with arbitrary geometries on unstructured meshes. Recently, we extended the capabilities of custEM by implementing multiple approaches for time-domain (transient) electromagnetic (TEM) and magnetotelluric (MT) data.

All modeling approaches rely on the finite-element modeling library FEniCS and the direct solver MUMPS. Of the implemented FE approaches, we prefer the total electric-field formulation using Nédélec basis functions. Further potential and magnetic-field approaches, either as total- or secondary-field formulation, are available as well. Second-order basis functions usually represent a good trade-off between accuracy and computational effort. We support general anisotropic petrophysical parameters, including the conductivity, the magnetic permeability, electric permittivity, and Cole-Cole parameters to simulate induced-polarization effects. We improved all sub-modules of custEM which enabled more robust, accurate, and computationally efficient simulations. It is well-known that solving 3D FE systems with direct solvers demands plenty of memory (RAM). Though particular simulations might require up to a few TB RAM, most geometries can be handled on computer architectures with a few tens of cores and about 250 GB RAM. In addition, reusing the factorization of the system matrix significantly accelerates the solution of problems with multiple right-hand sides which is beneficial for multiple transmitters, time-domain approaches using the implicit Euler or rational Arnoldi methods, or the computation of sensitivities.

We present simulations for the three fields of electromagnetic modeling to demonstrate the accuracy and computational performance of custEM. The CSEM example is motivated by multi-frequency semi-airborne surveys as conducted in the DESMEX project. A 3D LOTEM example serves to compare three different 3D time-domain modeling approaches. The most recent support for magnetotelluric data is demonstrated by comparing our solutions of the Dublin test model 1 with those provided by the community. These and further examples are available in the code repository and can be reproduced independently. In combination with the automated online-documention of the source code, the variety of provided modeling example can help interested users to gain first experiences in custEM. Our implementation can support the community in forward modeling studies, inverse modeling applications, cross-validations, as well as understanding or teaching purposes.

How to cite: Rochlitz, R. and Guenther, T.: 3D Finite-Element Modeling of Electromagnetic Data with the Open-Source Toolbox custEM 1.0, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11123, https://doi.org/10.5194/egusphere-egu21-11123, 2021.

EGU21-15471 | vPICO presentations | EMRP2.11

Frequency-domain electromagnetic induction inversion with randomized tensor decomposition

João Narciso, Mingliang Liu, Ellen Van De Vijver, Leonardo Azevedo, and Dario Grana

Near-surface systems can be complex and highly heterogeneous. The complex nature of these systems makes their numerical modelling a challenging problem in geosciences. Geophysical survey methods combined with direct measurements have been widely used to characterize the spatial distribution of the near-surface physical properties. Within this scope, geophysical inversion has been a preferable tool to predict quantitively the spatial distribution of the relevant near-surface properties. Ensemble-based data assimilation techniques are common geophysical inversion methods used in problems related to subsurface modelling and characterization. These methods allow the accurate prediction of the spatial distribution of the subsurface properties and have the ability to assess the uncertainty about the model predictions. However, these are computationally demanding inversion techniques, which makes their applicability to large data sets prohibitive.

This study presents the application of a computationally efficient ensemble-based data assimilation technique for inversion of a large-scale frequency-domain electromagnetic induction survey data set. The inversion method is based on randomized high-order singular value decomposition. We combine randomized linear algebra with high-order singular value decomposition, which allows to perform data assimilation in a low-dimensional model and data space. This inversion approach satisfies two objectives: it reduces the computational burden of the inversion and has the same characteristics as conventional ensemble-based data assimilation methods. The inversion method presented herein predicts the spatial distribution of subsurface electrical conductivity and magnetic susceptibility from frequency-domain electromagnetic induction data (as related to the in-phase and quadrature FDEM signal components).

The method is illustrated in a three-dimensional real case application where a set of geophysical and borehole data is available. The log-set composed by electrical conductivity and magnetic susceptibility is used as conditioning data to generate a prior ensemble of numerical three-dimensional models with geostatistical simulation. The predicted posterior distribution generates synthetic frequency-domain electromagnetic induction data that reproduces the observed data. The model predictions at a blind well location, not used in the generation of the prior ensemble, agree will with the observed log data, validating the quality of the applied method.

How to cite: Narciso, J., Liu, M., Van De Vijver, E., Azevedo, L., and Grana, D.: Frequency-domain electromagnetic induction inversion with randomized tensor decomposition, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15471, https://doi.org/10.5194/egusphere-egu21-15471, 2021.

EGU21-12425 | vPICO presentations | EMRP2.11

Complex resistivity inversion using controlled source electromagnetic data

Julien Porté, Jean-François Girard, and François Bretaudeau

In some Earth materials, induced polarization (IP) phenomena are occurring when an electric perturbation is applied. These mechanisms are described by a frequency dependent complex resistivity. The study of relaxation model parameters describing these phenomena allows to access indirectly to several properties of interest of the underground, as properties linked to the pore space geometry, fluid content or presence and discrimination of disseminated metallic particles. Nevertheless, complex resistivity is usually studied using electrical method with a direct current hypothesis, neglecting by the way electromagnetic induction that can occurs in the data. Thus, strong limitations appear to recover a complex resistivity image as EM induction increase with frequencies and larger offset.

 

We implemented a frequency dependent complex resistivity in POLYEM3D, a 3D finite-difference modelling and inversion code for controlled-source electromagnetic data (CSEM) in order to fully recover IP information contained in EM data. CSEM method is a resistivity imaging technique using multi-frequency electromagnetic signals fully taking into account EM induction with larger investigation depth. Following a preliminary sensitivity study, a multi-stages inversion strategy was defined to undertake the multi-parameters problem. Furthermore, to manage the increasing number of parameters, a second order polynomial parametrization is used to describe frequency variation of complex resistivity.

 

We show through 1D synthetic data inversions and preliminary 3D results that we are able to recover a complex resistivity and its frequency variation from CSEM data in the IP/EM coupling domain, when IP signals are sufficiently large compared to EM induction. Our inversion strategy allows then to access to IP parameters of the medium in an extended frequency domain as well as for greater depth of investigation. A 3D CSEM survey was undertaken in December 2020 on the former mining site of La Porte-Aux-Moines (Côtes-d'Armor, France) presenting strong IP responses, to validate our inversion method for a 3D in-situ dataset.

How to cite: Porté, J., Girard, J.-F., and Bretaudeau, F.: Complex resistivity inversion using controlled source electromagnetic data, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12425, https://doi.org/10.5194/egusphere-egu21-12425, 2021.

EGU21-2373 | vPICO presentations | EMRP2.11

Marine electromagnetic imaging and volumetric estimation of large-scale freshwater plumes offshore Hawai'i

Eric Attias, Steven Constable, Dallas Sherman, Khaira Ismail, Christopher Shuler, and Henrietta Dulai

Submarine groundwater discharge (SGD) is a flow of cold and buoyant freshwater from the seafloor the ocean surface. Because SGD contains carbon, nutrients, metals, and green-house gases, it changes the oceanographical and biochemical properties of coastal waters. Therefore, SGD is an important phenomenon that governs hydrological cycles at the land-to-ocean transition zone. Due to the high spatial distribution and variability of SGD at the ocean surface, it is nontrivial to map SGD seep location and fluxes using traditional oceanographic methods. Here, we present electromagnetic imaging of large freshwater plumes in high-resolution, offshore west of Hawai‘i island. Our electrical resistivity models detect multiple vertical freshwater plumes (SGD point-sources) as well as spatially distributed surface freshwater, extending to a distance of ~3 km offshore Hawai‘i. Plume-scale salinity distribution indicates that these plumes contain up to 87% of freshwater. Thus, a substantial volume of freshwater occupies Hawaiian water column plumes. Our findings provide valuable information to elucidate hydrogeologic and oceanographic processes affecting biogeochemical cycles in coastal waters worldwide. This is the first study to demonstrate the marine electromagnetic method’s capability to image and delineate freshwater plumes from the seafloor to the ocean surface.

Keywords: Freshwater Plumes, SGD, Hawai'i, Surface-towed CSEM, high-resolution 2D electrical imaging.   

How to cite: Attias, E., Constable, S., Sherman, D., Ismail, K., Shuler, C., and Dulai, H.: Marine electromagnetic imaging and volumetric estimation of large-scale freshwater plumes offshore Hawai'i, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2373, https://doi.org/10.5194/egusphere-egu21-2373, 2021.

EGU21-5080 | vPICO presentations | EMRP2.11

Controlled source electromagnetic measurements offshore the Maltese Islands: Implications for offshore freshened groundwater in a carbonate shelf

Amir Haroon, Aaron Micallef, Zahra Faghih, Marion Jegen, Katrin Schwalenberg, Jens Karstens, Christian Berndt, Xavier Garcia, Michel Kuehn, Enzo Rizzo, Nicoletta Chiara Fusi, Chibuzo Valeria Ahaneku, Lorenzo Petronio, and Bradley A. Weymer

Carbonate lithologies host considerable quantities of the Earth’s freshwater resources and partially supply a significant amount of the global population with drinkable water.  Although they comprise substantial amounts of the coastlines, it is not known if these carbonate lithologies can sustain freshened groundwater offshore, and if this can help meet future water demands in coastal regions. To date, predicting volumes of freshened groundwater within marine carbonates has been challenging. Here, we integrate controlled source electromagnetic profiles with seismic reflection and core log data to derive a lithological model for the eastern carbonate margin of the Maltese Islands, one of the most water-starved countries in the world. Electrical resistivity models are used to guide lithological inference where seismic data provide limited information due to the superimposed seafloor multiple. We show that resistivity values within the Upper Coralline and Globigerina Limestone formations exceed the measured resistivity of seawater-saturated core log samples by at least a factor of four. This could be indicative of offshore freshened groundwater that occupies the pore space of the low permeability limestone along the eastern Maltese shelf. To validate this observation without further ground-truthing data, we use extensive forward modelling to show that a similar resistivity footprint can be achieved by localized interbedded low-porosity or highly cemented units. However, the spatial extent of such units across the entire eastern Maltese margin is geologically improbable. This points to the occurrence of offshore freshened groundwater that was likely emplaced during the last sea-level lowstand.

How to cite: Haroon, A., Micallef, A., Faghih, Z., Jegen, M., Schwalenberg, K., Karstens, J., Berndt, C., Garcia, X., Kuehn, M., Rizzo, E., Fusi, N. C., Ahaneku, C. V., Petronio, L., and Weymer, B. A.: Controlled source electromagnetic measurements offshore the Maltese Islands: Implications for offshore freshened groundwater in a carbonate shelf, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5080, https://doi.org/10.5194/egusphere-egu21-5080, 2021.

EGU21-13581 | vPICO presentations | EMRP2.11

Results of ground and UAV based EM investigations at the gold promising area (Altay Republic, Russia)

Kseniia Antashchuk, Alexey Atakov, and Anton Kocherov

The results of different scale EM investigations at the gold prospective area in the Altay republic are considered. The study was aimed at the first one to detect the prospective area of gold-deposits location and also to test the UAV based EM system. At the first stage the AMT surveys along 20 km length line were implemented. Their results and geological information allowed us to delineated the small area with size about 2x4 km for detailed survey. The ERT, IP, magnetometry and ground and UAV based EM surveys were implemented. EM surveys were carried out using VLF field and CS. The horizontal electric dipole of 1.6 km in length was used as a source of EM field and it produced the signals of rectangular wave form at 500 Hz. Three magnetic components of EM field (Hx, Hy, Hz) were measured with sampling frequency 312 kHz. Data were obtained in the range 500 Hz – 100 kHz. There are the few VLF stations in the studied area and the general information were obtained from the measurements using CS technique. The comparison of ground-based EM soundings results with ERT data shows good correlation. In addition, the UAV based measurements possibility were shown and their results allow to map the general features of the area geological structure. The mineralized fault zones characterized by high conductivity and IP anomaly were delineated and they are the most promising for gold-deposits detection.

How to cite: Antashchuk, K., Atakov, A., and Kocherov, A.: Results of ground and UAV based EM investigations at the gold promising area (Altay Republic, Russia), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13581, https://doi.org/10.5194/egusphere-egu21-13581, 2021.

EGU21-15040 | vPICO presentations | EMRP2.11

Three-dimensional imaging of the Mérida Andes, Venezuela

José Cruces, Oliver Ritter, Ute Weckmann, Kristina Tietze, Naser Meqbel, and Michael Schmitz

The Mérida Andes are a 100 km wide mountain chain that extends from the Colombian/Venezuelan border to the Caribbean coast. To the north and south, the Mérida Andes are bound by hydrocarbon-rich sedimentary basins. Uplift of the mountains started in the late Miocene due to oblique convergence of the Caribbean and South American tectonic plates and the north-eastwards expulsion of the North Andean Block (NAB). This tectonic interaction fostered major strike-slip fault systems, with associated high seismicity, and the partitioning of the North Andean Block into smaller tectonic units, whose interaction accelerated the uplift of the Mérida Andes since the Plio-Pleistocene.

We present the three-dimensional inversion results of broadband magnetotelluric (MT) data from 72 sites gathered along a 240 km long profile across the central part of the MA, the Maracaibo (MB), and Barinas-Apure (BAB) foreland basins. Directionality and dimensionality analyses suggested 3D structures for the MA section, with the induction vectors indicating off-profile structures, particularly at long periods. Since the distribution of sites predominantly along a single profile can have adverse effects on the outcome of the 3D inversion, we rigorously tested all model features for robustness and excluded artefacts.

One of the main findings is a deep connection (> 10km) between the most prominent faults of the MA, the Valera and Boconó fault systems, with a deep off-profile conductor to the east of our profile. We interpret this conductive structure as a detachment surface of the Trujillo Block, which is part of the NAB and whose expulsion to the NE significantly influences the present-day geodynamic evolution of western Venezuela. A conductive zone under the Maracaibo Basin correlates spatially with the location of a Bouguer low. Both geophysical anomalies may be caused by a SE tilt of the Maracaibo Triangular Block under the Mérida Andes, bound by the north-western thrust system which could reach depths of 30 km.

How to cite: Cruces, J., Ritter, O., Weckmann, U., Tietze, K., Meqbel, N., and Schmitz, M.: Three-dimensional imaging of the Mérida Andes, Venezuela, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15040, https://doi.org/10.5194/egusphere-egu21-15040, 2021.

EGU21-12081 | vPICO presentations | EMRP2.11

Magnetotellurics in the Eger Rift: An overview of subsurface imaging of different tectonic features

Ute Weckmann, Anna Platz, Basel Aleid, Gregor Willkommen, Johannes Mair, Radek Klanica, Svetlana Kovacikova, and Paula Rulff

The Bohemian Massif represents the easternmost part of the geodynamically active European Cenozoic Rift System. This region hosts the contact between three tectonic units of the Variscan Belt, the NE-SW trending Eger Rift and the NNW-SSE striking Marianské Lázne fault. It is characterised by ongoing magmatic processes in the intra-continental lithospheric mantle, repeated earthquake swarms, extensive CO2 degassing in mineral springs and mofettes and the presence of Quaternary volcanoes. While the ICDP drilling programme utilizes information gathered within shallow boreholes in the region, we applied the Magnetotelluric (MT) method to obtain site characterizations in the vicinity of the proposed drill sites. The electrical conductivity has proven to be an important parameter to image the above-mentioned tectonic from the lower crust to the shallow subsurface as well as on a regional and a local scale. Here, we present 2D and 3D inversion models of different MT and Radio-MT (RMT) experiments to study e.g. the Hartouŝov mofette fields, the Quaternary scoria cones, the regional faults and their interplay. Thereby the experiments were designed that we can use lower frequency data from MT to support shallow 3D inversions of e.g. the scoria cones in the regions. The most prominent large-scale conductivity features map channels from the lower crust to the surface possibly forming pathways for fluids into the region of earthquake swarms, mofette fields and know spas. However, the locations of the scoria cones seem to be bound to regional fault zones.

How to cite: Weckmann, U., Platz, A., Aleid, B., Willkommen, G., Mair, J., Klanica, R., Kovacikova, S., and Rulff, P.: Magnetotellurics in the Eger Rift: An overview of subsurface imaging of different tectonic features, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12081, https://doi.org/10.5194/egusphere-egu21-12081, 2021.

EGU21-8772 | vPICO presentations | EMRP2.11

Exploring geothermal resources using active and passive EM methods in the coastal areas of volcanic islands

Simon Védrine, Pascal Tarits, Mathieu Darnet, François Bretaudeau, and Sophie Hautot

Electromagnetic geophysical exploration plays a key role in high-temperature geothermal projects to estimate the geothermal potential of a region. The objective of an EM campaign applied to high-temperature geothermal exploration is to obtain an image of the impermeable clay cap, the permeable geothermal reservoir, and the system's heat source at depth, as these three components of the overall geothermal system have distinct electrical signatures. However, deep electromagnetic imaging in the coastal areas of volcanic islands represents a major challenge due to the presence of strong cultural noise induced by urbanized areas concentrated around the coast, the proximity to the sea, strong variations of topography and bathymetry, the small size of targets and the heterogeneity of the near surface. Our objective is the multi-scale integration of airborne transient electromagnetism (ATEM), shallow marine and in land magnetotelluric (MT) and controlled source electromagnetism (CSEM) to improve the reconstruction of deep geological structures by inversion. The contribution of the CSEM method is the key to overcoming cultural electromagnetic noise and exploiting data acquired in urbanized areas. In order to study how to integrate the different EM data, we first apply our methodology to data from a geothermal exploration campaign carried out a few years ago in Martinique in the French West Indies. Then, we present results from runs with synthetic tests for a campaign planned this year in Guadeloupe, also in the French West Indie, whose objective is to increase the production capacity of an existing geothermal field.

How to cite: Védrine, S., Tarits, P., Darnet, M., Bretaudeau, F., and Hautot, S.: Exploring geothermal resources using active and passive EM methods in the coastal areas of volcanic islands, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8772, https://doi.org/10.5194/egusphere-egu21-8772, 2021.

EGU21-3403 | vPICO presentations | EMRP2.11

Three-Dimensional interpretation of broadband magnetotelluric data at Fogo Volcano, Azores Islands 

Colin Hogg, Duygu Kiyan, Volker Rath, Andreas Junge, Philip Hering, Cesar Castro, Robert Delhaye, Rita Carmo, Rita Marques, Rui Marques, and Fatima Viveiros

The architecture of volcanic systems is essential to know as (1) it yields knowledge on evolution of the volcanic system, thus improving our capability to project future behaviour; (2) as it provides insights regarding geohazards (such as seismic activity, landslides, increased outgassing), crucial for mitigating risk to human population; (3) as it contributes to the assessment of potential for renewable energy resources. High electrical conductivity values are typically associated with volcanic-hydrothermal systems and the magnetotelluric (MT) method has proven to be successful in mapping such conductivity contrasts and constraining volcanic processses.

The Azores archipelago (Portugal) is formed by nine volcanic islands located in the North Atlantic Ocean where the American, Eurasian, and African plates meet at a triple junction. São Miguel Island is the largest of the archipelago and hosts three trachytic polygenetic volcanoes: Sete Cidades, Fogo (Água de Pau) and Furnas. Following our earlier MT studies at Furnas, 44 high-quality (to ~1000s) broadband MT sites were collected during 2018 across Fogo Volcano and the adjacent Congro region that is prone to seismic swarm activity.

Our MT studies comprised two avenues: generating geoelectrical models that provided new insights into this unique setting, and investigating and assessing new tools for the MT community.

(1) Fogo has a resistive core and we do not see a magma chamber beneath.

(2) Shallow conductive channels are observed beneath Congro and their presence have been tested and validated through forward modelling and additional sensitivity tests.

(3) The MT results can be used to map clay alteration, with the highly conductive zone on the northern flank of Fogo corresponding to the smectite zone. The alteration temperature distribution is consistent with the formation temperature recorded within the area.

(4) A potential new geothermal resource has been identified. An area north of Ribeira Cha, on the southern flank of Fogo has very similar characteristics of the Ribeira Grande geothermal system that is located on the northern flank. This area may be key in increasing the energy self-sufficiency of the island.

Depth slices through the final 3-D MT inversion volume will be presented.

The new MT processing code of University of Frankfurt was compared against two long-standing codes commonly used in the MT community and it proved to yield superior responses for every site and examples will be presented. See presentation in this session “FFproc - an improved multivariate robust statistical data processing code for the estimation of MT transfer functions” (Castro et al).

A novel approach exploiting the Jacobian matrix elements for the 3-D MT inversion strategy will be presented. The Jacobian matrix elements map the relationship between the model and model responses, thus portions of the model with low sensitivities infer that the sensitivity structure is algorithmically influenced more strongly by the regularisation term than by the data-fitting term. Our results will show that the computation of the Jacobian matrix (albeit computationally expensive) is a powerful tool in aiding interpretation.

How to cite: Hogg, C., Kiyan, D., Rath, V., Junge, A., Hering, P., Castro, C., Delhaye, R., Carmo, R., Marques, R., Marques, R., and Viveiros, F.: Three-Dimensional interpretation of broadband magnetotelluric data at Fogo Volcano, Azores Islands , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3403, https://doi.org/10.5194/egusphere-egu21-3403, 2021.

EGU21-13168 | vPICO presentations | EMRP2.11

The resistivity structure of the Hekla volcano in South Iceland: first insights from electromagnetic investigations

Duygu Kiyan, Ásdís Benediktsdóttir, Gylfi P. Hersir, Magnús T. Guðmundsson, Christopher J. Bean, Colin Hogg, Steini Jonsson, and Jón Einar Jónsson

EGU21-15192 | vPICO presentations | EMRP2.11

Chasing the Magma Chamber: MT meets Geodynamics and Seismology – A numerical case study of magmatic plumbing at Oldoinyo Lengai Volcano

César Daniel Castro, Miriam Christina Reiss, Arne Spang, Philip Hering, Luca de Siena, Abo Komeazi, Yi Zhang, Georg Rümpker, Boris Kaus, and Andreas Junge

How well can geophysical methods image magmatic systems? Geophysical methods are commonly used to image magmatic systems; however, synthetic studies which give insights into the resolution of such methods and their interpretational scope are rare. Gravity anomalies, magnetotelluric, seismological and geodynamical modelling all have a different sensitivity to the rock parameters and are thus likely complementary methods. Our study aims to better understand their interplay by performing joint modelling of a synthetic magmatic system.  Our model setup of a magma chamber is inspired by seismological observations at the Natron plumbing system including active volcano Oldoinyo Lengai within the East African Rift system. The geodynamic modelling is guided by shear-wave velocity anomalies and it is constrained by a large Bouguer gravity anomaly which is modelled by a voxel-based gravity code. It yields the 3D distribution of several geological parameters (pressure, temperature, stress, density, rock type). The parameters are converted into a 3D resistivity distribution. By 3D forward modelling including the topography, synthetic MT transfer functions (phase tensor, induction vectors) are calculated for a rectangular grid of 441 sites covering the area. The variation of geodynamic parameters and/or petrological relations alters the related resistivity distribution and thus yields the sensitivity of MT responses to geodynamic parameters. In turn, MT observations may constrain geodynamic modelling by inverting MT transfer functions. The inversion is performed allowing for the recent seismicity distribution beneath the Natron plumbing system, assuming that active seismic areas are related to enhanced resistivity. The inversion is performed for a realistic distribution (in view of logistic accessibility) of about 40 MT sites.

By combining multiple forward models, this study yields insights into the sensitivity of different observables and thus provides a valuable base on how MT, gravity and seismological observations can help imaging a complex geological setting.

How to cite: Castro, C. D., Reiss, M. C., Spang, A., Hering, P., de Siena, L., Komeazi, A., Zhang, Y., Rümpker, G., Kaus, B., and Junge, A.: Chasing the Magma Chamber: MT meets Geodynamics and Seismology – A numerical case study of magmatic plumbing at Oldoinyo Lengai Volcano, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15192, https://doi.org/10.5194/egusphere-egu21-15192, 2021.

EGU21-4096 | vPICO presentations | EMRP2.11

Transient Electromagnetics at Stromboli volcano, Italy: a virtual and field experiment

Carolin Schneider, Klaus Spitzer, and Matthias Hort

Among others, the transient electromagnetic method (TEM) is used in investigating volcanic systems. It is capable of imaging conductive structures, such as hydrothermal systems and magmatic pathways, which are of great volcanological interest since they are attributed a key role in volcanic eruptions. Based on this motivation, we aim to investigate the medium depth resistivity structure of Stromboli volcano, Italy. Its persistent activity makes it an ideal target to investigate whether TEM can image changes in the subsurface conductivity structure associated with volcanic eruptions.
In order to determine a reasonable measurement configuration, we conduct so-called virtual experiments, i.e., three-dimensional TEM simulations on a digital elevation model. The sophisticated routine combines vectoral finite elements for spatial discretization and a time integration based on a Krylov subspace method. Furthermore, in order to minimize computational cost and run-time, automated adaptive mesh refinement is included.
Within the presentation, we introduce our modelling workflow and simulation routine, and link our numerical findings to the results of our field measurement at Stromboli volcano, Italy, in June 2019.

How to cite: Schneider, C., Spitzer, K., and Hort, M.: Transient Electromagnetics at Stromboli volcano, Italy: a virtual and field experiment, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4096, https://doi.org/10.5194/egusphere-egu21-4096, 2021.

EGU21-4824 | vPICO presentations | EMRP2.11

Opportunistic magnetotelluric transects from 3D CSEM surveys in the SW Barents Sea

Romain Corseri, Sverre Planke, Jan Inge Faleide, Kim Senger, Leiv Jacob Gelius, and Ståle Emil Johansen

Magnetotelluric (MT) data allow for electrical resistivity probing of the earth’s subsurface. Integration of resistivity models in passive margin studies could help disambiguate non-unique interpretations of crustal lithologies from seismic and potential field data. In this contribution, we present the first marine MT data in the Barents Sea, derived from industrial controlled-source electromagnetic (CSEM) surveys and characterize its quality, dimensionality and depth penetration to elaborate a modelling strategy. This MT database consists of 337 receivers located along 7 regional transects, emanating from ~70,000 km2 of 3D CSEM surveys acquired for hydrocarbon exploration from 2007 to 2019. High-quality MT data are extracted for periods ranging from 0.5 s to 5000 s with no apparent contamination by the active source, nor effects related to large time-gaps in data collection and variable solar activity. Along receiver profiles, abrupt lateral variations of apparent resistivity and phase trends coincide with major structural boundaries and underline the geological information contained in the data. Dimensionality analysis reveals a dichotomy between the “western” domain of the SW Barents Sea, dominated by a single N-S electromagnetic strike, and the “eastern” domain, with a two-fold, period-dependent strike. 35 receivers show 3D distortion caused by nearby bathymetric slopes, evidenced by elevated skew angle values. We delimit geographical areas where the 2D assumption is tenable and lay the foundation for future 2D and 3D MT modelling strategies in the SW Barents Sea. We performed 2D MT inversion along one of the regional transects, a ~220 km-long, E-W profile encompassing a major structural high and distal basin approaching the continent-ocean boundary. The resistivity model shows low crustal resistivity values (1-10 Ω.m) beneath the sedimentary cover in western distal basins, in stark contrast with high resistivity values (1000 - 5000 Ω.m) of the thick crystalline crust in the proximal domain on the structural high. We interpret this abrupt lateral resistivity variation as a crustal necking zone with a rapid transition to a hyperextended continental crust. In the proximal domain, a 50-100 Ω.m and 20 km-wide, intra-crust vertical contact hints at a plausible paleo-plate suture. Integration of resistivity with velocity, density and magnetic susceptibility models will further refine these tectonic models and related processes in the SW Barents Sea margin. Our methodology is applicable globally where 3D CSEM surveys are acquired and has a large potential for harvesting new knowledge on the electrical resistivity properties of the lithosphere.

How to cite: Corseri, R., Planke, S., Faleide, J. I., Senger, K., Gelius, L. J., and Johansen, S. E.: Opportunistic magnetotelluric transects from 3D CSEM surveys in the SW Barents Sea, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4824, https://doi.org/10.5194/egusphere-egu21-4824, 2021.

EGU21-8946 | vPICO presentations | EMRP2.11

Marine electromagnetic forward modeling in a resistivity model constrained by seismic and well log data from a field at Campos basin SE-Brazil

Artur Benevides, Naser Meqbel, Williams Lima, Sergio Fontes, Gary Egbert, Paulo Werdt, and Emanuele La Terra

This work presents electromagnetic (EM) responses using both marine magnetotelluric (MMT) and controlled source electromagnetic (MCSEM) methods applied to a resistivity model typical of the Campos basin. The Campos basin is located in the Brazilian east margin, with origin in the Neocomian stage of the Cretaceous period 145–130 million years ago during the breakup of the supercontinent Gondwana (South America and Africa split). The clastic reservoirs in this basin have been the largest oil producer in Brazil for the past three decades and the present challenge moves to deeper waters, well known for great challenges in exploration enforced by the complex geology posed by the tectonics associated with giant saline bodies. The seismic reflection is the highest resolution geophysical method and the most used tool in hydrocarbon (HC) exploration; however, it finds difficulties in generating good images in complex geological environments (i.e., associated with the presence of salt and volcanic rocks) and is not a direct hydrocarbon indicator. EM methods, in contrast, are sensitive to resistive variations, and can therefore aid in indicating a reservoir HC-filled and/or in the obtaining subsurface images that can be integrated in joint approaches with seismic for producing less ambiguous interpretations. A set of geophysical data available in the study area includes: CSEM stations, 2D seismic lines, and 3D seismic cube, in addition to 33 well logs. The studies are still in an early stage and the main objective now is to evaluate the responses of the MMT and MCSEM methods in different scenarios involving hydrocarbon accumulations in a thin post-salt Maastrichtian reservoir. The EM forward modeling responses was performed using the modified version of Modular System for EM inversion (ModEM code) within a research project at Observatório Nacional – Brazil. The resistivity model used in this study incorporates vertical transverse isotropy (VTI) in resistivity and is derived from the interpretation of the seismic data and well logs present in the study area. The obtained EM responses demonstrate the effectiveness of the method for detecting thin reservoirs HC-filled, when compared to an environment without accumulation, and the exercise is valuable to compare and understand the real data collected in the study area.

How to cite: Benevides, A., Meqbel, N., Lima, W., Fontes, S., Egbert, G., Werdt, P., and La Terra, E.: Marine electromagnetic forward modeling in a resistivity model constrained by seismic and well log data from a field at Campos basin SE-Brazil, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8946, https://doi.org/10.5194/egusphere-egu21-8946, 2021.

EGU21-12542 | vPICO presentations | EMRP2.11

Exploring the near surface geothermal structure at Mt. Meager, British Columbia, Canada

Fateme Hormozzade Ghalati, James A. Craven, Dariush Motazedian, Stephen E. Grasby, Eric Roots, Vicki Tschirhart, SeyedMasoud Ansari, and Jon Liu

Mount Meager is located ~150 km north of Vancouver, British Columbia Canada, and is a part of the Garibaldi volcanic belt. Exploration at Mount Meager for geothermal energy resources has been ongoing since 1974 and has shown, based on well data, that there is a permeable zone at a depth of 1200-1600 m and that the reservoir has a temperature of 270 °C near 2500 m depth. In this study, we have utilized recordings and related information from a new network of 84 audio-magnetotelluric (AMT) stations collected during the summer of 2019 plus 37 stations from previous studies to investigate the geothermal potential of the area around Mount Meager and Pylon peak. We used Phoenix Geophysics’ MTU-5C recording equipment and their proprietary software for data processing, separating extensive noise from the signal, to calculate the components of the natural electrical and magnetic signals in the frequency domain. After manual processing and editing, the data showed good quality in the frequency range of 1 to 1000 Hz. The ModEM inversion algorithm (Egbert and Kelbert, 2012) was then used to model the data. Modelling started using a coarse grid mesh with different starting resistivities, and then a finer grid size and topography was added to refine the model. The preliminary result of this 3D inversion defines the shape and location of conductors in the study area. The results show a conductor at a depth 2000 m located to the southwest of Mount Meager. Comparison of the 3D model and the geological setting of the area demonstrated that this conductor shallows toward the southern portion of the No-good Fault.

How to cite: Hormozzade Ghalati, F., Craven, J. A., Motazedian, D., Grasby, S. E., Roots, E., Tschirhart, V., Ansari, S., and Liu, J.: Exploring the near surface geothermal structure at Mt. Meager, British Columbia, Canada, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12542, https://doi.org/10.5194/egusphere-egu21-12542, 2021.

EGU21-8885 | vPICO presentations | EMRP2.11

Plumbing and architecture of a crustal mineralizing system in the Archean Superior Province, Canada

Eric Roots, Graham Hill, Ben M. Frieman, James A. Craven, Richard S. Smith, Andrew J. Calvert, Phil Wannamaker, and Virginie Maris

The role of melts and magmatic/metamorphic fluids in mineralization processes is well established. However, the role of crustal architecture in defining source and sink zones in the middle to lower crust remains enigmatic. Integration of three dimensional magnetotelluric (MT) modelling and seismic reflection data across the Archean Abitibi greenstone belt of the Superior Province, Canada, reveals a ‘whole-of-crust’ mineralizing system and highlights the controls by crustal architecture on metallogenetic processes. Electrically conductive conduits in an otherwise resistive upper crust are coincident with truncations and offsets of seismic reflections that are mostly interpreted as major brittle-ductile fault zones. The spatial association between these features and low resistivity zones imaged in the 3D models suggest that these zones acted as pathways through which fluids and melts ascended toward the surface. At mid-crustal levels, these ‘conduit’ zones connect to ~50 km long, north-south striking conductors, and are inferred to represent graphite and/or sulphide deposited from cooling fluids. At upper mantle to lower crustal depths, east-west trending conductive zones dominate and display shallow dips. The upper mantle features are broadly coincident with the surface traces of the major deformation zones with which a large proportion of the gold endowment is associated. We suggest that these deep conductors represent interconnected graphitic zones perhaps augmented by sulphides that are relicts from metamorphic fluid and melt emplacement associated primarily with the later stages of regional deformation.  Thus, from the combined MT and seismic data, we develop a crustal-scale architectural model that is consistent with existing geological and deformational models, providing constraints on the sources for and signatures of fluid and magma emplacement that resulted in widespread metallogenesis in the Abitibi Subprovince.

How to cite: Roots, E., Hill, G., Frieman, B. M., Craven, J. A., Smith, R. S., Calvert, A. J., Wannamaker, P., and Maris, V.: Plumbing and architecture of a crustal mineralizing system in the Archean Superior Province, Canada, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8885, https://doi.org/10.5194/egusphere-egu21-8885, 2021.

EGU21-13382 | vPICO presentations | EMRP2.11

Electrical properties of the lithosphere in the western desert, Egypt, using magnetotelluric sounding

Hossam Marzouk, Tarek Arafa-Hamed, Michael Becken, Mohamed Abdel Zaher, and Matthew Comeau

We present electrical resistivity models of the crust and upper mantle estimated from 2D inversions of broadband magnetotellurics (MT) data acquired from two profiles in the western desert of Egypt, which can contribute to the understanding of the structural setup of this region. The first profile data are collected from 14 stations along a 250 km profile, in EW direction profile runs along latitude ~25.5°N from Kharga oasis to Dakhla oasis. The second profile comprises 19 stations measured along a 130 km profile in NS direction centered at longitude 28°E and crossing the Farafra. The acquisition for both profiles continued for 1 to 3 days at each station, which allowed for the calculation of impedances for periods from 0.01 sec up to  4096 sec at some sites. The wide frequency band corresponds to a maximal skin depths of up to 150 km that can provide penetration to the top of the asthenosphere. The inversion models display high-conductivity sediments cover at the near surface (<1-2 km), which can be associated with the Nubian aquifer. Along the EW-profile from Kaharge to Dhakla, the crustal basement is overly highly resistive and homogeneous und underlain by a more conductive lithospheric mantle below depths of 30-40 km. Along the N-S profile across Farafra, only the southern portion exhibits a highly resistive crust, whereas beneath Farafra northwards, moderate crustal conductivities are encountered. A comparison has been made between the resultant resistivity models with the 1° tessellated updated crust and lithospheric model of the Earth (LITHO1.0) which was developed by Pasyanos, 2014 on the basis of seismic velocity data. The obtained results show a remarkable consistency between the resistivity models and the calculated crustal boundaries. Especially at the Kharga-Dakhla profile a clear matching can be noticed at the upper and lower boundaries of a characteristic anomaly with the Moho and LAB boundaries respectively.

How to cite: Marzouk, H., Arafa-Hamed, T., Becken, M., Abdel Zaher, M., and Comeau, M.: Electrical properties of the lithosphere in the western desert, Egypt, using magnetotelluric sounding, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13382, https://doi.org/10.5194/egusphere-egu21-13382, 2021.

EGU21-15690 | vPICO presentations | EMRP2.11

Crustal Characterization of Portugal's mainland based on Magnetotelluric measurements. 

Pedro Baltazar-Soares, Francisco Martinez Moreno, Joana Alves Ribeiro, Fernando Monteiro Santos, Maria Alexandra Pais, and Paulo Ribeiro

In the last decades, the phenomena of Geomagnetic Induced Currents (GICs) have received special attention as one of the main hazards of Space Weather and has been widely investigated. In the high and mid-latitudes, these large GICs can flow in power systems and become problematic and even severe enough to cause a complete system shutdown. Two major factors determine GICs: (1) the strength and orientation of the electric field in the power system, which depends on the ionospheric and magnetospheric currents as well as on the crust and mantle conductivity; and (2) the electric power network characteristics. The Earth's conductivity can be obtained based on geophysical measurements that give the distribution of the conductivity in-depth and laterally. A realistic model of conductivity can be built based on the interpretation of Magnetotelluric (MT) soundings. The power of this geophysical method resides in the fact that it uses a natural source of energy, which allows estimating the conductivity distribution from a dozen of meters to some kilometres in depth.

We present a 3D resistivity model of the entire Portugal mainland based on more than 40 broadband MT soundings spaced 50x50km. The present study aims to contribute to a better understanding of Portugal's crust and its main geological structures. As a more practical application, knowledge of the presence of resistivity/conductivity bodies is important to obtain more precise GICs estimations. 

 

How to cite: Baltazar-Soares, P., Martinez Moreno, F., Alves Ribeiro, J., Monteiro Santos, F., Pais, M. A., and Ribeiro, P.: Crustal Characterization of Portugal's mainland based on Magnetotelluric measurements. , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15690, https://doi.org/10.5194/egusphere-egu21-15690, 2021.

EGU21-12413 | vPICO presentations | EMRP2.11

Lower crustal low-resistivity zones caused by compaction-induced fluid localization and stagnation – recent results from electromagnetic data in an intracontinental setting

Matthew Joseph Comeau, Michael Becken, James A. D. Connolly, Alexander Grayver, Alexey V. Kuvshinov, Johannes Käufl, Erdenechimeg Batmagnai, Shoovdor Tserendug, and Sodnomsambuu Demberel

We investigate how a conceptual hydrodynamic model consisting of fluid localization and stagnation by thermally activated compaction can explain low-resistivity anomalies observed in the lower crust (>20 km depth). Electrical resistivity models, derived from magnetotelluric data collected across the intracontinental Bulnay region, a subset of a larger regional array across central Mongolia, are generated. They reveal low-resistivity (3 - 30 Ωm) domains with a width of ~25 km and a vertical extent of <10 km in the lower crust, with their tops ~5 km below the brittle-ductile transition zone. In 3-D these features appear as laterally extended (tube-like) structures, 300 km long, rather than disconnected ellipsoids. The features are oriented parallel to the adjacent Bulnay fault zone segments and perpendicular to the far-field compressive tectonic stress (i.e., northward motion from China and Tibet). These low-resistivity domains are consistent with the presence of saline metamorphic fluids. Deeper features imaged with the data include a large upper mantle conductor that we attribute to an asthenospheric upwelling, and thin lithosphere, related to intraplate surface uplift and volcanism, in agreement with recent geodynamic modelling of lithospheric removal in this region.

Based on the observed thermal structure of the crust, and assuming the mean stress at the brittle-ductile transition is twice the vertical load, the hydrodynamic model predicts that fluids would collect in zones <9 km below the brittle-ductile transition zone, and the zones would have a vertical extent of ~9 km, both in agreement with the resistivity models across the Bulnay region. The hydrodynamic model also gives plausible values for the activation energy for viscous creep (270 - 360 kJ/mol), suggesting that the mechanism is dislocation creep.

From the electrical resistivity models, the lower crustal viscous compaction-length is constrained to be ~25 km - in this region. Within the conceptual model, this length-scale is entirely consistent with independent estimates for the specific hydraulic and rheological properties of this region. In fact, this can be used to independently constrain acceptable ranges for the lower crustal effective viscosity, which is found to be low (on the order of 10^18 Pas). Accordingly, the results indicate that low-salinity fluids (likely 1 - 0.01 wt% NaCl), and correspondingly low porosities (likely 5 - 0.1 vol%), are the most plausible. These key findings suggest partial melts are not favoured to explain the anomalies. Overall, the results of this contribution imply that it is tectonic and compaction processes that control lower crustal fluid flow, rather than lithological or structural heterogeneity.

How to cite: Comeau, M. J., Becken, M., Connolly, J. A. D., Grayver, A., Kuvshinov, A. V., Käufl, J., Batmagnai, E., Tserendug, S., and Demberel, S.: Lower crustal low-resistivity zones caused by compaction-induced fluid localization and stagnation – recent results from electromagnetic data in an intracontinental setting, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12413, https://doi.org/10.5194/egusphere-egu21-12413, 2021.

We have developed and tested a new frequency-domain, spherical harmonic-finite element approach to the inverse problem of global electromagnetic (EM) induction. It is based on the quasi-Newton minimization of data misfit and regularization, and uses the adjoint approach for fast calculation of misfit gradients in the model space. Thus, it allows for an effective inversion of satellite-observed magnetic field induced by tidally driven flows in the Earth's oceans in terms of 3-D structure of the electrical conductivity in the upper mantle. Before proceeding to the inversion of Swarm-derived models of tidal magnetic signatures, we have performed a series of parametric studies, using a 3-D conductivity model WINTERC-e as a testbed.

The WINTERC-e model has been derived using state-of-the-art laboratory conductivity measurements of mantle minerals, and thermal and compositional model of the lithosphere and upper mantle WINTERC-grav. The latter model is based on the inversion of global surface waveforms, satellite gravity and gradiometry measurements, surface elevation, and heat flow data in a thermodynamically self-consistent framework. Therefore, the WINTERC-e model, independent of any EM data, represents an ideal target for synthetic tests of the 3-D EM inversion.

We tested the impact of the satellite altitude, the truncation degree of the spherical-harmonic expansion of the tidal signals, the random noise in data, and of the sub-continental conductivity on the ability to recover the sub-oceanic upper-mantle conductivity structure. We demonstrate that with suitable regularization we can successfully reconstruct the 3D upper-mantle conductivity below world oceans.

How to cite: Šachl, L., Velímský, J., and Fullea, J.: Inversion of the satellite observations of the tidally induced magnetic field in terms of 3-D upper-mantle electrical conductivity: Method and synthetic tests, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7139, https://doi.org/10.5194/egusphere-egu21-7139, 2021.

EGU21-10679 | vPICO presentations | EMRP2.11

An adaptive inversion algorithm for one-dimensional magnetotelluric problems

Huang Chen, Zhengyong Ren, and Jingtian Tang

      As we know, the traditional one-dimensional (1-D) magnetotelluric (MT) regularization inversion needs the geometry model of the 1-D Earth conductivity model, i.e., the number of layers and the thickness of each layer to be given in advance and cannot be changed during the inversion. In this way, too few layers cannot approximate the 1-D conductivity model accurately, while too many layers will increase the non-uniqueness of the inversion problem and hence may result in unreasonable results. Aiming to solve this issue, an adaptive inversion algorithm has been proposed for 1-D MT problems, where the layer number and the thickness of each layer can be adjusted automatically during the inversion process. To this end, three pseudo a-posterior error estimators has been proposed to guide the adjustment of the 1D geometry model, which are based on the gradient of the data misfit term of the penalty function, the diagonal elements of the model resolution matrix, and the weighted elements of the sensitivity matrix, respectively. The inversion results of the synthetic and field data by using our proposal adaptive inversion algorithm and the traditional regularization inversion not only validate the proposed algorithm, but also show that our proposed algorithm can obtain more accurate and reasonable results than traditional one. Subsequently, the proposed algorithm will be extended for 3-D magnetotelluric inversion problems soon.

How to cite: Chen, H., Ren, Z., and Tang, J.: An adaptive inversion algorithm for one-dimensional magnetotelluric problems, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10679, https://doi.org/10.5194/egusphere-egu21-10679, 2021.

I present a newly developed 3D forward modeling algorithm for controlled-source electromagnetic data. The algorithm is based on the finite-difference method, where the source term vector is redefined by combining a modified boundary condition vector and source term vector. The forward modeling scheme includes a two-step modeling approach that exploits the smoothness of the electromagnetic field. The first step involves a coarse grid finite-difference modeling and the computation of a modified boundary field vector called radiation boundary field vector. In the second step, a relatively fine grid modeling is performed using radiation boundary conditions. The fine grid discretization does not include stretched grid and air medium. The proposed algorithm derives computational efficiency from a stretch-free discretization, air-free computational domain, and a better initial guess for an iterative solver. The numerical accuracy and efficiency of the algorithm are demonstrated using synthetic experiments. Numerical tests indicate that the developed algorithm is one order faster than the finite-difference modeling algorithm in most of the cases analyzed during the study. The radiation boundary method concept is very general; hence, it can be implemented in other numerical schemes such as finite-element algorithms.

How to cite: Dehiya, R.: A modified-boundary condition algorithm for efficient 3D forward modeling of CSEM data, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11690, https://doi.org/10.5194/egusphere-egu21-11690, 2021.

EGU21-13265 | vPICO presentations | EMRP2.11

Investigation of a multiphysics joint inversion framework

Dmitry Molodtsov, Duygu Kiyan, and Christopher Bean

We present a 3-D multiphysics joint inversion framework that in a certain sense is a trade-off between “simultaneous” and “cooperative” approaches to data integration. Using a variable splitting approach, inverse problems are solved by individual inverse solvers, on individual grids, while coupling combined with interpolation is implemented separately. Up to date, first-arrival seismic tomography, gravity and magnetotelluric inverse problems are included in this framework. Magnetotelluric inversion uses the NLCG method implemented in the ModEM code (Kelbert et al., 2014). Seismic tomography is based on the Gauss-Newton method with a finite-difference eikonal solver and posterior ray tracing. Gravity inversion uses the conjugate gradient method with wavelet compression of the sensitivity matrix. Structure coupling is based on mixed-norm regularization inducing joint sparsity between the models. Among particular functionals that were studied, following numerical experiments, joint total variation and joint minimum support have proved to be the most efficient options. In the numerical experiments, we invert synthetics simulating regional datasets observed in Ireland.

How to cite: Molodtsov, D., Kiyan, D., and Bean, C.: Investigation of a multiphysics joint inversion framework, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13265, https://doi.org/10.5194/egusphere-egu21-13265, 2021.

We present an accurate and fast finite element solver for global electromagnetic induction forward modeling problems in spherical Earth. We solve for the electric field equation using the first-order Nedelec elements. The magnetic field is then obtained by computing the curl of the electric field. The computational domain composed of the air space and the conductive Earth is discretized by disjoint unstructured tetrahedral elements. To improve the accuracy with an optimal number of unknowns, we propose a simple two-step goal-oriented adaptive mesh refinement (AMR) strategy. In the first step, an h-type AMR procedure is used to obtain an optimal finite element mesh. The mesh refinement is accomplished by bisection to generate a set of hierarchal tetrahedral meshes. The AMR procedure is driven by a goal-oriented error estimator, which is based on face jumps of normal components of current density. In the second step, we adopt the high-order finite elements at the last iteration to update the accuracy of final numerical solutions. This simple two-step adaptive strategy takes advantage of both h-type AMR and high-order basis functions, and in the meanwhile, it is also computationally economical. To improve efficiency, the solver is parallelized with an MPI-based domain decomposition technique. The sophisticated auxiliary space preconditioned linear solver is adopted to efficiently solve the linear system of equations. This new solver is verified on both semi-analytic and realistic 3-D Earth models. It can be used as a core to derive the inversion of global electromagnetic induction data.

 

How to cite: Yao, H., Ren, Z., and Tang, J.: A parallel adaptive finite element solver for global electromagnetic induction modeling using hierarchal tetrahedral meshes , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13751, https://doi.org/10.5194/egusphere-egu21-13751, 2021.

EGU21-14002 | vPICO presentations | EMRP2.11

3-D CSEM inversion for the complex model with topography using an efficient dual parallel approach

Zhengguang Liu, Zhengyong Ren, Jingtian Tang, and Huang Chen

    There is a significant interest in improving the efficiency of 3-D CSEM inversion and obtaining more reliable inversion results. A 3-D CSEM inversion code using unstructured tetrahedral elements has been developed in order to consider the topographic effect by directly incorporating it into computational grids. In the forward modeling, the electric dipole source is divided into a set of short electric dipoles to simulate its practical shape, size and attitude. We adopt the edge-based finite-element method to discretize the electric field equation. In the inversion, the inversion grids are entirely independent of the forward grids. The lower and upper bounding constraints on model parameters are used to improve the reliability of the inversion result further. We use the Gauss-Newton algorithm to minimize the inversion objective function and obtain the underground conductivity model. The calculation of the forward modeling and the sensitivity matrix spends most of the time in the inversion. At present, most inversion codes use frequency-based parallel methods to accelerate the inversion, to further improve the efficiency of 3D CSEM inversion, except for the frequency-based parallel methods, we use the open-source software METIS to divide the model into several parts and then use the MPI-based parallel toolkits (such as PETSc and MUMPS) to solve the forward linear equations. The same parallel scheme can also be used to calculate the sensitivity matrix. Finally, we can further improve the efficiency of 3-D CSEM inversion by the dual parallel strategy based on the frequency and domain decomposition.

How to cite: Liu, Z., Ren, Z., Tang, J., and Chen, H.: 3-D CSEM inversion for the complex model with topography using an efficient dual parallel approach, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14002, https://doi.org/10.5194/egusphere-egu21-14002, 2021.

EGU21-2339 | vPICO presentations | EMRP2.11

In-depth comparison of the spectral-element and finite-element method for 3D CSEM forward modelling

Michael Weiss, Paula Rulff, and Thomas Kalscheuer

We developed two forward modelling approaches to simulate 3-dimensional land-based controlled-source electromagnetic (CSEM) problems in frequency domain with hexahedral spectral-element meshes and tetrahedral finite-element meshes. In recent years, the geo-electromagnetic community made a lot of progress in modelling and inversion of EM data in three dimensions using a variety of approaches. The available software is used to verify the accuracy of newly developed codes, which apply e.g. different element shapes or interpolation schemes. However, a direct comparison in terms of advantages and disadvantages of different modelling strategies, especially discretisation methods in 3D, is often not focused on in publications.

Having two modelling codes and their developers available at the same place, gives us the unique opportunity to compare the approaches in a very detailed way. Our spectral-element as well as our finite-element solution is based on Galerkin’s weighted residual method and we solve the electromagnetic diffusion equations for the total electric field on the element edges.
The main differences between both codes are the choice and order of the interpolation functions and the discretisation of the modelling domain employing hexahedral and tetrahedral elements. While the tetrahedral meshes used in our finite-element approach are known for being able to properly resolve complex structures in the subsurface, this issue is addressed in the spectral-element method by utilising curvilinear instead of orthogonal hexahedral elements.

In this contribution, we focus on the comparison of both approaches for a simple 1D model and a complex 3D model in terms of accuracy, effort in mesh generation and computational resources such as simulation time and memory requirement. Moreover, we contrast the influence of mesh discretisation on the solution for the two methods as well as the order of approximation. A preliminary test simulation of a  model consisting of a conductive body buried within a resistive background covered by a thin conductive layer yielded comparable results in terms of accuracy. It also revealed significant differences concerning the mesh discretisation meaning the solution's dependency on the meshing of the model domain.

Acknowledgements: This work was partly funded by Uppsala’s Center for Interdisciplinary
Mathematics and the 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: Weiss, M., Rulff, P., and Kalscheuer, T.: In-depth comparison of the spectral-element and finite-element method for 3D CSEM forward modelling, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2339, https://doi.org/10.5194/egusphere-egu21-2339, 2021.

EMRP2.14 – Open Session in Geomagnetism

EGU21-14660 | vPICO presentations | EMRP2.14 | Highlight

NanoMagSat, a 16U nanosatellite constellation high-precision magnetic project to initiate permanent low-cost monitoring of the Earth’s magnetic field and ionospheric environment

Gauthier Hulot, Jean-Michel Léger, Lasse B. N. Clausen, Florian Deconinck, Pierdavide Coïsson, Pierre Vigneron, Patrick Alken, Arnaud Chulliat, Christopher C. Finlay, Alexander Grayver, Alexey Kuvschinov, Nils Olsen, Erwan Thébault, Thomas Jager, François Bertrand, and Tobias Häfner

The geomagnetic field has been continuously monitored from low-Earth orbit (LEO) since 1999, complementing ground-based observatory data by providing calibrated scalar and vector measurements with global coverage. The successful three-satellite ESA Swarm constellation is expected to remain in operation up to at least 2025. Further monitoring the field from space with high-precision absolute magnetometry beyond that date is of critical importance for improving our understanding of dynamics of the multiple components of this field, as well as that of the ionospheric environment. Here, we will report on the latest status of the NanoMagSat project, which aims to deploy and operate a new constellation concept of three identical 16U nanosatellites, using two inclined (approximately 60°) and one polar LEO, as well as an innovative payload including an advanced Miniaturized Absolute scalar and self-calibrated vector Magnetometer (MAM) combined with a set of precise star trackers (STR), a compact High-frequency Field Magnetometer (HFM, sharing subsystems with the MAM), a multi-needle Langmuir Probe (m-NLP) and dual frequency GNSS receivers. The data to be produced will at least include 1 Hz absolutely calibrated and oriented magnetic vector field (using the MAM and STR), 2 kHz very low noise magnetic scalar (using the MAM) and vector (using the HFM) field, 2 kHz local electron density (using the m-NLP) as well as precise timing, location and TEC products. In addition to briefly presenting the nanosatellite and constellation concepts, as well as the evolving programmatic status of the mission (which already underwent a consolidation study funded by the ESA Scout programme), this presentation will illustrate through a number of E2E simulations the ability of NanoMagSat to complement and improve on many of the science goals of the Swarm mission at a much lower cost, and to bring innovative science capabilities for ionospheric investigations. NanoMagSat could form the basis of a permanent collaborative constellation of nanosatellites for low-cost long-term monitoring of the geomagnetic field and ionospheric environment from space.

How to cite: Hulot, G., Léger, J.-M., Clausen, L. B. N., Deconinck, F., Coïsson, P., Vigneron, P., Alken, P., Chulliat, A., Finlay, C. C., Grayver, A., Kuvschinov, A., Olsen, N., Thébault, E., Jager, T., Bertrand, F., and Häfner, T.: NanoMagSat, a 16U nanosatellite constellation high-precision magnetic project to initiate permanent low-cost monitoring of the Earth’s magnetic field and ionospheric environment, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14660, https://doi.org/10.5194/egusphere-egu21-14660, 2021.

A digital map of the anomalous magnetic field  (AMF) of Russia has been created over 12 years in the monitoring (update) mode. The map was built from the level of the normal magnetic field Т n VSEGEI-1965  at a scale of 1: 2,500,000 using materials that were not previously involved in the process of summary mapping, taking into account modern digital technologies. The base of digital cartographic data contains grids on the network of 2,500 ×2,500 and 5,000×5,000 m and cartographic projects in * .mxd.
The anomalous component is of particular interest in the study of geodynamic processes and dynamic environments in the earth's crust and upper mantle. It is believed that the anomalous (short-wave or high-frequency) component, being a quasi-stationary (Lugovenko V.N., 1982) function of the general geomagnetic field, almost does not change over time. However, when calculating it, the primary role is played by the correct registration of the secular variation and the normal field, which change both in time and in space, and these changes are closely related to the dynamic processes inside the Earth. The works of T. Nagata (1969), F. Stacey (1974, 1977), Yu.P. Skovorodkin and L.S. Bezugloy (1980), V.A. Shapiro (1983) and others showed that the anomalous magnetic field of the Earth is also characterized by temporary changes associated with the dynamics of field sources, manifested in anomalies of the secular course. There is a connection between the secular variation anomalies and regional medium-scale anomalies. Within the Manchazh regional anomaly, the anomalous magnetic field increases monotonically at a rate of up to ±5 nT per year. It has been established that the source of the Manchazh anomaly is a block of rocks with increasing remanent magnetization, the mechanism of which is still unclear. The relationship between AMF changes with changes in the seismic regime and with individual earthquakes is evidenced by changes in the amplitudes of temporary changes in the local field from 5-8 nT at the Carpathian geodynamic test site and up to 30-80 nT during the Moneron earthquake on southern Sakhalin. Changes up to the first tens of nT AMFs were recorded several days before the Tashkent earthquake (Ulomov, 1967). During this earthquake, the author of this article observed the glow of the atmosphere, which indicates strong short-term changes in the variable geomagnetic field, which caused ionization processes in the surface layers of the atmosphere.
The Earth's magnetic field is 99% generated by its internal sources and reacts sensitively to nonequilibrium phase transitions of a different hierarchical class, which are the basis for the self-organization of the planet Earth system. On the map of magnetic anomalies of Russia, geostructures of different orders of rectilinear, circular, arcuate mosaic forms of anomalies are clearly distinguished, grouped into systems, the shape and size of which allows to reasonably judge the geodynamic conditions of their formation.

 

How to cite: Litvinova, T.: An updated summary digital map of the anomalous magnetic field of Russia as a base for geo-modeling, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3292, https://doi.org/10.5194/egusphere-egu21-3292, 2021.

EGU21-5305 | vPICO presentations | EMRP2.14

Shannon Information of the geomagnetic field during the excursions

Saioa A. Campuzano, Angelo De Santis, Martina Orlando, F. Javier Pavón-Carrasco, and Alberto Molina-Cardín

The Shannon Information or Information Content is a statistical measure of a system characterising its properties of organisation (maximum value) or disorder (minimum value). Once it is introduced the scalar potential of the geomagnetic field in terms of a spherical harmonic expansion, it is straightforward to define the Shannon Information by an expression including the Gauss coefficients [De Santis et al., EPSL, 2004]. Some recent models of the past geomagnetic field, including also the two most recent excursions, i.e. Laschamp (~41 ka) and Mono Lake (~34 ka) events, allow us to calculate the Shannon Information in the periods of those events and compare each other. It is expected that when approaching to excursions, the Shannon Information decreases, i.e. the disorder of the system increases. From the behaviour in time of the Shannon Information calculated from the Gauss coefficients of three geomagnetic field reconstructions that span the last excursions, i.e. IMOLE, GGF100k and LSMOD2, it is observed a decrease of the Shannon Information that seems to anticipate the occurrence of the impending excursions some time in advance. This result must be taken with caution because the reconstructions used are based on sedimentary data, which could present some smoothing effects related to the acquisition of the magnetisation mechanism.

How to cite: Campuzano, S. A., De Santis, A., Orlando, M., Pavón-Carrasco, F. J., and Molina-Cardín, A.: Shannon Information of the geomagnetic field during the excursions, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5305, https://doi.org/10.5194/egusphere-egu21-5305, 2021.

EGU21-4237 | vPICO presentations | EMRP2.14

Eccentric dipole of the geomagnetic field during the last reversal, last excursions, and the most significant Holocene anomalies

Alicia González-López, Saioa A. Campuzano, Pablo Rivera, Alberto Molina-Cardín, F. Javier Pavón-Carrasco, and M. Luisa Osete

The geomagnetic field is commonly approximated to a geocentric tilted dipole. However, a next step in the approach of the geomagnetic field is the eccentric dipole which takes the first and second terms of the spherical harmonic representation of the geomagnetic field. In this work, we analyze the behavior of the eccentric dipole during the last reversal (Matuyama – Brunhes, 780 ka), the last excursions (Laschamp, 41 ka, and Mono Lake, 34 ka), and during two interesting features of the geomagnetic field observed during the Holocene (the South Atlantic Anomaly, from 1840 AD or older, and the Levantine Iron Age Anomaly, around 1000 BC). The last reversal and excursions are studied by using the IMMAB4 and LSMOD2 paleoreconstructions, respectively. We found that for these events the center of the eccentric dipole follows a common longitude path. The Holocene anomalies have been analyzed by using two of the most up-to-date paleoreconstructions for the last 3 millennia: the SHAWQ2k and the SHAWQ Iron Age paleoreconstructions. A common longitude path has not been observed between these anomalies.

How to cite: González-López, A., A. Campuzano, S., Rivera, P., Molina-Cardín, A., Pavón-Carrasco, F. J., and Osete, M. L.: Eccentric dipole of the geomagnetic field during the last reversal, last excursions, and the most significant Holocene anomalies, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4237, https://doi.org/10.5194/egusphere-egu21-4237, 2021.

EGU21-12312 | vPICO presentations | EMRP2.14

Geophysical study of viability for a new geomagnetic observatory at São Marcos (Portugal)

M. Alexandra Pais, Paulo Ribeiro, Katia Pinheiro, Marcos Vinicius, and Juergen Matzka

The magnetic observatory of Coimbra (IAGA code COI) measures the geomagnetic field components since 1866. These long time series were disturbed by the urban expansion in Coimbra city, causing anthropogenic perturbations in particular to the high frequency spectral band of the vertical Z component.

We plan to move the observatory to another site with less magnetic disturbances. The new site is about 15 km west of Coimbra over limestone Cretaceous terrains, in São Marcos (SMC). It is far from important industrial facilities, high voltage lines, and DC electrified railways. This farm is under administration of the University of Coimbra and is surrounded by walls.  

Both magnetic surveys and electrical soundings were carried out at SMC. Results show low values for the magnetic gradients and resistivity profiles of relatively low to moderate values (~ 10-1000 ohm.m), typical of the studied lithological types (sandstone, marl and limestone). In this study we characterize the geomagnetic field as measured at SMC, in comparison with COI and SPT (San Pablo/Toledo) in Spain, the nearest observatory in the INTERMAGNET network of magnetic observatories.

How to cite: Pais, M. A., Ribeiro, P., Pinheiro, K., Vinicius, M., and Matzka, J.: Geophysical study of viability for a new geomagnetic observatory at São Marcos (Portugal), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12312, https://doi.org/10.5194/egusphere-egu21-12312, 2021.

EGU21-2446 | vPICO presentations | EMRP2.14

The Earth's magnetic field variations at the Algerian magnetic observatory of Tamanrasset from 1932 to 2020.

Lemgharbi Abdenaceur, Hamoudi Mohamed, Abtout Abdeslam, Abdelhamid Bendekken, Ener Aganou, Hemmi Abderrahman, and Mansouri Abdallah

In order to understand the spatial and temporal behavior of the Earth's magnetic field, scientists, following C.F. Gauss initiative in 1838 have established observatories around the world. More than 200 observatories aiming to continuously record, the time variations of the magnetic field vector and to maintain the best standard of the accuracy and resolution of the measurements.

This study focused on the acquisition and analysis of the magnetic data provided by the Algerian magnetic observatory of Tamanrasset (labelled TAM by the International Association of Geomagnetism and Aeronomy). This observatory is located in southern Algeria at 5.53°E longitude, 22.79°N Latitude. Its altitude is 1373 meters above msl. TAM is continuously running since 1932, using old brand variometers, like Mascart and La Cour with photographic recording at the very beginning. Nowadays modern electronic equipment are used in the framework of INTERMAGNET project. Very large geomagnetic database collected over a century is available. We will describe the history and the various improvement of the methods and instrumentation.

Preliminary analysis of time series of the observatory data allowed to distinguish two kinds of data: the first type, with low resolution, collected between 1932 and 1992. This data set comes from the annual, monthly, daily and hourly means. The second one with high resolution is represented by minutes and seconds sampling rate since 1993 when TAM was integrated to the world observatory network, INTERMAGNET. Part of the second dataset contains many gaps. We try to fill these gaps thanks to mathematical methods. Absolute measurements and repeat station data allow better accuracy in the secular variations and an improved regional model.

Keywords: TAM observatory, temporal variation, terrestrial magnetic field, secular variations, INTERMAGNET.

How to cite: Abdenaceur, L., Mohamed, H., Abdeslam, A., Bendekken, A., Aganou, E., Abderrahman, H., and Abdallah, M.: The Earth's magnetic field variations at the Algerian magnetic observatory of Tamanrasset from 1932 to 2020., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2446, https://doi.org/10.5194/egusphere-egu21-2446, 2021.

EGU21-8971 | vPICO presentations | EMRP2.14

Declination magnetic charts for Mexico since 1907.0 to 2010.0

Alejandro Paredes-Arriaga, Ana Caccavari-Garza, Esteban Hernández-Quintero, and Gerardo Cifuentes-Nava

We present the construction of magnetic declination charts corresponding to five epochs for last one hundred years; this was result of recovery and statistical analysis of historical magnetic data. The charts were made with the records of magnetic repeat stations reoccupation, distributed throughout the country, the goal was to observe and study the geomagnetic field morphology and their space-temporal variations in Mexico. We aimed to systematize an optimal numerical method for the spatial estimation to minimize the error given the average data for any Mexican magnetic chart: forty magnetic repeat stations and only one magnetic observatory. Also, the charts were compared with the original charts made in its corresponding epoch. The charts quality was improved and the historic geomagnetic information preserved, considering the invaluable record of historical magnetic measurements that exist in Mexico.

How to cite: Paredes-Arriaga, A., Caccavari-Garza, A., Hernández-Quintero, E., and Cifuentes-Nava, G.: Declination magnetic charts for Mexico since 1907.0 to 2010.0, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8971, https://doi.org/10.5194/egusphere-egu21-8971, 2021.

EGU21-9106 | vPICO presentations | EMRP2.14

Imaging the Mt. Melbourne Volcanic Field (Northern Victoria Land, Antarctica): a Hamiltonian Monte Carlo approach applied to high-resolution aeromagnetic data

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

The Mt. Melbourne Volcanic Complex (MMVC) is located in Northern Victoria Land (Antarctica) along the western flank of the West Antarctic Rift System, at the boundary with the Transantarctic Mountains. It is constituted by two main volcanic areas, i.e. the Mt. Melbourne Edifice (MME) and the Cape Washington Shield (CWS), and some other minor centres.

To date, the inner structure of this volcanic complex is still poorly known, being the direct geological information on site confined to either glacial erratics or a few rock outcrops not hidden by the ice sheet. Consequently, even the temporal building up and evolution of the MMVC as well as its primary magmatic source are still under investigation (debated).

Recently, we attempted to define the geological structure of the MMVC by means of digital enhancement and forward modeling performed on a high-resolution aeromagnetic dataset (Ghirotto et al. 2020, EGU). Coupling both information derived from past geological/geophysical studies and unpublished magnetic susceptibility measurements from rock samples collected in the field, we proposed two models to explain the chronological evolution of the MME and CWS. These models involve either i) major magmatic events occurred in periods of both normal and reverse magnetic polarity or ii) only magmatic flows with normal polarity.

To gain further insights into the geological structure and the geodynamic evolution of the MMVC in relation to the two proposed models, we develop here a Hamiltonian Monte Carlo (HMC) algorithm (Fichtner et al. 2018) based on the probabilistic approach to inverse problems. To date, this methodology has never been applied to aeromagnetic data for geological studies. In detail, the above proposed models provide some soft a priori information from which to start exploring potential solutions. The parameterization of the volcanic area is defined in terms of 2-D polygonal bodies, representing e.g. magmatic lava flows, where the unknown parameters are represented by both the position of the vertices and/or the magnetization (induced and/or remnant), resulting in a non-linear forward model. The HMC algorithm requires the computation of gradients of the posterior probability density (PPD), i.e., derivatives of the objective functional with respect to the position of vertices of the bodies and magnetization, in order to better move the inversion process toward high-probability areas in the model space manifold. We implement such calculations using automatic differentiation, a tool which is very accurate and fast compared to other approaches such as finite difference. The result of the inversion is then a collection of models representing the PPD, from which statistical analysis can provide measures of uncertainty and plausible geological scenarios.

In this study we present some preliminary results of applying the above-mentioned methodology, which finally could help unravel the framework of the MMVC.

How to cite: Ghirotto, A., Zunino, A., Armadillo, E., Crispini, L., and Ferraccioli, F.: Imaging the Mt. Melbourne Volcanic Field (Northern Victoria Land, Antarctica): a Hamiltonian Monte Carlo approach applied to high-resolution aeromagnetic data, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9106, https://doi.org/10.5194/egusphere-egu21-9106, 2021.

There has long been interest in the sources of ionospheric variability, especially in how this variability can be caused by the dynamics of the neutral atmosphere. Many studies have found planetary wave fluctuations in ionospheric parameters, especially f0F2, with periods ranging from 2 to 20 days. We investigate variations of the geomagnetic field and critical frequency of the F2-layer in the range of planetary waves in winter. We used the data of geomagnetic monitoring at the Central Geophysical Observatory at Belsk of Institute of Geophysics of the Polish Academy of Sciences (Poland, Belsk) and the results of high-frequency sounding of the ionosphere in the form of ionograms obtained by the Space Research Center of the Polish Academy of Sciences (Poland, Warsaw).  In the spectra of temporal variations of the geomagnetic field and the critical frequency of the ionospheric F2 layer in the range of planetary waves periods in winter season, we found both harmonics associated with the modulation effect of longer-period (annual and 11-year) variations and tidal effects, and a harmonic corresponding to a quasi 16-day planetary wave. Possible mechanisms of their manifestations in the upper atmosphere are discussed.

How to cite: Riabova, S. and Shalimov, S.: Investigation of the ionospheric plasma parameters variations in the range of planetary waves periods using Warsaw ionosonde and Belsk observatory data, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14185, https://doi.org/10.5194/egusphere-egu21-14185, 2021.

EGU21-11937 | vPICO presentations | EMRP2.14

Micromagnetic simulation of taenite particles

José Devienne, Thomas Berndt, and Wyn Williams

The cloudy zone (CZ), an intergrowth structure of Fe-rich and Ni-rich phases that forms during slow cooling of iron meteorites are potential recorders of  their parent body’s thermal and magnetic history. The ability of the cloudy zone’s principal magnetic minerals, taenite and tetrataenite, to reliably record ancient magnetic fields from the early solar system has, however, insufficiently been investigated. In this work we performed a series of micromagnetic simulations in order to assess the magnetic stability of taenite grains. Micromagnetic simulations allow to investigate the changes in the magnetic state in taenite as a function of the grain size: in ellipsoidal grains below 68 nm (equivalent sphere volume diameter, ESVD) a single domain state dominates.  At 68 nm (ESVD) a “flowering” state starts, and further increase in size (> 75 nm) gives rise to a single vortex state. Contrary to common conception, theoretical evaluation of relaxation times for taenite grains based on micromagnetics leads to values that exceed the age of solar system, which makes taenite, not just its ordered equivalent tetrataenite, a reliable paleomagnetic recorder.

How to cite: Devienne, J., Berndt, T., and Williams, W.: Micromagnetic simulation of taenite particles, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11937, https://doi.org/10.5194/egusphere-egu21-11937, 2021.

EGU21-7173 | vPICO presentations | EMRP2.14

Kalmag: a high spatio temporal model of the Geomagnetic field

Julien Baerenzung and Matthias Holschneider

We present a new high resolution model of the Geomagnetic field spanning the last 121 years. The model derives from a large set of data taken by low orbiting satellites, ground based observatories, marine vessels, airplane and during land surveys. It is obtained by combining a Kalman filter to a smoothing algorithm. Seven different magnetic sources are taken into account. Three of them are of internal origin. These are the core, the lithospheric  and the induced / residual ionospheric fields. The other four sources are of external origin. They are composed by a close, a remote and a fluctuating magnetospheric fields as well as a source associated with field aligned currents. The dynamical evolution of each source is prescribed by an auto regressive process of either first or second order, except for the lithospheric field which is assumed to be static. The parameters of the processes were estimated through a machine learning algorithm with a sample of data taken by the low orbiting satellites of the CHAMP and Swarm missions. In this presentation we will mostly focus on the rapid variations of the core field, and the small scale lithospheric field.  We will also discuss the nature of model uncertainties and the limitiations they imply.

How to cite: Baerenzung, J. and Holschneider, M.: Kalmag: a high spatio temporal model of the Geomagnetic field, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7173, https://doi.org/10.5194/egusphere-egu21-7173, 2021.

EGU21-10207 | vPICO presentations | EMRP2.14

Ionospheric precursors of earthquakes from satellites

Angelo De Santis, Saioa A. Campuzano, Gianfranco Cianchini, Domenico Di Mauro, Dedalo Marchetti, Martina Orlando, Loredana Perrone, Alessandro Piscini, Dario Sabbagh, Maurizio Soldani, Xuhui Shen, and Zeren Zhima

In-situ magnetic field and electron density, as observed by Swarm and CSES satellites, are analyzed to identify possible anomalies in geomagnetic quiet time with respect to the ionospheric background. To avoid detecting possible anomalies induced by auroral activity we investigate regions between +50 and -50 degrees in magnetic latitude. Then a superposed epoch and space approach is applied to this anomaly dataset with respect to their time and space distance from shallow M5.5+ earthquakes occurred in about last 6 years. A comparison with analogous homogeneous random distribution of anomalies shows that the real anomaly concentrations found before the occurrence of earthquakes are statistically significant. In addition, we find that, in general, the anticipation times of the ionospheric precursors scale with the earthquake magnitude, confirming the validity of the Rikitake law for ionospheric signals, previously valid for ground precursors. We also find that the anomaly duration seems to depend on the magnitude of the impending earthquake. Finally, we propose a simple scheme of potential earthquake forecast on the base of the previously mentioned characteristics.

How to cite: De Santis, A., Campuzano, S. A., Cianchini, G., Di Mauro, D., Marchetti, D., Orlando, M., Perrone, L., Piscini, A., Sabbagh, D., Soldani, M., Shen, X., and Zhima, Z.: Ionospheric precursors of earthquakes from satellites, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10207, https://doi.org/10.5194/egusphere-egu21-10207, 2021.

EGU21-8931 | vPICO presentations | EMRP2.14

A preliminary investigation of dynamical complexity in Swarm Dst-like time series using information theory techniques

Georgios Balasis, Constantinos Papadimitriou, Stelios M. Potirakis, Adamantia Zoe Boutsi, Ioannis A. Daglis, Omiros Giannakis, Paola De Michelis, and Giuseppe Consolini

For 7 years now, the European Space Agency’s Swarm fleet of satellites surveys the Earth’s magnetic field, measuring magnetic and electric fields at low-Earth orbit (LEO) with unprecedented detail. We have recently demonstrated the feasibility of Swarm measurements to derive a Swarm Dst-like index for the intense magnetic storms of solar cycle 24. We have shown that the newly proposed Swarm Dst-like index monitors magnetic storm activity at least as good as the standard Dst index. The Swarm derived Dst index can be used to (1) supplement the standard Dst index in near-real-time geomagnetic applications and (2) replace the ‘prompt’ Dst index during periods of unavailability. Herein, we employ a series of information theory methods, namely Hurst exponent and various entropy measures, for analyzing Swarm Dst-like time series. The results show that information theory techniques can effectively detect the dissimilarity of complexity between the pre-storm activity and intense magnetic storms (Dst < 150 nT), which is convenient for space weather applications.

How to cite: Balasis, G., Papadimitriou, C., Potirakis, S. M., Boutsi, A. Z., Daglis, I. A., Giannakis, O., De Michelis, P., and Consolini, G.: A preliminary investigation of dynamical complexity in Swarm Dst-like time series using information theory techniques, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8931, https://doi.org/10.5194/egusphere-egu21-8931, 2021.

EGU21-15443 | vPICO presentations | EMRP2.14

Swarm Mission: instruments performance, data availability, quality and future evolutions

Enkelejda Qamili, Filomena Catapano, Lars Tøffner-Clausen, Stephan Buchert, Christian Siemes, Anna Mizerska, Jonas Bregnhøj Nielsen, Thomas Nilsson, Jan Miedzik, Maria Eugenia Mazzocato, Lorenzo Trenchi, Jerome Bouffard, Anja Stromme, Pierre Vogel, and Berta Hoyos ortega

The European Space Agency (ESA) Swarm mission, launched on November 2013, continue to provide very accurate measurements of the strength, direction and variation of the Earth’s magnetic field. These data 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 a status update of the Swarm Instrument performance after seven years of operations. Moreover, we will provide full details on the new Swarm Level 1b product baseline of Magnet and Plasma data which will be generated and distributed soon to the whole Swarm Community.  Please note that the main evolutions to be introduced in the Swarm L1B Algorithm are: i) computation of the Sun induced magnetic disturbance (dB_Sun) on the Absolute Scalar Magnetometer (ASM) and Vector Field Magnetometer (VFM) data; ii) computation of systematic offset between Langmuir Probes (LP) measurements ad ground observations derived from Incoherent Scatter Radars (IRS) located at middle, low, and equatorial latitudes. These and further improvements are planned to be included in the upcoming versions of the Swarm Level 1b products, aiming at achieving the best data quality for scientific applications.

How to cite: Qamili, E., Catapano, F., Tøffner-Clausen, L., Buchert, S., Siemes, C., Mizerska, A., Bregnhøj Nielsen, J., Nilsson, T., Miedzik, J., Mazzocato, M. E., Trenchi, L., Bouffard, J., Stromme, A., Vogel, P., and Hoyos ortega, B.: Swarm Mission: instruments performance, data availability, quality and future evolutions, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15443, https://doi.org/10.5194/egusphere-egu21-15443, 2021.

ESA's Swarm mission continues to deliver excellent data providing insight into a wide range of geophysical phenomena. The mission is an important asset whose data are used within a number of critical resources, from geomagnetic field models to space weather services. As the product portfolio grows to better deliver on the mission's scientific goals, we face increasing complexity in accessing, processing, and visualising the data and models. ESA provides “VirES for Swarm” [1] (developed by EOX IT Services) to help solve this problem. VirES is a web-based data retrieval and visualisation tool where the majority of Swarm products are available. VirES has a graphical interface but also a machine-to-machine interface (API) for programmable use (a Python client is provided). The VirES API also provides access to geomagnetic ground observatory data, as well as forwards evaluation of geomagnetic field models to give data-model residuals. The "Virtual Research Environment" (VRE) adds utility to VirES with a free cloud-based JupyterLab interface allowing scientists to immediately program their own analysis of Swarm products using the Python ecosystem. We are augmenting this with a suite of Jupyter notebooks and dashboards, each targeting a specific use case, and seek community involvement to grow this resource.

[1] https://vires.services

How to cite: Smith, A. and Pačes, M.: VirES for Swarm & Virtual Research Environment: Software, guides & infrastructure to boost accessibility of Swarm, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9272, https://doi.org/10.5194/egusphere-egu21-9272, 2021.

EMRP3.1 – Magnetic Chronostratigraphy

The extent of Greater India with precise and accurate chronological control is a key issue that concerns the spatio-temporal pattern and tectonic models of the India-Asia collision. Here we carried out a detailed magnetostratigraphic and paleomagnetic study on the Upper Cretaceous oceanic red beds (CORBs) (Chuangde Formation) exposed in the Tethyan Himalaya terrane. The high temperature (650‒690°C) magnetic components are isolated from two separated sections at Cailangba and display both normal and reverse polarities, which were used to construct magnetic polarity sequences of the sections that can be subsequently correlated to the geomagnetic polarity time scale (GPTS) to better estimate the age of the rocks. With the aid of previously published biostratigraphy by Chen et al. (2011, Sedimentary Geology), the polarity magnetozones of the Cailangba B section are correlated to chron C32r.2r (74.3–74.0 Ma) and the upper part of chron C33n (79.9–74.3 Ma), and the single normal polarity magnetozone of the Cailangba A section is correlated to the upper part of chron C33n (79.9–74.3 Ma). As a result, the CORBs in the Cailangba A and B sections represent the time interval of 76.2–74.0 Ma by magnetobiostratigraphy. Two independent methods of inclination shallowing correction were tested, which all indicate a bias inclination of ~70%. After inclination shallowing correction, the mean inclination increased to ‒35.0°, giving what we propose to be a high-quality Late Cretaceous paleopole of 40.8°N/256.3°E, A95 =1.8°. Our findings indicate that the Indian passive continental margin was situated at a paleolatitude of 19.4° ± 1.8°S at ~75 Ma. These data suggest that Greater India extended about 715 ± 374 km farther north from the present northern margin of India in the Late Cretaceous, implying a latitudinal width of 3641 ± 308 km for the Neo-Tethys Ocean that still separated the Lhasa terrane of southern part of the Asian plate and the Greater India.

How to cite: Yuan, J., Yang, Z., and Deng, C.: Paleomagnetism of the Upper Cretaceous oceanic red beds in southern Tibet, China: Implications for the extent of Greater India at ~75 Ma, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3685, https://doi.org/10.5194/egusphere-egu21-3685, 2021.

Deep-sea sediment sometimes lacks biostratigraphic or radiometric age constraints. Chemical stratigraphy and magnetostratigraphy is useful for dating it. Oxic pelagic clay contains Fe-Mn oxyhydroxides that can retain seawater 187Os/188Os values, and its age can be estimated by fitting the isotopic ratios to the seawater 187Os/188Os curve. On the other hand, the stability of Fe-Mn oxyhydroxides is sensitive to redox change, and it is not clear whether the original 187Os/188Os values are always preserved in sediments. However, due to the lack of independent age constraints, the reliability of 187Os/188Os ages of pelagic clay have never been tested. Magnetostratigraphy is often unsuccessful for pelagic clay older than a few Ma, which has been attributed to diagenesis. Here we report multiple polarity reversals in ca. 35 Ma pelagic clay around Minamitorishima Island, which is inconsistent with a 187Os/188Os age model. In a ~5 m thick interval, previous studies correlated 187Os/188Os data to a brief (<1 million years) isotopic excursion in the late Eocene. Paleomagnetic measurements revealed at least 12 polarity zones in the interval, indicating a >2.9 – 6.9 million years duration. Quartz and feldspars content showed that while the paleomagnetic chronology gives reasonable eolian flux estimates, the 187Os/188Os chronology leads unrealistically high values. These results suggest that the low 187Os/188Os signal has diffused from an original thin layer to the current ~5 m interval, causing an underestimate of the deposition duration. The preservation of the polarity patterns indicates that a mechanical mixing such as bioturbation cannot be the main process for the diffusion, so diagenetic re-distribution of Fe-Mn oxyhydroxides and associated Os may be responsible. The paleomagnetic chronology presented here also demands reconsiderations of the timing, accumulation rate, and origins of the high content of rare-earth elements and yttrium in pelagic clay around Minamitorishima Island. It is also indicated that old oxic pelagic clay can be a faithful paleomagnetic recorder, and success of magnetostratigraphy depends on sedimentation rate and polarity length rather than diagenesis.

Usui, Y., Yamazaki, T. Earth Planets Space 73, 2 (2021). https://doi.org/10.1186/s40623-020-01338-4

How to cite: Usui, Y. and Yamazaki, T.: Magnetostratigraphic evidence for post-depositional distortion of osmium isotopic records in pelagic clay: implications for mineral flux estimates, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3695, https://doi.org/10.5194/egusphere-egu21-3695, 2021.

EGU21-6381 | vPICO presentations | EMRP3.1

A new reference loess-paleosol archive spanning the last 850 kyrs near Pleven (Bulgaria) – first results

Christian Laag, Diana Jordanova, France Lagroix, Neli Jordanova, and Yohan Guyodo

Loess-paleosol sequences (LPSs) are proven valuable archives for continental paleoclimatic reconstructions. However, studied LPSs worldwide, spanning multiple glacial-interglacial cycles, are seldomly sampled and analyzed at a continuous high resolution. Exceptionally, in a quarry setting near the city of Pleven (Bulgaria), a new LPS, with a thickness of 27 m, was continuously sampled at a 2 cm resolution resulting in 1340 bulk-samples. We present herein first rock magnetic results suggesting that the site archives aeolian deposition and soil formation over the last 850 kyrs.  Room temperature bulk mineral magnetic parameters including magnetic susceptibility, hysteresis loop derived parameters, IRM, and ARM (underway) were acquired on all samples. Variations in mineral magnetic data clearly show the alternation of strongly developed paleosols overlying loess units indicative of interglacial and glacial climate cycles. We created a correlative age model by comparing Xferri/Ms to inverted LR04 benthic oxygen isotope ratios and adjustments undertaken by the Imbrie & Imbrie ice model. This initial correlative age model leads to an assumed continuous dust accumulation for the last 850 kyrs, from MIS 19 to present. In addition to the regionally widely observed L2-tephra, which is observed outcropping along the Pleven LPS, several other sharp spikes in concentration dependent magnetic characteristics suggest that the sedimentary record had preserved also other tephra layers, clearly identified in the magnetic record due to the accomplished high-resolution sampling design. Additional geochemical and mineralogical data are however necessary for an unequivocal source (age) identification of these events. A tentative scheme of a possible correspondence with well dated tephra layers from sedimentary core at Fucino Basin is established. It implies the occurrence of westerly wind directions during the last 850 kyrs in SE Europe. In summary, the Pleven LPS provides new insights into late-Pleistocene climatic regimes, prevailing wind directions and preservation of tephra layers, essential for further correlative terrestrial-aeolian-coupled age models, regional stratigraphic correlations and paleoclimate reconstructions.

How to cite: Laag, C., Jordanova, D., Lagroix, F., Jordanova, N., and Guyodo, Y.: A new reference loess-paleosol archive spanning the last 850 kyrs near Pleven (Bulgaria) – first results, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6381, https://doi.org/10.5194/egusphere-egu21-6381, 2021.

EGU21-9036 | vPICO presentations | EMRP3.1

Devonian magnetostratigraphy: new data and old problems

Annique van der Boon, Andy Biggin, Daniele Thallner, Mark Hounslow, Jerzy Nawrocki, Kristyan Wójcik, Mariusz Paszkowski, Peter Königshof, Tim de Backer, Pavel Kabanov, Sofie Gouwy, Richard Vandenberg, and Richard Bono

The global polarity time scale (GPTS) is relatively unconstrained for the Paleozoic, particularly the Devonian. Constraining the GPTS and reversal frequency for the Devonian is crucial for the understanding of the behaviour of Earth’s magnetic field. Furthermore, construction of a GPTS for the Paleozoic could provide a valuable tool for age determination in other studies. However, most paleomagnetic data from the Devonian is problematic. The data are difficult to interpret and don’t have a single easy to resolve (partial or full) overprint. Paleointensity studies suggest that the field was much weaker than the field of today, which could have been accompanied by many reversals (a hyperreversing field). In order to improve the geomagnetic polarity time scale in the Devonian, and quantify the number of reversals in this time, we sampled three Devonian sections in Germany, Poland and Canada. These sections show evidence that the rocks were not significantly heated, and they show little evidence for remineralisation. This minimises the chance the rocks were remagnetised after the Devonian. Our data show that even when rocks are well qualified to have reliably recorded the Devonian field, the interpretation is not straightforward. We also discuss problems with the Devonian GPTS as presented in the geologic timescale.

How to cite: van der Boon, A., Biggin, A., Thallner, D., Hounslow, M., Nawrocki, J., Wójcik, K., Paszkowski, M., Königshof, P., de Backer, T., Kabanov, P., Gouwy, S., Vandenberg, R., and Bono, R.: Devonian magnetostratigraphy: new data and old problems, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9036, https://doi.org/10.5194/egusphere-egu21-9036, 2021.

EGU21-9682 | vPICO presentations | EMRP3.1

New advances on the magnetic chronostratigraphy of cave sediments in Atapuerca Gran Dolina, Spain

Josep M Pares, Mathieu Duval, Isidoro Campaña, José M. Bermúdez de Castro, and Eudald Carbonell

Magnetostratigraphy has proven to be a powerful and versatile method as well the first line of defence for dating sediments. When properly anchored to the Geomagnetic Polarity Time Scale (GPTS), chron boundaries provide a basis for numerical dating by correlating the local magnetostratigraphy to the GPTS. A caveat and intrinsic limitation when anchoring magnetic stratigraphy to the GPTS is that we deal with essentially a binary code, a sequence of normal and reverse polarity zones. To overcome such limitation biostratigraphy or (ideally) numerical (absolute) age dating is required. Unfortunately, numerical dating of sediments is typically hampered by the lack of amenable minerals for the application of standard methods such as Ar-Ar, requiring thus the use of less conventional methods. Burial dating is possible using methods such as Electron Spin Resonance (ESR) on optically bleached quartz grains. Similar to luminescence, ESR is a paleodosimetric method that provides the time elapsed since the last exposure of quartz grains to natural sun light. Cave sediments are particularly amenable for paleodosimetric methods, as sediments are preserved in the dark and the ESR signal should survive over the geologic history of the deposits. On the down side, we date the moment when the quartz grain enters the karst system, not its deposition. If the transit time is too long, this might be an issue and we would be significantly overestimating the true burial age. Caves at Atapuerca (N Spain) hold the richest Quaternary paleontological record in Eurasia, including fossils and lithic tools. Sediments in these caves have been traditionally dated via magnetostratigraphy by identifying the Matuyama-Brunhes reversal (0.78 Ma) thus providing the Lower to Middle Pleistocene boundary. Nevertheless, the appearance of older sediments in the caves required the combination of paleomagnetism with methods such as ESR to interpret older intra-Matuyama Subchrons. In the deepest levels of the Gran Dolina cave, close to the floor of the cavity, a number of short intervals of normal polarity have been identified in the fluviatile sediments belonging to TD1 unit, which we interpret in terms of Subchrons using ESR ages of quartz grains. We will discuss both paleomagnetic data and interpret the magnetic polarity stratigraphy in the view of the ESR ages obtained from the Multiple Centre (MC) approach. 

How to cite: Pares, J. M., Duval, M., Campaña, I., Bermúdez de Castro, J. M., and Carbonell, E.: New advances on the magnetic chronostratigraphy of cave sediments in Atapuerca Gran Dolina, Spain, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9682, https://doi.org/10.5194/egusphere-egu21-9682, 2021.

EGU21-12503 | vPICO presentations | EMRP3.1

Magnetostratigraphy from the Hominin Sites Paleolakes Drilling Project (HSPDP) drill cores, low latitudes reorientation

Mark Jan Sier, Guillaume Dupont-Nivet, Cor Langereis, and Andy Cohen and the HSPDP science team

The Hominin Sites Paleolakes Drilling project (HSPDP) has collected around 2000 meters of drilled cores in lake sediment in Kenya and Ethiopia. All cores were drilled near important sites in human evolution with as main goal to help us better understand the influence of climate change on our evolutionary past.

An important first step in this research is building an age-model for these cores with magnetostratigraphy being important building block. However, building a magnetostratigraphy for the HSPDP cores is not straightforward. Due to the rotational movement of the coring process the azimuthal orientations of the cores is lost. This hinders the construction of magnetostratigraphy based of correctly orientated paleomagnetic samples. For high latitudes a high quality magnetostratigraphy can be reconstructed on the basis of the inclination of the paleomagnetic direction.

However, at low latitudes near the equator the inclination of the (paleo) magnetic field are near zero. As a result a magnetostratigraphy on the basis of inclination alone cannot be made.

In this presentation we discuss two methods that can be used to build a core based magnetostratigraphy at low latitudes. First, the anisotropy of the magnetic susceptibility (AMS) can be used in certain cases to reorientate the paleomagnetic samples by identifying the bedding of the sediments throughout the core.

Second, the present/recent low temperatures –low coercivity (LT/LC) overprint can be used to reorientate the paleomagnetic directions by orientating these LT/LC components towards the north and recalculate the paleomagnetic directions.

Both methods have been used on the ICDP Hominin Sites Paleolakes Drilling Project (HSPDP) cores taken in Ethiopia and Kenia with varied success. Here we will present data of four HSPDP cores as case study to help illustrate the effectiveness of these two methods for building a magnetostratigraphy for low latitude cores.

How to cite: Sier, M. J., Dupont-Nivet, G., Langereis, C., and Cohen, A. and the HSPDP science team: Magnetostratigraphy from the Hominin Sites Paleolakes Drilling Project (HSPDP) drill cores, low latitudes reorientation, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12503, https://doi.org/10.5194/egusphere-egu21-12503, 2021.

EGU21-13264 | vPICO presentations | EMRP3.1

The record of the Eocene-Oligocene Transition and the “Grande Coupure”. Magnetostratigraphic constraints from the Ebro Basin revisited.

Miguel Garcés, Elisabet Beamud, Miguel López-Blanco, Manuel Gómez, Elisenda Costa, Alberto Sáez, and Luis Cabrera

Magnetostratigraphy is the key to put disparate chronological pieces together in a consistent chronostratigraphic framework. Provided that a long continuous record of reversals can be obtained from the sedimentary record, a correlation with the GPTS may be established. Magnetostratigraphy provides added value to the chronology as long as it keeps certain independence from external age constraints, such as bioevents calibrated elsewhere or radiochronologic data.

An independent correlation is meant to not be anchored to a given chron on the basis of an external age constrain. Our experience recommends that external age constraints are best taken with flexibility, allowing for the searching of a best fit between the magnetic polarity sequence (in meters) and the GPTS (in million-years). This rationale relies on the fact that the Geological Time Scale is the tool that allows earth-scientist of many varied disciplines to understand and discuss about the dimension of time. But the time scale calibration is a task in continuous refinement. As the accuracy and precision of the dating tools increases, our ability to unravel lag times in geological processes increases too. As more refined data is produced, the calibration of the time scale reveals as an ongoing task rather than a final product.

Here we present the case of the Eocene-Oligocene Transition (EOT) as recorded in alluvial-lacustrine sediments of the eastern Ebro Basin. An earlier work provided a magnetostratigraphic correlation that was in agreement with small-mammals biostratigraphic data. A key constraint to this study was the Santpedor locality, which yielded a characteristic post-Grand Coupure small mammal assemblage, then attributed to the lowest Oligocene.

An extended record of the magnetostratigraphy has challenged the earlier correlation and puts forward an alternate scenario that reveals a misfit with earlier and recent biochronological interpretations of the fossil mammal record. The significance of this discrepancy in terms of heterochrony of biostratigraphic events, the punctuated character of faunal replacement across the EOT, and time lags between the marine and continental realms may need to be addressed.

How to cite: Garcés, M., Beamud, E., López-Blanco, M., Gómez, M., Costa, E., Sáez, A., and Cabrera, L.: The record of the Eocene-Oligocene Transition and the “Grande Coupure”. Magnetostratigraphic constraints from the Ebro Basin revisited., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13264, https://doi.org/10.5194/egusphere-egu21-13264, 2021.

A detailed paleomagnetic study of Tarkhanian sediments of Skelya section was carried out with the goal to obtained magnetostratigraphy data. The Skelya section is located on the Azov sea side of Kerch peninsula, Crimea (45o42′N, 36o53′E). The Tarkhanian sediments of Skelya section are represented mainly of clays and have a total thickness of ~ 100 m. According to GTS (2012), the Tarkhanian stage of Miocene is related to the lower part of the Langhian of the General Stratigraphic Scale. Standard paleomagnetic measurements have been carried out to investigate magnetic parameters: natural remenent magnetization, magnetic susceptibility, saturation remanent magnetization, anhysteretic   remanent magnetization varied through out the section. The remanent coercitivity force, determined from backfield demagnetization measurements, range between ~34 and 67 mT.   The composition of the ferromagnetic fraction was examined using temperature dependences of saturation remanent magnetic moment. The thermomagnetic analysis showed that the blocking temperatures are  about 320 oC and 410-470 oC and  greigite and titanomagnetite  are the main carriers of NRM in the section.  The biplot of  IRM-100 mT / SIRM versus  ARM40mT /SARM showed that the ratios fall down into the field around the titanomagnetite and greigite areas. The pseudo-single domain  state of titanomagnetite and greigite was determined from their Mrs/Ms and Bcr/Bc ratios by Day-plot. Paleomagnetic studies have shown that the interval of the Kuvinian beds in its upper part is composed of sediments of reversal polarity magnetization. The rocks of the Terskian and Argunian beds are characterized by intervals of normal and reversed polarity magnetization. This work was supported by Russian Science Foundation, project № 19-77-10075.

How to cite: Pilipenko, O. and Rostovtseva, Y.: Paleomagnetism and magnetostratigraphy of Tarkhanian sediments of the Eastern Paratethus (Skelya section, Kerch peninsula, Crimea)  , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14134, https://doi.org/10.5194/egusphere-egu21-14134, 2021.

EGU21-15419 | vPICO presentations | EMRP3.1

New and revised magnetostratigraphic age constraints on the Akchagylian (late Pliocene) five-fold expansion of the Caspian Sea.

Sergei Lazarev, Klaudia Kuiper, Oriol Oms, Maia Bukhsianidze, Davit Vasilyan, Elisabeth Jorissen, and Wout Krijgsman

The late Pliocene Akchagylian transgression in the Caspian Basin led to a five-fold increase of the Caspian Sea surface water, extending the basin to the vast areas of Central Asia, Caucasian foreland (Kura Basin) and the Russian Plate. It also changed the regional climatic conditions by making the Pliocene glaciation milder. Later, establishment of hydrological connection between the Caspian Sea and the global ocean known as the “Akchagylian flooding” enabled active fauna migrations transforming the paleoecology of the region. Despite a relatively well constrained palaeoenvironmental history, the Akchagylian still lacks a univocal age model and two major age constraints exist - the “long” (3.6-1.8 Ma) and the “short” Akchagylian (2.7-2.1 Ma). In this study, we resolve the age contradictions by magnetostratigraphic and 40Ar/39Ar dating of several sections in the Kura Basin. With our new data, we further revise magnetostratigraphy and 40Ar/39Ar constraints in 25 sections across the Kura Basin and Turkmenistan. We propose a new unified age model for the Akchagylian Stage: 1. Akchagylian transgression at 2.95±0.02 Ma; 2. Caspian-Arctic connection (2.75–2.45 Ma); 3. “Desalinated” Akchagylian between 2.45-2.13 Ma; 4. Akchagylian-Apsheronian boundary at 2.13 Ma correlated to the Reunion subchron (C2r.1n). Our data shows, that magnetostratigraphy requires a careful assessment of sedimentation rates and support from other proxies such as sedimentology, biostratigraphy and radioisotopic dating. The new ages constrain a much shorter (2.95–2.1 Ma) Akchagylian than in previously mentioned regional geological time scales (3.6–1.8 Ma) and strongly appeal to reconsider the ages of numerous archaeological and mammalian sites in the south Caspian region.

How to cite: Lazarev, S., Kuiper, K., Oms, O., Bukhsianidze, M., Vasilyan, D., Jorissen, E., and Krijgsman, W.: New and revised magnetostratigraphic age constraints on the Akchagylian (late Pliocene) five-fold expansion of the Caspian Sea., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15419, https://doi.org/10.5194/egusphere-egu21-15419, 2021.

EGU21-15845 | vPICO presentations | EMRP3.1

Duration of the Carbon Isotope Excursion in the Zhuya Group (Patom Basin, South Siberia)

Dmitriy Rud'ko, Sergey Rud'ko, Andrey Shatsillo, Boris Pokrovskiy, Ivan Fedyukin, Irina Latysheva, and Aleksander Rimskiy

Probably the largest negative δ13С anomaly in Earth history called the “Shuram” excursion (SE) had taken place in the Ediacaran period. Determining the duration of SE is needed to resolve its nature and for the stratigraphic correlation of Ediacaran rocks. The cyclostartigraphic method allows to precisely determine the accumulation rates of ancient deposits (the theoretical error is up to 10,000 years), but the testing of the accuracy of the cyclostratigraphic method usually based on biostratigraphy and geochronology meets difficulties for the Precambrian deposits. The reliability of cyclostartigarphic estimates of the SE duration can be determined by the convergence of cyclostratigraphic results obtained from distant sections on different continents and in sections representing different depositional environments. Recently limitations on the SE duration have been obtained in Australia, California, Oman, and China. Recently limitations on the SE duration have been obtained in Australia, California, Oman, and China. Here we present the first cyclostratigraphic estimates of the SE duration from the Zhuya Group of the Patom basin in South Siberia.

Two sections of the Zhuya Group were studied, both recording the decrease of the δ13С values up to -12 ‰  in the nadir point and then increase till -9 ‰. In both sections, the cyclicity of variations in magnetic susceptibility (MS) was studied. The first section (57 m, Nikolskoe Fm.) represents sediments deposited on the slope of the carbonate platform. Spectral analysis of the MS variations revealed peaks above 95% significance level on the period lengths of 11.5, 1.73, 1.04, 0.67, 0.51 m with ratios 1/6.6 /11/17/22.3 respectively. This cyclicity is interpreted as a reflection of orbitally forced climate changes, where the longest-period variations correspond to short eccentricity cycles (100 ky). Then, the studied interval lasted approximately 500 ky, and the duration of the entire Nikolskaya Fm., corresponding to the lower third of the SE, is about 2.5 My.

The second section belongs to the Torgo Fm. in the Berezovskaya depression, which is the epicontinental part of the Patom Basin. MS variations in the studied 14.2 m interval shows significant peaks at period lengths of 2.3, 0.74, 0.51, 0.38, 0.28, 0.27, 0.25, 0.20 m with ratios 1.00/3.13/4.52/6.10/8.03/8.48/9.19/11.52. In this section, we also interpret the longest-period of the MS variations as a reflection of cycles of short eccentricity (100 ka). Then, the duration of the studied interval is 613 ky.  and the SE duration in the whole 200 meters of the Torgo Fm. is estimated as 8.6 My.

 

The obtained preliminary results are in good agreement with those from Australia (ca. 8 My), Oman (7.7 +/- 0.2 My), North America (8.2 +/- 1.2 My), and China (9.1 +/- 1 My). Thus, the influence of Milankovitch's orbital cycles on the formation of carbonate deposits of the Late Precambrian seems to be quite convincing, and the cyclostratigraphic estimate of the duration of the SE about 10 Ma is more and more reliable.

Research supported by the Russian Science Foundation (project № 20-77-10066)

How to cite: Rud'ko, D., Rud'ko, S., Shatsillo, A., Pokrovskiy, B., Fedyukin, I., Latysheva, I., and Rimskiy, A.: Duration of the Carbon Isotope Excursion in the Zhuya Group (Patom Basin, South Siberia), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15845, https://doi.org/10.5194/egusphere-egu21-15845, 2021.

EGU21-15933 | vPICO presentations | EMRP3.1

Magnetostratigraphic dating of late Miocene megalake regressions in Central Eurasia 

Wout Krijgsman, Dan Palcu, Irina Patina, Ionuț Șandric, Sergei Lazarev, Iuliana Vasiliev, and Marius Stoica

The largest megalake in the record formed in Eurasia during the late Miocene, when the epicontinental Paratethys Sea became tectonically-trapped and disconnected from the global ocean. The Paratethys megalake was characterized by several episodes of hydrological instability and partial desiccation, but the chronology, magnitude and impacts of these paleoenvironmental crises are poorly known. The Panagia section on the Taman Peninsula of Russia is the only place known to host a continuous sedimentary record of the late Miocene hydrological crises of Paratethys. Paleomagnetic measurements allow the development of a polarity pattern that can be used to date the regression events. The Panagia polarity pattern consists of 17 polarity intervals, 9 of normal polarity and 8 of reversed polarity, plus 4 additional short-term polarity fluctuations, that are inferred to correspond to the 11-7.5 Ma interval. We identified four major regressions that correlate with aridification events, vegetation changes and faunal turnovers in large parts of Europe. Our paleogeographic reconstructions reveal that Paratethys was profoundly transformed during the regression episodes, losing ~1/3 of the water volume and ~70% of its surface during the most extreme events. The remaining water was stored in a central salt-lake and peripheral desalinated basins while vast regions (up to 1.75 million km2) became emerged land, suitable for the development of forest-steppe landscapes. The dry episodes of the megalake match with climate, food-web and landscape changes throughout Eurasia but the exact triggers and mechanisms remain to be resolved.

How to cite: Krijgsman, W., Palcu, D., Patina, I., Șandric, I., Lazarev, S., Vasiliev, I., and Stoica, M.: Magnetostratigraphic dating of late Miocene megalake regressions in Central Eurasia , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15933, https://doi.org/10.5194/egusphere-egu21-15933, 2021.

EMRP3.2 – Open Session in Paleomagnetism and Environmental Magnetism

EGU21-460 | vPICO presentations | EMRP3.2

Authigenic nanoscale magnetite within methanic marine sediments

Zhiyong Lin, Xiaoming Sun, Andrew Roberts, Harald Strauss, Benjamin Brunner, and Jörn Peckmann

Magnetic studies of methanic sediments focus mainly on magnetic iron sulfide (greigite, 3C pyrrhotite) formation and magnetic iron oxide (magnetite, titanomagnetite) dissolution, which mainly result from the release of hydrogen sulfide during sulfate-driven anaerobic oxidation of methane. In some instances, authigenic fine-grained magnetite within methanic environments is recognized from magnetic parameters, but the mechanisms for explaining its occurrence remain unclear. We report a novel authigenic nanoscale magnetite source in methanic marine sediments. The magnetite occurs in large concentrations in multiple horizons in a 230-m long sediment core with gas hydrate-bearing intervals. In contrast to typical biogenic magnetite produced by magnetotactic bacteria and dissimilatory iron-reducing bacteria, most particles have sizes of 200-800 nm and many are aligned in distinctive structures that resemble microbial precipitates. This new type of magnetite is interpreted to be a by-product of microbial iron reduction within methanic sediments. It will record younger paleomagnetic signals than surrounding sediments, which is important for paleomagnetic interpretations in methanic sediments.

How to cite: Lin, Z., Sun, X., Roberts, A., Strauss, H., Brunner, B., and Peckmann, J.: Authigenic nanoscale magnetite within methanic marine sediments, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-460, https://doi.org/10.5194/egusphere-egu21-460, 2021.

EGU21-1980 | vPICO presentations | EMRP3.2

Tracing Ti-rich titanomagnetite oxidation with low-temperature magnetic measurements

Andrei Kosterov, Leonid Surovitskii, Valerii Maksimochkin, Svetlana Yanson, and Aleksey Smirnov

Ti-rich titanomagnetite is a primary magnetic mineral in submarine basalts, as well as in some terrestrial volcanic rocks. On geological timescale, it undergoes a slow oxidation forming titanomaghemites. This natural process may be modeled to some extent by a prolonged annealing at moderately elevated temperatures. We test this by treating at 355°C for 4, 40, 110, and 375 hours a sample of submarine basalt containing titanomagnetite of approximate TM46 composition with Curie temperature of 205°C. To characterize the oxidation products emerged during annealing, we have carried out magnetic measurements between at cryogenic temperatures between 1.8 K and 300 K and at high temperatures up to 700°C.

Temperature dependences of magnetic susceptibility measured in an argon atmosphere reveal that annealing for 4 hours already leads to the formation of new magnetic phases (Phases 1 and 2 thereafter) with Curie temperatures of 420°C and 590°C, respectively. At the same time, a phase close to the initial titanomagnetite still remains in a noticeable amount, although its Curie point also shifts towards higher temperatures. Upon further annealing, the initial titanomagnetite completely disappears, the Curie temperature of Phase 1 increases, reaching 500°C after 375 hours, and the Curie temperature of Phase 2 remains practically unchanged. Phase 1 appears unstable to heating to 700°C in argon atmosphere. In samples annealed for up to 110 hours, Phase 1 disappears on cooling, and a phase with the same Curie temperature as the initial titanomagnetite reemerges. In the sample annealed for 375 hours, traces of Phase 1 are still visible in the cooling branch of the susceptibility vs. temperature curve, and the Curie temperature of the reemerged initial-like phase is 250°C. The newly formed Phase 2 remains stable when heated to 700°C in argon.

Effect of prolonged annealings is clearly seen in low-temperature magnetic properties. In the fresh sample, about one quarter of magnetization acquired at 1.8 K is demagnetized by 5 K. This feature holds for the annealed samples as well. The titanomagnetite phase in the fresh sample manifests itself in a magnetic transition at 58 K. Below this temperature, the FC and ZFC curves sharply diverge, as previously observed for titanomagnetites of intermediate composition. For the annealed samples, the shape of ZFC and FC curves and the ratio between them remain generally similar to those observed for the fresh sample, but there are also several differences. The magnetic transition temperature shifts to ~45 K, while the curves’ shape above the transition changes from concave-up to concave-down. RT-SIRM cycle to 1.8 K in zero field for the fresh sample has a characteristic convex shape and is almost reversible. Magnetization at 1.8 K is about 20% higher than the initial value at 300 K, and magnetization loss after the cycle is only 2-3%. The shape of RT-SIRM cycles changes progressively with increasing annealing time, the degree of irreversibility increasing to ~30% for the sample annealed for 375 hours. 

This study is supported by Russian Foundation of the Basic Research, grants 19-05-00471 and 20-05-00573.

How to cite: Kosterov, A., Surovitskii, L., Maksimochkin, V., Yanson, S., and Smirnov, A.: Tracing Ti-rich titanomagnetite oxidation with low-temperature magnetic measurements, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1980, https://doi.org/10.5194/egusphere-egu21-1980, 2021.

EGU21-4094 | vPICO presentations | EMRP3.2

An inclination-only fold test

Yiming Ma, Tianshui Yang, Huapei Wang, Shaochen Hu, Mengqing Wang, and Di Zou

The fold test is inevitably used to determine whether a remanence is acquired prefold. The remanence declinations recorded in rocks may be affected by local rotation. Thus, it is difficult to restore the structure correction’s original orientation, leading to a possible incorrect conclusion of the fold test. However, the tilt correction of inclination is immune to the influence of local rotations. Therefore, we propose a more straightforward fold test based on the inclination-only mean. Examples are given to verify the validity of the inclination-only fold test, which can be applied to data affected by a possible local rotation. The inclination-only fold test should be used to determine the contribution of inclinations. The combined use of the 3-D and inclination-only fold tests is required to evaluate paleomagnetic results from the orogen or sampling sections with unrecognizable plunging folds or near the faults.

How to cite: Ma, Y., Yang, T., Wang, H., Hu, S., Wang, M., and Zou, D.: An inclination-only fold test, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4094, https://doi.org/10.5194/egusphere-egu21-4094, 2021.

EGU21-5351 | vPICO presentations | EMRP3.2

Reliability of palaeomagnetic poles from sedimentary rocks

Bram Vaes, Shihu Li, Cor Langereis, and Douwe van Hinsbergen

Palaeomagnetic poles form the building blocks of apparent polar wander paths and are used as primary input for quantitative palaeogeographic reconstructions. The calculation of such poles requires that the short-term, palaeosecular variation (PSV) of the geomagnetic field is adequately sampled and averaged by a palaeomagnetic dataset. Assessing to what extent PSV is recorded is relatively straightforward for rocks that are known to provide spot readings of the geomagnetic field, such as lavas. But it is unknown whether and when palaeomagnetic directions derived from sedimentary rocks represent spot readings of the geomagnetic field and sediments are moreover suffering from inclination shallowing, making it challenging to assess the reliability of poles derived from these rocks. Here, we explore whether a widely used technique to correct for inclination shallowing, known as the elongation-inclination method (E/I), allows us to formulate a set of quality criteria for (inclination shallowing-corrected) palaeomagnetic poles from sedimentary rocks. The E/I method explicitly assumes that a sediment-derived dataset provides, besides flattening, an accurate representation of PSV. We evaluate the effect of perceived pitfalls for this assumption using a recently published dataset of 1275 individual palaeomagnetic directions of a >3 km-thick succession of ~69-41.5 Ma red beds from the Gonjo Basin (eastern Tibet), as well as synthetic data generated with the TK03.GAD field model. The inclinations derived from the uncorrected dataset are significantly lower than previous estimates for the basin, obtained using coeval lavas, by correcting inclination shallowing using anisotropy-based techniques, and by predictions from tectonic reconstructions. We find that the E/I correction successfully restores the inclination to values predicted by these independent datasets if the following conditions are met: the number of directions N is at least 100, the A95 cone of confidence falls within a previously defined A95min-max reliability envelope, no negative reversal test is obtained and vertical-axis rotation differences within the dataset do not exceed 15°. We propose a classification of three levels (A, B, and C) that should be applied after commonly applied quality criteria for paleomagnetic poles are met. For poles with classification ‘A’, we find no reasons to assume insufficient quality for tectonic interpretation. Poles with classification ‘B’ could be useful, but have to be carefully assessed, and poles with classification ‘C’ provide unreliable paleolatitudes. We show that application of these criteria for datasets of other sedimentary rock types classifies datasets whose reliability is independently confirmed as ‘A’ or ‘B’, and that demonstrably unreliable datasets are classified as ‘C’, confirming that our criteria are useful, and conservative. The implication of our analysis is that sediment-based datasets of quality ‘A’ may be considered statistically equivalent to datasets of site-mean directions from rapidly cooled igneous rocks like lavas and provide high-quality palaeomagnetic poles.

How to cite: Vaes, B., Li, S., Langereis, C., and van Hinsbergen, D.: Reliability of palaeomagnetic poles from sedimentary rocks, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5351, https://doi.org/10.5194/egusphere-egu21-5351, 2021.

EGU21-7593 | vPICO presentations | EMRP3.2

Paleomagnetic study in lava and volcanoclastic materials of the Late-Carboniferous-Permian Cadí basin (Central-Eastern Pyrenees)

Ana Simon-Muzas, Antonio M Casas-Sainz, Ruth Soto, Elisabet Beamud, Belén Oliva-Urcia, Emilio L Pueyo, and Josep Gisbert

This work presents preliminary paleomagnetic results from the Cadí basin, one of the Pyrenean Late-Carboniferous-Permian basins.

These basins are the consequence of a complex tectonic evolution. At the outset, they were controlled by an extensional or transtensional regime during the progressive dismantling of the Variscan chain that gradually changed to a strike-slip fault regime. Afterwards, during Pyrenean compression (Late Cretaceous to Cenozoic) most of these basins were inverted and transported southwards as a part of the basement units. All of them are characterized by a similar subsidence pattern interpreted as graben or half-graben continental troughs. In a close relationship with its genesis, during Late-Carboniferous-Permian times, several magmatic events took place. Because of that, fluvio-lacustrine sediments present lateral changes to volcanic and volcanoclastic deposits and materials are sometimes cut by intrusions.

The studied zone, the Cadí basin, is located at the eastern Pyrenees, south of the Axial Zone, and shows a high variability of Late-Carboniferous-Permian volcanic products in E-W continuous outcrops that reach several hundred meters of thickness with a good preservation and exposition. The Late Carboniferous-Permian evolution of the Cadí basin was strongly controlled by tectonics and volcanism. In order to characterize this stage and its volcanic rocks, we applied in a previous work the anisotropy of magnetic susceptibility (AMS) technique to determine the primary magnetic fabric of lava flows and volcanoclastic materials. A dominant WNW-ESE direction of the magnetic lineation was recognized related to the paleoflow direction. Paramagnetic minerals and magnetite were recognized as the main carriers of the AMS.

In this work we present the paleomagnetic results from 15 sites (about 150 specimens) in volcanic, volcanoclastic and intrusive materials sampled along four N-S cross sections. The obtained results indicate that the carrier of the magnetization are both magnetite and haematite. The thermal demagnetization of samples shows a paleomagnetic component with unblocking temperatures from 480ºC with reversed polarity and very low inclination after bedding correction. In some sites a normal polarity component is also recognized. These results seem to be coherent with a magnetization coetaneous with the emission and deposition of these materials during Permian times. In cases where the samples record a normal component is important to take into account the complex structural situation of these outcrops.

How to cite: Simon-Muzas, A., Casas-Sainz, A. M., Soto, R., Beamud, E., Oliva-Urcia, B., Pueyo, E. L., and Gisbert, J.: Paleomagnetic study in lava and volcanoclastic materials of the Late-Carboniferous-Permian Cadí basin (Central-Eastern Pyrenees), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7593, https://doi.org/10.5194/egusphere-egu21-7593, 2021.

Shipborne ex-situ oxygen measurements in mid-ocean ridge flank sediment cores from the eastern low-latitude North Pacific (Clarion-Clipperton Zone) revealed a downward increase of pore-water oxygen above the sediment-crust interface (Mewes et al., 2016, Kuhn et al., 2017). This inverse redox zonation is caused by an upward diffusion of oxygen (and other solutes) from fluids circulating through the underlying 20 Mio. Year old and still cooling ocean crust. In consequence, these sediments experience a cyclic change in redox-conditions from oxic seafloor conditions at the top through mostly suboxic conditions throughout the sediment column back to oxygen-rich pore water in the last few sediment meters above the rock basement.

We studied paleomagnetic records and bulk magnetic properties of three gravity cores from such settings that were collected during RV Sonne expedition SO-240 in 2015 and obtained high-quality magnetostratigraphic records covering the past 3.2 Ma. The generally very good preservation and interpretability of our reversal and RPI records, however, conflicts with a well-defined, but irregular ‘ghost event’ of normal polarity within the upper Gilbert reversed C2Ar section. This magnetic polarity and intensity artifact cannot be explained by sediment tectonics, but coincides with the present depth of the lower suboxic-to-oxic redox boundary. Although chemical overprinting could be considered as an obvious explanation of such findings, bulk magnetic analyses (FORCs, thermomagnetics) infer no diagenetic alteration of the magnetic minerals. Over the entire paleomagnetic record, bacterial magnetite appears to be the predominant NRM carrier. We therefore introduce a novel conceptual model of secondary biogenic magnetite formation at crustal depth, hypothesizing that microaerophilic magnetotactic bacteria live and biomineralize not only in the shallow subsurface, but also near the deep oxygen above the sediment-crust interface.

 

References

Mewes, K., Mogollón, J.M., Picard, A., Rühlemann, C., Eisenhauer, A., Kuhn, T., Ziebis, W., Kasten, S., 2016. Diffusive transfer of oxygen from seamount basaltic crust into overlying sediments: An example from the Clarion-Clipperton Fracture Zone. Earth and Planetary Science Letters 433, 215-225.

Kuhn, T., Versteegh, G.J.M., Villinger, H., Dohrmann, I., Heller, C., Koschinsky, A., Kaul, N., Ritter, S., Wegorzewski, A.V., Kasten, S., 2017. Widespread seawater circulation in 18-22 Ma oceanic crust: Impact on heat flow and sediment geochemistry. Geology 45, 799-802.

 

 

 

How to cite: Höfken, A., von Dobeneck, T., and Kasten, S.: Magnetostratigraphic effects and artifacts of an inverse redox zonation in bottom-up oxygenated East Pacific mid-ocean ridge flank sediments, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9458, https://doi.org/10.5194/egusphere-egu21-9458, 2021.

EGU21-10336 | vPICO presentations | EMRP3.2

Paleomagnetism of the Cretaceous Alkaline Magmatism of the Lusitanian Basin, Portugal

Rafael Dinis, Eric Font, Marta Neres, Morgan Ganerød, Elsa Gomes, Pedro Terrinha, and Elisa Sanchez-Moreno

The paleogeographic reconstructions of the Iberian plate during the opening of the Atlantic Ocean is still poorly constrained. Major limitations include the paucity of high quality paleomagnetic poles and geochronological constrains, the occurrence of widespread remagnetization events, and controversial seafloor magnetic anomalies. Recent studies provided new high quality paleomagnetic poles from intrusive rocks (sills) dated at 88 and 94 Ma, which contributed to improve the calibration of the apparent polar wander path of Iberia at this time interval. These intrusive rocks are part of the Cretaceous Alkaline Magmatic Pulse that occurred between 72 and 94 Ma, and that is expressed by sills and lava flows cropping out in the Lusitanian Basin of Portugal. Here we provided new paleomagnetic, rock magnetic and anisotropy of magnetic susceptibility (AMS) data of two sills apparently contemporaneous of the Cretaceous Magmatic Alkaline Pulse, namely the Anços sill in the city of Mafra and the Lomba dos Piano sill in the vicinity of the city of Sintra. Rock magnetic experiments consisted in the acquisition and unmixing of isothermal remanent magnetization curves, thermomagnetic analyses, and hysteresis curves, complemented by petrographic analyses. Results indicate that the main magnetic carrier is a mixture of SD to MD titanomagnetite. The magnetic fabric of the Anços sill is oblate in both sills and sub-horizontal k3 eigenvectors indicate that no major titling occurred after the intrusion of the rocks. The Lomba do Piano sill shows more scattered eigenvector directions with a mixture of oblate and prolate fabrics. After alternating field demagnetization, all samples show high-quality and reliable magnetic vectors, with a mean characteristic remanent magnetization orientated Dec=346.88º, I=42.66º (n/N=219/228; k=78.19; a95=1.8º) for Anços and Dec=351.12º, I=48.90º (n/N=142/143; k=94.03; a95=1.23º) for Lomba dos Pianos. All magnetic vectors show a normal (positive) polarity, characteristic of the Cretaceous Normal Polarity Superchron. The corresponding virtual geomagnetic poles (VGP) are Plong=212.62º and Plat=72.03º (N=219, K=98.81, A95=0.96º) for the Anços sill and Plong=212.12º and Plat=78.35º (N=142, K=74.22, A95=1.38º) for the Lomba dos Pianos sill. The Anços VGP plots close to the poles of the Paços d’Ilhas (PI, 88 Ma) and Foz da Fonte (FF; 94 Ma) sills previously published. However, the VGP of the Lomba dos Pianos has a distinct and lower paleolatitude, questioning the contribution of paleosecular variations (PSV). We applied the method of the A95 envelope and find that PSV has not been minimized in the studied sills, which can be explained by the rapid cooling of this kind of rocks. We compiled paleomagnetic data of all sills to provide a more robust paleomagnetic pole for the interval of 88-94 Ma.

 

This work was supported by the project FCT/UIDB/50019/2020 – IDL funded by FCT

How to cite: Dinis, R., Font, E., Neres, M., Ganerød, M., Gomes, E., Terrinha, P., and Sanchez-Moreno, E.: Paleomagnetism of the Cretaceous Alkaline Magmatism of the Lusitanian Basin, Portugal, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10336, https://doi.org/10.5194/egusphere-egu21-10336, 2021.

EGU21-13644 | vPICO presentations | EMRP3.2

Skewness Pole from Magnetic Anomaly C21n Implies Rapid Early Eocene True Polar Wander

Daniel Woodworth, Richard Gordon, and Kevin Gaastra

Skewness analysis of marine magnetic anomalies is the most misunderstood methodology in paleomagnetism. Such analysis has several advantages. First, marine magnetic anomalies innately average secular variation. Second, paleomagnetic poles determined by analysis of their skewness are not biased by overprints. Third, skewness analysis can determine high precision paleomagnetic poles. Specifically, skewness analysis of magnetic anomalies recording Late Cretaceous and early to mid-Cenozoic seafloor spreading between the Pacific and Farallon plates, because of their geometry with respect to the paleo-spin axis, results in high-precision paleomagnetic poles. These anomalies in many cases span ~140° of effective remanent inclination over a span of ~40° of latitude, reducing uncertainty by a factor of ~0.3 when mapping from direction space to pole space (Zheng et al. 2018).

Paleomagnetic poles have been previously determined from skewness analysis for six Pacific plate anomalies: C32n (74-71 Ma), C31n-C27r (60-63 Ma), C26r (62-59 Ma), C25r (59-58 Ma), C24r (57-54 Ma), C20r (46-43 Ma), and C12r (33-31 Ma). The younger group, C20r and C12r, together with independent paleo-spin axis estimates from the paleo-distribution of sediment accumulation rates from 12-46 Ma, define an approximately stationary paleo-spin axis location relative to the Pacific hotspots but offset from the current spin axis by 3°. The older group, 74-54 Ma, also shows that the Pacific hotspots remained approximately stationary relative to an additional paleo-spin axis location separated by 8° from the 12-46-Ma paleo-spin axis, implying an episode of reorientation of the entire solid earth – i.e., true polar wander (TPW) – of ~8° over at most 8 Ma between 54 and 46 Ma, or a rate of TPW of ~1°/Ma or more.

To constrain the timing and rate of reorientation, we analyze anomaly C21n (47-46 Ma), the youngest anomaly inside the 54-46-Ma interval. We incorporate 33 total-intensity ship- and 11 vector aero-magnetic track lines and find a well-constrained paleomagnetic pole near 77N, 23E in the fixed-Pacific plate reference frame.

Our new paleomagnetic pole is consistent with a prior, more uncertain, 48-Ma paleo-spin axis location from the paleo-distribution of sediment accumulation rates. When reconstructed into the Pacific hotspot reference frame, our new paleomagnetic pole lies close to the younger 46 to 12-Ma TPW stillstand location, indicating that true polar wander was completed by 47 Ma, if not earlier. Thus the ~8° shift occurred in, at most, 6.0 Ma at a rate of at least ~1.3°/Ma, and potentially even faster. The lower bound of ~1.3°/Ma of TPW indicate that Early Eocene TPW is comparable to the rate of present-day TPW (~1.1°/Ma extrapolated from geodetic data (Argus and Gross, 2004)). This new pole bounds the Early Eocene TPW episode between approximately the old and young ends of the Early Eocene Climatic Optimum (EECO; 53.2-49.1 Ma (Westerhold et al. 2018)). Thus, there may be a link between Early Eocene TPW and important climate events, such as the frequency of hyperthermals and the onset of Eocene cooling. In addition, TPW was likely complete before the 47.4-Ma age of the bends in Pacific plate hotspot chains (Gaastra & Gordon, this meeting).

How to cite: Woodworth, D., Gordon, R., and Gaastra, K.: Skewness Pole from Magnetic Anomaly C21n Implies Rapid Early Eocene True Polar Wander, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13644, https://doi.org/10.5194/egusphere-egu21-13644, 2021.

EGU21-15842 | vPICO presentations | EMRP3.2

Are carbonates from the India-Asia collision remagnetized ? 

Pierrick Roperch and Guillaume Dupont-Nivet

Widespread carbonate rocks from the Tibetan plateau have been extensively used to constrain terrane paleolatitudes involved in the India-Asia collision. However, their reliability in preserving a primary magnetization has been recently put into question.  A transformation of pyrite to magnetite has been recently proposed as a cause for late re-magnetizations in Paleocene Tethyan Himalaya carbonates (1) and late Triassic carbonates from the Qiantang (2), thus discarding such Characteristic Remanent Magnetizations (ChRM) for tectonic purposes. We have re-examined the paleomagnetic data obtained on late Triassic carbonate rocks from the Qiantang. Our SEM observations indicate pristine pyrite in non-weathered carbonate rocks. Optical microscope observations in reflected light demonstrate that pyrite, when it is weathered, is transformed to iron hydroxides minerals but not to magnetite. This is at odds with previously proposed pyrite to magnetite transformation hypothesis mainly based on interpretations of Scanning Electron Microscope data (SEM/EDS). We thus interpret the ChRM more likely related to an early diagenetic magnetization of Late Triassic age. Knowing that the arguments put forward for a remagnetization of Triassic carbonates are the same as those proposed for the remagnetization of Paleocene carbonates, the ChRM in some Paleocene carbonates could also be of early diagenetic origin. However, there is also a growing number of studies where remagnetization is obvious in the Tethyan Himalaya and undetected remagnetizations (3) are likely the cause of the large differences in the estimation of the size of Greater India. These examples show the urgent need to publish the complete demagnetization dataset in an open database like MAGIC or the FAIR data initiative from (4) in order to reassess previous interpretations if we want to solve problems like the size of Greater India and hypothesis like the Greater India basin.

(1) doi:10.1002/2016JB013662 ; (2) doi:10.1016/j.epsl.2019.06.035 ; (3) doi:10.1016/j.epsl.2020.116330; (4) doi:10.1029/2019GC008838.

 

How to cite: Roperch, P. and Dupont-Nivet, G.: Are carbonates from the India-Asia collision remagnetized ? , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15842, https://doi.org/10.5194/egusphere-egu21-15842, 2021.

EGU21-3158 | vPICO presentations | EMRP3.2

Rock magnetism as a tool for investigating the use of archaeological artefacts from baked clay

Evdokia Tema, Enzo Ferrara, Lorenzo Zamboni, Marica Venturino, Margherita Reboldi, Albert Egea Guevara, and Lluís Casas

Even though multidisciplinary approaches applied to the investigation of archaeological findings are widely used, the use of rock magnetic properties is still poorly exploited in the determination of the use of ancient artefacts. In this study, we present the results of a combined archaeological, morphological and magnetic analyses applied on the ring-shape clay artefacts found at the archaeological site of Villa del Foro, in Northern Italy. The materials studied are dated between the sixth and the first half of fifth century BC and are found in large quantities in different trenches of the archaeological excavation. To investigate their thermal history and to exploit their possible use as kiln supports, cooking stands, or loom weights, we have investigated their natural remanent magnetization (NRM) and the magnetic mineralogy changes occurred during laboratory heating. Magnetic analysis used for the determination of the firing temperatures show thermal stability up to 500-600 o C, while further laboratory heating at 700 o C introduces magnetic alteration. Thermal demagnetization of the samples generally shows a strong and stable thermal remanent magnetization. In few cases, a clear secondary component is present, suggesting partial re-heating or displacement at temperatures ranging from 200 oC to 450 °C. Such secondary magnetic component can be indicative of a secondary heating or of a displacement of the rings from their initial firing position while still hot. Even though the studied rings belong to casually different morphological typologies, no connection among type and magnetic behavior was observed, suggesting that the ring’s morphology does not define neither their production conditions nor the final use of the artefacts. The estimated firing temperatures of around 600-700 oC are compatible with the heating of the rings during their manufacture rather than related to cooking activities. In combination with the archaeological evidence and the morphological analysis it is thus suggested that the rings were used as weight looms and baked only during their production procedures. Such a pilot study can be used as reference for the identification of similar objects found in Italy and Europe during the Iron Age and confirms the great potential of rock magnetic analysis in the investigation of the technology and use of ancient baked clays.

How to cite: Tema, E., Ferrara, E., Zamboni, L., Venturino, M., Reboldi, M., Egea Guevara, A., and Casas, L.: Rock magnetism as a tool for investigating the use of archaeological artefacts from baked clay, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3158, https://doi.org/10.5194/egusphere-egu21-3158, 2021.

EGU21-9845 | vPICO presentations | EMRP3.2

Fired clay from Neolithic houses – the role of past environment revealed in mineral magnetic properties

Neli Jordanova, Diana Jordanova, Deyan Lesigyarski, and Maria Kostadinova-Avramova

Human behavior and especially the use of fire increasingly influence our environment during the Anthropocene epoch. Balkan Peninsula is on the road of the ancient human dispersal during the Neolithic period. Burnt Neolithic remains are often related to ancient houses which ended their existence as a result of extensive fire. Materials from burnt clay remains from house destructions originating from 18 Neolithic sites from Bulgaria were studied using rock magnetism. Mineral magnetic studies and equivalent firing temperature estimates were carried out. The aim of the study was to explore the magnetic signature of fired clay materials in relation to the most important environmental factors. The main magnetic minerals identified were magnetite, maghemite and hematite, in several cases also epsilon-Fe2O3. Magnetic susceptibility enhancement is dependent on the raw clay mineralogy and the firing intensity, being higher for sites developed on loess materials. Sites located in river valleys from South Bulgaria show lower susceptibility enhancement. Magnetic susceptibility  and percent frequency dependent magnetic susceptibility at site level were considered in relation to the climatic conditions during the Neolithic as revealed by anthracological studies already published for the study region. Firing temperature estimates, comprising 198 single determinations in total, vary in the range 580 – 1050°C across the sites. Estimated average firing temperatures at site’s level showed higher values in Early Neolithic sites (Tfire average=815°C) as compared to Late Neolithic ones (Tfire average = 746°C and 713°C). Several possible hypotheses for the trend observed are considered: difference in climate regimes across the territory leading to different “fire weather”; difference in the vegetation fuel used in house construction; and intentional burning of Early Neolithic houses. This study is financially supported by the project KP-06-COST/2, funded by the Bulgarian National Science Fund.

How to cite: Jordanova, N., Jordanova, D., Lesigyarski, D., and Kostadinova-Avramova, M.: Fired clay from Neolithic houses – the role of past environment revealed in mineral magnetic properties, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9845, https://doi.org/10.5194/egusphere-egu21-9845, 2021.

EGU21-3142 | vPICO presentations | EMRP3.2

The magnetic properties of natural pigments: preliminary analyses for their identification in Fine Arts

Chiara Anselmi, Antonio Sgamellotti, Manuela Vagnini, and Aldo Winkler

A multidisciplinary approach, including compositional, spectroscopic and microscopic methodologies, is often used for the analysis and identification of pigments in Fine Arts. Although a large part of widely used natural and synthetic pigments contain Fe-oxides and hydroxides, their magnetic characterization is still poorly explored. The application of rock magnetism analyses through fast, cheap and non-destructive measurements, can be instead useful for the identification and discrimination of pigments through their distinctive magnetic properties.

In this preliminary study, the magnetic properties of several iron-based commercial pigments together with paintings models and supports, were analyzed.

In order to investigate the compositional differences of pigments by means of their magnetic behavior, the magnetic susceptibility, the hysteresis properties and the magnetic susceptibility variation at low and high temperature were measured on selected samples.

All the pigments showed different magnetic properties, mainly related to variable proportions of magnetite, hematite and maghemite as the main magnetic carriers.

Further studies will be addressed to define a protocol for applying the magnetic techniques to the characterization of pigments, including tests on samples produced by different brands and different periods, with the final aim of integrating the magnetic measurements with the different spectroscopic techniques commonly employed for the preservation and the analysis of cultural heritage.

How to cite: Anselmi, C., Sgamellotti, A., Vagnini, M., and Winkler, A.: The magnetic properties of natural pigments: preliminary analyses for their identification in Fine Arts, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3142, https://doi.org/10.5194/egusphere-egu21-3142, 2021.

EGU21-2973 | vPICO presentations | EMRP3.2

Linking cave sediments and soil magnetism in the Urșilor cave (Romania)

Cristian George Panaiotu, Cristian Necula, Relu D. Roban, Alexandru Petculescu, Ionut-Cornel Mirea, Luchiana Faur, and Silviu Constantin

Cyclical changes in the magnetic mineral assemblages have been observed in numerous sedimentary records confirming the relationship between rock magnetism and past global change. Several studies have shown that the magnetic susceptibility data of cave sediments reflect both long- and short-term climatic oscillations. These magnetic susceptibility variations are attributed to changes in climate-controlled pedogenesis which influence the production of low coercivity magnetic mineral phases, magnetite, and maghemite outside the cave. These soils with climate-dependent magnetic properties are then washed, blown, or tracked into the cave where they accumulate, creating the changes observed in rock magnetic data. We present a rockmagnetism study of the sediments from the Urșilor cave and the soils above the cave. Our focus is the detailed characterization of the ferromagnetic mineralogy preserved in the cave sediments and its links with potential soil sources. In the cave, we sampled four sections (2-3 m high) consisting mainly of silts and clays, with some sand layers. The age of the sediments is older than 40 ka. At the surface, we sampled various types of soils from 9 sites. For all samples, we measured: variation of magnetic susceptibility with frequency (976 and 15616 Hz), the anisotropy of magnetic susceptibility, isothermal remanent magnetization, and anhysteretic remanent magnetization. Because soils are characterized by the presence of superparamagnetic magnetite produced by pedogenesis which can be detected by the frequency dependence of magnetic susceptibility, we also measured the frequency dependence of soils and selected cave sediment samples at 13 frequencies (between 128 and 512000 Hz). Multi-frequencies measurements of the magnetic susceptibility of recent soils show that all the sampled soils have a strong frequency dependence indicating the presence of superparamagnetic particles produced by pedogenesis. Most of the sediment samples have an important frequency dependence similar to the one observed in the recent soils. As a preliminary conclusion, we can state that most of the fine cave sediments contain superparamagnetic particles, which can be probably attributed to soils transported into the cave by erosion. These results suggest that during the deposition of high magnetic susceptibility sediments it was a climate favorable for intense pedogenesis. The interpretation of the intervals with lower values of magnetic susceptibility is still under investigation to decide if represents a climatic signal or a change in the dynamics of sediment transport. Acknowledgment: The research leading to these results has received funding from the EEA Grants 2014-2021, under Project contract no. EEA-RO-NO-2018-0126.

How to cite: Panaiotu, C. G., Necula, C., Roban, R. D., Petculescu, A., Mirea, I.-C., Faur, L., and Constantin, S.: Linking cave sediments and soil magnetism in the Urșilor cave (Romania), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2973, https://doi.org/10.5194/egusphere-egu21-2973, 2021.

EGU21-16462 | vPICO presentations | EMRP3.2

Environmental magnetism study of Lake Cadagno, Switzerland

Artin Ali, Andrea Biedermann, Jasmine Berg, Mark Lever, and Hendrik Vogel

Climate affects the mineralogy and grain size of sediments deposited in lakes. These properties are reflected in the sediment magnetic properties and can be characterized using magnetic methods. As part of the Cadagno-Project, which recovered several gravity and piston cores spanning the entire lake history from the deglacial to the present from the deepest part of permanently stratified Lake Cadagno, which is due to its peculiar water column chemistry considered an early Earth ocean analogue, our study aims to define changes in climate conditions during sedimentation. Here, we present a rock magnetic dataset (low-field magnetic susceptibility and its temperature dependence, anhysteretic and isothermal remanent magnetization (ARM, IRM), acquired in various fields, AF demagnetization, and hysteresis loops) that helps characterize the concentration, mineralogy, and grain size of magnetic carriers, and their variability with depth. Susceptibility, ARM, and IRM were measured on core sediments down to a depth of 886 cm below the lake bottom, providing a high-resolution record of the sedimentary environment of Lake Cadagno over the last 11,000 years. In addition to these depth profiles, detailed rock magnetic experiments were conducted at specific depths. The cores consist of pelagic sediments, flood turbidites, and late glacial sediments. In order to determine the characteristics of the background sedimentation, only turbidite-free intervals were included in this study. The depth profiles of susceptibility, ARM and IRM have approximately similar variations with depth. They show distinct peaks at the upper parts of the pelagic sediments (156-158 cm below the lake bottom,   ̴1280-1320 cal. Yr Bp) and of the late glacial sediments (826-844 cm below the lake bottom), which can be interpreted as increased concentration of ferromagnetic minerals or as a change in the magnetic mineralogy, in addition to decreasing trend in the background. Several intervals within the pelagic sediments are dominated by low-coercivity minerals (<10 mT), while higher coercivity grains (10–100 mT) contribute significantly at (150-170, 418-448 and 719-735 cm below the lake bottom). Magnetic grain size was analyzed using a Day plot, and shows that single domain magnetite dominates at (844 cm) below the lake bottom, indicating the presence of magnetotactic bacteria, which are believed to dwell mainly in the oxic–anoxic interface where chemical gradients are high. These results provide important constraints on the environmental conditions and climate change recorded by the magnetic minerals in Lake Cadagno.

How to cite: Ali, A., Biedermann, A., Berg, J., Lever, M., and Vogel, H.: Environmental magnetism study of Lake Cadagno, Switzerland, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16462, https://doi.org/10.5194/egusphere-egu21-16462, 2021.

EGU21-10914 | vPICO presentations | EMRP3.2

Reconsidering the paleoclimate implications of magnetic susceptibility records of Holocene loessic soils

Diana Jordanova and Neli Jordanova

Mass specific magnetic susceptibility variations with depth along soil profiles developed on loess parent material is one of the most frequently used physical parameters in local, regional and global correlations of loess deposits. It is also utilized as a paleo-precipitation proxy, defined either as absolute difference between susceptibilities of the enhanced B-horizon and parent loess, or as relative enhancement using ratios of magnetic parameters. These different approaches in the application of magnetic susceptibility as paleoclimate proxy lead us to perform a comparative study on a number of Holocene soil profiles developed on loess from European loess area and the Chinese Loess Plateau (CLP). We made a compilation of data including 20 profiles from North Bulgaria, 28 profiles from Eastern and Central Europe; and 26 profiles from the CLP. Minimum magnetic susceptibilities of the last glacial loess (Xmin) from the compiled data base for European and Chinese profiles show climate related variability, revealing multi linear relationship with both present day MAP and MAT values for the corresponding locations. Strong deviations of Xmin from this dependence display sites located at low elevation river terraces, Black sea coast and possessing large content of coarse silt and sand fractions. Pedogenic magnetic susceptibility (Xpedo) defined as (Xmax - Xmin) with  Xmax determined from the youngest part (last 1500 – 2000 years B.P.) of the Holocene magnetic susceptibility records of Chinese sections and absolute Xmax of the European sites show systematic dependence on modern MAP and MAT values. This dependence is uniform for all sites with steppe vegetation, while higher scatter and steeper regression trends are observed for sites under mixed (steppe – forest) and forest vegetation. The study is financially supported by project No KP-06-N34/2 funded by the Bulgarian National Science Fund.

How to cite: Jordanova, D. and Jordanova, N.: Reconsidering the paleoclimate implications of magnetic susceptibility records of Holocene loessic soils, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10914, https://doi.org/10.5194/egusphere-egu21-10914, 2021.

EGU21-1758 | vPICO presentations | EMRP3.2

Reconstruction of the Pleistocene climate change in the Middle Dnieper area on the basis of rock magnetic, palaeopedological and pollen studies of two reference loess-palaeosol sequences

Dmytro Hlavatskyi, Natalia Gerasimenko, Volodymyr Bakhmutov, Oleksandr Bonchkovskyi, Ievgen Poliachenko, Viktor Shpyra, Illya Kravchuk, and Semen Cherkes

Palaeoclimatic reconstructions including rock magnetic, magnetostratigraphic, palaeopedological and pollen studies were performed on two reference Ukrainian loess-palaeosol sequences at Stari Kaydaky and Vyazivok, located within the Middle Dnieper River Basin. The results have shown that rock magnetic palaeoenvironmental proxies, primarily magnetic susceptibility (MS), are clearly correlated with the marine isotope record (MIS), being enhanced in the palaeosols compared to the loesses. The background susceptibilities of the Stari Kaydaky and Vyazivok sections are both in the range of 7×10-8 to 10×10-8 m3kg-1, which is a half of those at the Danube Basin loess sites. The good correspondence of the MS curve with changes in the marine δ18O signal, provides a strong evidence for correlating the Pryluky/Kaydaky (S1) pedocomplex at Stari Kaydaky and Vyazivok to MIS 5, and the welded Zavadivka (S3/S4) pedocomplex to MIS 9–11. In the Upper Pleistocene deposits, the well developed Luvisol of the Kaydaky unit has a pollen succession of the last interglacial, with high percentages of broadleaf trees’ pollen indicating the early and late temperate phases of the interglacial. Relatively high pollen percentages of broadleaf trees are observed in the Greyzem of Lower Pryluky subunit (the correlative of MIS 5c) and indicate the south-boreal, transitional to temperate, climate. The Upper Pryluky Cambisols and Chernozems (MIS 5a) formed under boreal forest-steppe and steppe, as well as the Vytachiv soils (L1S1, MIS 3). Pollen assemblages from the Uday (L1L2) and Bug (L1L1) loesses reflect tundrasteppe in the north of the studied area and dry periglacial steppe in its southern part. The Late Middle Pleistocene is represented by three interglacial pedocomplexes (correlatives of MIS 11, 9 and 7), with the larger proportions of broad-leaved trees’ pollen in the lower forest soils. Judging from these indices, the climates were warmer than during the last interglacial. Only the upper soils of the MIS 7 formed in the cooler climate. Indicative Pterocarya pollen occur in the MIS 11 soil. During the Early Middle Pleistocene, the temperate climate was typical for the pedocomplex correlated with MIS 13, whereas the older red-brown and dark-brown clayey soils formed in warm-temperate climate (their palynospectra include pollen of thermophilic Neogene relics). At Vyazivok, the Matuyama/Brunhes (M/B) boundary has been detected in the lower part of the Shyrokyne (S7?) soil unit, providing an ultimate chronological benchmark of 780 ka. The Kryzhanivka pedocomplex, located below the M/B boundary, formed in subtropical climate. Preliminary investigations at Stari Kaydaky did not detect the M/B boundary within the studied upper part of the Shyrokyne unit. Pseudo-single-domain/multi-domain magnetite and hematite particles are the main magnetic carriers of the loess and palaeosol ChRMs. The reconstructions demonstrate a correspondence between the pollen and soil sequences and other palaeoenvironmental records including rock magnetic data.

The research was supported by the National Research Foundation of Ukraine grant 2020.02/0406 "Magnetic proxies of palaeoclimatic changes in the loess-palaeosol sequences of Ukraine".

How to cite: Hlavatskyi, D., Gerasimenko, N., Bakhmutov, V., Bonchkovskyi, O., Poliachenko, I., Shpyra, V., Kravchuk, I., and Cherkes, S.: Reconstruction of the Pleistocene climate change in the Middle Dnieper area on the basis of rock magnetic, palaeopedological and pollen studies of two reference loess-palaeosol sequences, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1758, https://doi.org/10.5194/egusphere-egu21-1758, 2021.

EGU21-14416 | vPICO presentations | EMRP3.2 | Highlight

Magnetic properties of PM in indoor/outdoor domestic environments 

Aruã Da Silva Leite, Melina Macouin, Sonia Rousse, Jean-François Leon, Loïc Drigo, and Ricardo Ivan Ferreira da Trindade

The finer fraction of the particulate matter (PM) is the most harmful health wise, as it has more capacity to reach deeper parts of the respiratory system. Among other constituents, PM also contains iron oxides, allowing for the use of magnetic methods in its investigation as proxies for the whole of PM. Those methods present advantages in comparison to traditional ones, being quick, cost effective and sensible to investigate iron oxides among PM. 

To better understand the risks related to PM exposition in the domestic context, the assessment of magnetic parameters may be used in outdoor and indoor environments, giving us information on the concentration of iron oxides (and consequently, PM) and its dispersion from one environment to the other. 

We developed a citizen sciences experiment in the city of Toulouse, France. Tree barks were used as bio-collectors. Garlands composed of tree bark pieces were distributed to the population in May-2019, and placed in both indoors and outdoors of flats and homes to capture PM. They were retrieved after one year. Measurement of magnetic susceptibility, ARM, SIRM, S -ratio and estimation of superparamagnetic concentration were performed. A total of 86 bio-collectors kits were successfully analyzed. The preliminary results indicate a higher concentration of iron oxides outdoors, with a mean difference between outdoor and indoor measurements of 6.58x10-9m3/kg and 1.38x10-5Am2/kg in susceptibility and SIRM respectively. The concentration of the SP fraction also follows this trend of higher outdoor values. The magnetic mineralogy is mostly dominated by low coercivity magnetite-like carriers.

 
 

How to cite: Da Silva Leite, A., Macouin, M., Rousse, S., Leon, J.-F., Drigo, L., and Ivan Ferreira da Trindade, R.: Magnetic properties of PM in indoor/outdoor domestic environments , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14416, https://doi.org/10.5194/egusphere-egu21-14416, 2021.

EGU21-8263 | vPICO presentations | EMRP3.2

To what extent does the environmental magnetism technique acts as a way of assessing the uptake of atmospheric particles by plants?

Sarah Letaïef, Pierre Camps, Thierry Poidras, Patrick Nicol, Delphine Bosch, and Olivier Bruguier

Numerous studies have already shown the possibility of tracing the sources, the
compositions, and the concentration of atmospheric pollutants deposited on plant
leaves. In environmental geochemistry, inter-element and isotope ratios from
chemical element assays have been used for these purposes. Alternatively,
environmental magnetism represents a quick and inexpensive asset that is
increasingly used as a relative indicator for concentrations of air pollutant on bio
accumulator surfaces such as plants. However, a fundamental issue is still pending:
Do plants in urban areas represent a sink for fine particles that is sufficiently effective
to improve air quality? This is a very topical issue because some studies have shown
that the foliage can trap fine particles by different dry deposition processes, while
other studies based on CFD models indicate that plant hedges in cities can hinder
the atmospheric dispersion of pollutants and therefore increase pollution at the level of
emission sources such as traffic. To date, no consensus was made because several
factors not necessary well known must be taken into account, such as, PM
concentration and size, prevailing wind, surface structures, epicuticular wax, to
mention just a few examples. A first step toward the understanding of the impact of
urban greens on air quality is the precise determination of the deposition velocity (Vd)
parameter. This latter is specific for each species and it is most of the time
underestimated in modeling-based studies by taking standard values.
In that perspective, we built a wind tunnel (6 m long, 86 cm wide and 86 cm high) to
perform analogical experiments on different endemic species. All parameters are
controlled, i.e, the wind speed, the nature and the injection time of pollutants (Gasoline
or Diesel exhausts, brakes or tires dust, etc…). We can provide the PM concentrations
upwind and downwind of natural reconstituted hedges by two dustmeters (LOACs -
MétéoModem). Beforehand, parameters such as the hedge resistance (%) or the leaf
area index (LAI) have been estimated for each studied specie to allow comparability
between plants removal potential. The interest would ultimately combine PM
concentration measured by size bins from the LOACs with magnetic measurements
(ARM, IRM100mT, IRM300mT and SIRM) of plant leaves. The idea is to check whether it
would be possible to precisely determine in situ the dust removal rate by urban greens
with environmental magnetism measurements. Up to now, we have carried out on
different endemic species such as Elaeagnus x ebbingei leaves and Mediterranean
pine needles, the results of which will be presented.

How to cite: Letaïef, S., Camps, P., Poidras, T., Nicol, P., Bosch, D., and Bruguier, O.: To what extent does the environmental magnetism technique acts as a way of assessing the uptake of atmospheric particles by plants?, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8263, https://doi.org/10.5194/egusphere-egu21-8263, 2021.

EGU21-3817 | vPICO presentations | EMRP3.2 | Highlight

Magnetic monitoring of PM filters from Air Monitoring Stations during the COVID-19 lockdown in Rome, Italy (March 10th - May 18th, 2020)

Aldo Winkler, Antonio Amoroso, Alessandro Di Giosa, and Giada Marchegiani

An extensive survey of the magnetic properties of PM filters from selected air monitoring stations in Rome and other localities in Latium Region (Sacco Valley, Civitavecchia, Fiumicino) was conducted for outlining the impact of the lockdown measures on air quality.

The magnetic measurements highlighted a relevant content of magnetic minerals, mostly attributable to traffic related sources, on the filters from two stations in Rome and two stations from the urban areas of Civitavecchia and Fiumicino.

The PM filters from the Sacco Valley showed reduced concentrations of magnetic minerals, compared to Rome, however higher than the Castel Di Guido and Civitavecchia Sant'Agostino control stations.

The daily PM concentration data did not generally correlate with the mass susceptibility data, indicating that PM was often dominated by non-ferromagnetic contents, presumably due to wind-driven natural dusts, as stressed by the frequent anticorrelation between mass magnetic susceptibility and PM concentration.

In Magnagrecia air quality station, Rome, the average values ​​of the concentration depending magnetic parameters resulted about a half of those measured in 2005 on the filters from the same station.

From the Day plot, the filters with higher magnetic susceptibility values showed relatively coarse magnetite-like particles as the main magnetic minerals, ascribable to non-exhaust PM emissions from brakes.

This study confirmed that the interpretation of PM concentration during the lockdown is not straightforward and depends on many factors, such as natural inputs, resuspension and local conditions; anyway, magnetic analyses confirmed to be a valuable tool in PM source apportionment and concentration data interpretation.

How to cite: Winkler, A., Amoroso, A., Di Giosa, A., and Marchegiani, G.: Magnetic monitoring of PM filters from Air Monitoring Stations during the COVID-19 lockdown in Rome, Italy (March 10th - May 18th, 2020), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3817, https://doi.org/10.5194/egusphere-egu21-3817, 2021.

EMRP3.5 – Paleogeography of the Neoproterozoic and links between the surface and deep Earth

EGU21-10714 | vPICO presentations | EMRP3.5

Congo-São Francisco at 1.11 Ga and the megacontinent Umkondia 

Johanna Salminen

Currently three supercontinent cycles have been identified and existed supercontinents named from youngest to oldest: Pangea, Rodinia and Nuna/Columbia. Recently Wang et al. (2020) suggested that supercontinent amalgamation were each preceded by ~200 Myr by the assembly of long-lasting megacontinent aking to Gondwana.

The Congo-São Francisco (C/SF) craton is a main building block in Gondwana due to its central location, but its participation to Rodinia is controversial. Salminen et al. (2018) presented 1.11 Ga paleomagnetic and geochronological data from a prominent Epembe-Huila swarm of gabbronoritic dykes in the southern part of the Congo craton in Namibia and in Angola. This paleomagnetic pole yields a relatively low paleolatitude for the C/SF craton at ca. 1.11 Ga and permits a direct connection between Congo and Kalahari cratons. This connection supports an earlier qualitative comparison (Ernst et al., 2013), that the mafic Epembe-Huila swarm was an integral component of the Umkondo Large Igneous Province (LIP). The 1.11 Ga Umkondo LIP is widespread across Kalahari craton, and coeval mafic magmatism has been identified in several of the world’s other late Mesoproterozoic cratons: Laurentia, India, Amazonia, and Antarctica (Grunehogna). Were these coeval provinces spatially linked at the time of emplacement during the amalgamation of Rodinia? Robust paleomagnetic and geochronological data from Laurentia and Kalahari have demonstrated substantial separation between those two blocks at 1.11 Ga (Swanson-Hysell et al., 2015). However, based on similar tholeiitic magmatism Choudhary et al. (2019) proposed that Kalahari and C/SF together with Amazonia and northern India constituted “Umkondia” at 1.11 Ga. It has been proposed that Umkondia occupied an intermediary “megacontinental” role in the Nuna-Rodinia transition analogous to Gondwana in Rodinia-Pangea evolution (Wang et al., 2020). Contradicting Gondwana the proposed Umkondia was not long-lasting, since it has been proposed that Kalahari and Congo separated after 1.10 Ga to form a vast ocean (ca. 6000 km) during the formation of Rodinia and widespread juvenile intra-oceanic magmatism along the present-day central Brazil indicates a large ca. 0.94 Ga ocean between C/SF and Amazonia (Cordani et al., 2003).

 

Choudhary et al. 2019. Precambrian Research 332, 105382.

Cordani et al. 2003. Gondwana Research 6, 275-283.

Ernst et al. 2003. Lithos 174 1-14.

Salminen et al. 2018. Geology 46, 1011-1014.

Swanson-Hysell et al. 2015. Geophysical Journal International 203, 2237-2247.

Wang et al. 2020. Geology 49, https://doi.org/10.1130/G47988.1

 

How to cite: Salminen, J.: Congo-São Francisco at 1.11 Ga and the megacontinent Umkondia , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10714, https://doi.org/10.5194/egusphere-egu21-10714, 2021.

EGU21-13336 | vPICO presentations | EMRP3.5

Paleomagnetism of the ~860 Ma Manso dyke swarm, West Africa: implications for the assembly of Rodinia

Paul Yves Jean Antonio, Lenka Baratoux, Ricardo Ivan Ferreira Trindade, Sonia Rousse, Anani Ayite, Cristiano Lana, Mélina Macouin, Emmanuel Williams Kobby Adu, Caroline Sanchez, Marco Silva, Anne-Sophie Firmin, Carmen Irène Martinez Dopico, Arnaud Proietti, Prince Ofori Amponsah, and Patrick Asamoah Sakyi

The West African Craton (WAC) is one of the major cratons in the Rodinia jigsaw puzzle (~1000–750 Ma). In the Rodinian models, the position of West Africa is mainly constrained by the assumption that it had been a partner of Amazonia since the Paleoproterozoic. Unfortunately, no paleomagnetic data are available for these cratons when the Rodina supercontinent is considered tectonically stable (~1000-750 Ma). Thus, every new reliable paleomagnetic pole for the West African Craton during the Neoproterozoic times is of paramount importance to constrain its position and testing the Rodinia models. In this study we present a combined paleomagnetic and geochronological investigation for the Manso dyke swarm in the Leo-Man Shield, southern West Africa (Ghana). The ~860 Ma emplacement age for the NNW-trending Manso dykes is thus well-constrained by two new U-Pb apatite ages of 857.2 ± 8.5 Ma and 855 ± 16 Ma, in agreement with baddeleyite data. Remanence of these coarse-to-fine grained dolerite dykes is carried by stable single to pseudo-single domain (SD-PSD) magnetite. A positive baked-contact test, associated to a positive reversal test (Class-C), support the primary remanence obtained for these dykes (13 sites). Moreover, our new paleomagnetic dataset satisfy all the seven R-criteria (R=7). The ~860 Ma Manso pole can thus be considered as the first key Tonian paleomagnetic pole for West Africa. We propose that the West Africa-Baltica-Amazonia-Congo-São Francisco were associated in a long-lived WABAMGO juxtaposition (~1100–800 Ma).

Keywords: West Africa, Neoproterozoic, Tonian, Rodinia, paleomagnetism.

 

How to cite: Antonio, P. Y. J., Baratoux, L., Trindade, R. I. F., Rousse, S., Ayite, A., Lana, C., Macouin, M., Adu, E. W. K., Sanchez, C., Silva, M., Firmin, A.-S., Martinez Dopico, C. I., Proietti, A., Amponsah, P. O., and Sakyi, P. A.: Paleomagnetism of the ~860 Ma Manso dyke swarm, West Africa: implications for the assembly of Rodinia, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13336, https://doi.org/10.5194/egusphere-egu21-13336, 2021.

EGU21-4671 | vPICO presentations | EMRP3.5

The Neoproterozoic geomagnetic field: new insights from a high-resolution paleomagnetic study in South China

Justin Tonti-Filippini, Boris Robert, Élodie Muller, Michael Wack, Xixi Zhao, and Stuart Gilder

The paleomagnetic record during the middle Neoproterozoic (~825-780 Ma) displays rapid apparent polar wander variations leading to large discrepancies in paleogeographic reconstructions. Some authors propose that these data may represent true polar wander events, which correspond to independent motion of the mantle and lithosphere with respect to Earth’s rotation axis. An alternative explanation might be a perturbation of the geomagnetic field, such as a deviation from a predominantly dipole field or a hyper-reversing field. To test these hypotheses, we sampled 1200 oriented cores over a stratigraphic height of 100 metres in sedimentary rocks of the 820-810 Ma Laoshanya Formation in South China. We will present preliminary paleomagnetic and rock magnetic analyses together with results of petrologic and geochemical experiments to better understand the origin of the paleomagnetic signal.

How to cite: Tonti-Filippini, J., Robert, B., Muller, É., Wack, M., Zhao, X., and Gilder, S.: The Neoproterozoic geomagnetic field: new insights from a high-resolution paleomagnetic study in South China, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4671, https://doi.org/10.5194/egusphere-egu21-4671, 2021.

EGU21-12614 | vPICO presentations | EMRP3.5

Geochronological and paleomagnetic studies of small carbonatite intrusions of the Udzha uplift (Tomtor massif, northeast of the Siberian platform)

Aleksandr Pasenko, Ivanov Alexey, Malyshev Sergey, and Travin Alexey

Paleomagnetic data obtained from Neoproterozoic glacial and glacier-associated sedimentary rocks indicate that they were formed at near equatorial latitudes. Based on these data, the Snowball Earth hypothesis was proposed [Kirschvink, 1992]. According to this hypothesis, during the Neoproterozoic glaciations, the entire planet (including the oceans) was completely covered with ice. Although evidence is emerging that does not support this hypothesis, there is still no conclusive evidence that it is not true [Sansjofre et al., 2011].

It is worth noting that the Snowball earth hypothesis is based on paleomagnetic data. At the same time, the available paleomagnetic data for the Neoproterozoic-Early Cambrian [Meert, Van der Voo, 2001; Shatsillo et al, 2005; Abrajevitch, Van der Voo, 2010; Pavlov et al., 2018] difficult to interpret in terms of the Geocentric Axial Dipole hypothesis. This imposes serious restrictions on the possibility of correctly constructing paleomagnetic reconstructions.

For the development and testing of a model of the geomagnetic field of the Neoproterozoic, it is necessary to obtain a lot of high-quality paleomagnetic data. Data from well-dated magmatic bodies are especially valuable.

Within the framework of this work, we obtained paleomagnetic data from three carbonatite dikes (7 to 30 cm thickness) exposed in the Udzha river bank on the Udzha uplift in the northeastern part of the Siberian platform. These dikes are associated with the large alkaline Tomtor massif located 15 km to the west. Ar/Ar dating of phlogopite megacrysts gives an intrusion age of the dikes of 706.1±8.8 Ma. Coordinates of the virtual geomagnetic pole, calculated from the direction of the high-temperature component of magnetization: Φ=-20.7°; Λ=88.6°; Α95=3.4°.

Our report will present preliminary interpretation of these data, as well as their comparison with paleomagnetic data on close-aged objects in Siberia.

The research was supported by the Russian Science Foundation grant (19-77-10048).

References:

How to cite: Pasenko, A., Alexey, I., Sergey, M., and Alexey, T.: Geochronological and paleomagnetic studies of small carbonatite intrusions of the Udzha uplift (Tomtor massif, northeast of the Siberian platform), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12614, https://doi.org/10.5194/egusphere-egu21-12614, 2021.

The Ediacaran (635-541 Ma) is the last geological period of the Precambrian during which major changes occurred in the superficial layers of the Earth (biosphere, cryosphere, oceans, atmosphere). The paleomagnetic data from the main continents of this epoch display very fast polar wander excursions, which seemed to occur simultaneously on several continents. Two main competing hypotheses have been proposed in the literature to explain these data: (1) very fast True Polar Wander episodes (TPW), which represent the global movement of the mantle and the crust with respect to the Earth's spin axis, or (2) perturbations of the Earth’s magnetic field. On geological timescales, the TPW is speed-limited to some degrees per million years while magnetic field changes could be much faster (degrees per kyrs). The velocity of the polar wander excursions of the Ediacaran is therefore a critical parameter to distinguish these two families of solutions. The volcanic rocks of the Ouarzazate group (575-545 Ma) in the Anti-Atlas belt recorded a large polar wander excursion from ~571 to ~565 Ma, which is also observed in Laurentia and Baltica at about the same time. Because the age uncertainties are too high, the existing SHRIMP U-Pb ages obtained on zircons are not precise enough to distinguish these two hypotheses. In this study, we bring new high-precision CA ID-TIMS ages on zircons from seven tuff layers that recorded the rapid paleomagnetic variations. Our results show, in most of the samples, a large spread in age, indicating either the presence of inherited zircons or strong Pb loss in some of the zircons. Four of the samples display a good consistency in the zircon ages, and could represent the age of the tuff emplacement. In this presentation, we will discuss the two hypotheses based on these new geochronological constraints.

How to cite: Robert, B., Corfu, F., and Blein, O.: New age constraints on the Ouarzazate Group (Morocco): implications on the hypothesis of True Polar Wander during the Ediacaran, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9564, https://doi.org/10.5194/egusphere-egu21-9564, 2021.

The Neoproterozoic is marked by unusual perturbations of the climate system (global glaciations), biogeochemical cycles (e.g. oxygenation, Carbon), and life diversity that will lead to a world as we know it today. These upheavals can be considered from the point of view of paleogeographic reconstructions to decipher the forcing mechanisms and consequences. The paleopositions of the continents and their geology impact the continental weathering, a fundamental element in the feedbacks driving the climate. Besides, the position of the supercontinent Rodinia and the nature of its margins influence degassing, itself a major factor in biogeochemical cycles. Neoproterozoic paleogeographic reconstructions are based on some reliable paleomagnetic data and geological evidence of kinship between the cratons. Uncertainties in Neoproterozoic paleogeographies hinder our understanding of the relationship between deep Earth and superficials layers. Notably, the positions of the cratons that will constitute the Arabian-Nubian shield are poorly constrained. The paleomagnetic results we obtained in Oman on well-dated mafic dykes, indicate a mid-latitude position at the dawn of the Sturtian glaciation. These results show the potential of these small cratons, which represent a zone of arcs and arc collisions, in our understanding of the geodynamics of the Rodinia supercontinent. We then propose a refined configuration at ca. 720 Ma, highlighting the extent of snowball Earth deposits.

How to cite: Macouin, M., Rousse, S., and Antonio, P.: Solving uncertainties in Neoproterozoic paleogeographic reconstructions: the key to understanding the links between the Earth's outer and inner envelopes?, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14668, https://doi.org/10.5194/egusphere-egu21-14668, 2021.

EGU21-10078 | vPICO presentations | EMRP3.5

Implications of the detrital zircon record for global plate tectonic reconstructions in deep time

Dongchuan Jian, Simon Williams, Shan Yu, and Guochun Zhao

Full-plate reconstructions describe the history of both past continental motions and how plate boundaries have evolved to accommodate these motions. The fluxes of material into and out of the mantle at plate boundaries is thought to deeply influence the evolution of deep Earth structure, surface environments and biological systems through deep time. Traditionally, plate tectonic reconstructions have relied on geophysical data from the oceans, which provides details of how Pangea broke apart (since ca. 200 Myr) while paleomagnetism is the primary quantitative constraint prior to Pangea formation. However, these data do not directly constrain the extent of subduction zones or other plate boundaries, so reconstructing the past plate configurations of past supercontinents must rely on alternative methods. One source of data that can resolve this problem is to use observations from detrital zircons. Previous studies have proposed classification schemes to determine tectonic settings where samples were deposited, based on the different characteristic shapes of detrital zircon age spectra found in convergent, collisional and extensional settings.

Here, we investigate the applicability of this method to test and refine global full-plate tectonic reconstructions in deep time, using a published database of zircon ages. We first use reconstructions for relatively recent times (<100 Ma), where reconstructions are reasonable well constrained, to evaluate the effectiveness of the classification method. For older times, where uncertainties in the reconstructions are far larger, we can use the results to discriminate between competing models. We analysed the proximity between reconstructed plate boundaries and zircon sample sites assigned to different tectonic classifications, and found that the classification method does well (~64-79% success depending on distance threshold used) in distinguishing convergent settings. The ability of the classification to define extensional settings such as rift basins is less clear, though samples in this class do lie preferentially further from convergent settings. Based on these insights, we apply the method to evaluate full-plate reconstructions for the Neoproterozoic as well as other competing models for the configuration of Rodinia.

How to cite: Jian, D., Williams, S., Yu, S., and Zhao, G.: Implications of the detrital zircon record for global plate tectonic reconstructions in deep time, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10078, https://doi.org/10.5194/egusphere-egu21-10078, 2021.

EGU21-9494 | vPICO presentations | EMRP3.5

MORGEN – The Mid Ocean Ridge GENerating algorithm

Thomas van der Linden and Douwe van Hinsbergen

Paleo-digital elevation models (paleoDEM) based on plate tectonic and paleogeographic reconstructions use age grids of ocean floor to determine ocean bathymetry. In recent years, such age grids have also been developed for now-subducted oceans from the far geological past, as far back as the Neoproterozoic, using geology and paleomagnetism-based estimates of ocean opening. In such reconstructions, mid ocean ridges are drawn based on estimated Euler poles and rotations, and conceptual knowledge on the geometry consisting of spreading ridges and transform faults.

Current procedures to draw mid ocean ridges in plate tectonic reconstructions are laborious, as new ridges are drawn every time the Euler pole location changes. Fortunately this is also a task that can be automated. We have written an algorithm using pyGPlates that takes as input a smooth curve at the approximate position of the reconstructed mid ocean ridge at the moment of its formation, and then calculates spreading and transform segments according to their typical geometries in modern oceans, assuming symmetric spreading. The algorithm allows gradual readjustment of ridge orientations upon Euler pole changes comparable to documented cases in the modern oceans (e.g., in the Weddell Sea). The algorithm also contains modules that can convert the calculated mid ocean ridges with other plate boundaries to boundary topologies – which can be used as input for the recently published TracerTectonics algorithm, produce isochrons which can be converted to age grids, check for subduction of isochrons and subsequently create bathymetry grids. We illustrate the use of the MORGEN algorithm with recently published reconstructions of subducted, as well as future oceans.

How to cite: van der Linden, T. and van Hinsbergen, D.: MORGEN – The Mid Ocean Ridge GENerating algorithm, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9494, https://doi.org/10.5194/egusphere-egu21-9494, 2021.

EMRP3.6 – The short- and long-term variation of the Earth's magnetic field: new data and models

A recent trend in paleomagnetism is the study of samples of ever decreasing sizes, going down to (sub)millimeter scales and even microscopic scales, including single-silicate-crystals and meteorites. Microscopic imaging has shown that some of these micro-scale samples appear to be much closer to ideal single-domain (SD) paleomagnetic recorders than bulk rocks. Small samples with large numbers of SD particles do, however, pose the problem of magnetic interactions affecting their paleomagnetic recording fidelity. We show that clusters of particles are common in micro-scale samples and that these interactions do affect thermoremanent magnetization (TRM) acquisition. We further show through numerical simulations that such interacting clusters may be difficult to detect in traditional experiments (such as FORC diagrams), but may nonetheless lead to over- or underestimates in paleointensities.

How to cite: Berndt, T., Han, C., and Devienne, J.: The Bigger, The Better? – Paleomagnetic recording fidelity of weakly interacting clusters of particles, and implications for micro-scale paleomagnetism, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1116, https://doi.org/10.5194/egusphere-egu21-1116, 2021.

EGU21-1669 | vPICO presentations | EMRP3.6

Paleomagnetism of terrigenous rocks of the Volynian series (Ediacaran) from Podolia (Ukraine)

Ievgen Poliachenko, Semyon Cherkes, and Dmytro Hlavatskyi

We present the results of palaeomagnetic study of Ediacaran terrigenous rocks from the SW part of East European Craton (EEC), Podolia (Ukraine). Samples are represented by red tufits of Grushkinska suite by Volhynian series, which is comparable to the upper part of the Ediacaran age by the international stratigraphic scale. Samples for paleomagnetic studies were taken at the reference section of the Grushkinsky suite of the Volhynia series in the village of Grushka (48.45°N 28°E). A total of 50 oriented core samples were selected. For the entire collection of samples, the standard procedure for paleomagnetic studies was applied. The samples underwent stepwise temperature demagnetization.  Demagnetization showed that all samples are completely demagnetized at a temperature close to 700°C. The results of demagnetization showed that additionally to the viscous components of the magnetization released up to 200°C, four more stable components of NRM are released: CLM-1–component, relatively low temperature, in the range of deblocking temperatures of 200–360°C. It is characterized by south-south-west declination and negative inclination (D/I = 197.9/-28.6); CLM-2–component, is allocated in the same temperature range as component CLM-1 (200-360°С), is characterized by south-south-west declination and positive inclination (D/I = 202.4/31); CMH–component, is strictly allocated in the range of unlocking temperatures of 590–630°C. It is characterized by northwestern declination and positive inclination (D/I = 311/18.9); CH –component, a bipolar high-temperature component, is released in the temperature range of 650–700°C. The middle direction of the forward and reverse polarity is characterized by north-north-west declination and positive inclination (D/I = 296.4/71.2). The directions of normal and reverse polarity of this component are closely antipodal and successfully pass the reversal test (γ/γc = 7.85/8.82), class “B” in accordance with [McFadden & McElhinny, 1990].

The coordinates of the virtual geomagnetic poles for the two low-temperature components, respectively, are located close to the Permian (Φ/Λ = -53.7/357.9) and Silurian part (Φ/Λ = -21.8/4.9) of the apparent polar wander path for the EEC [Torsvik et al., 2012]. The VGP, calculated from the middle-high-temperature component, is located in the Caribbean region (Φ/Λ = 33.8/271.4) and the VGP for the relatively high temperature component is located in the eastern part of the North Atlantic (Φ/Λ = -52.5/149.1) that close to the another paleomagnetic determinations with ages about 550 Ma and 570 Ma respectively for different parts of EEC.

New data demonstrate the palaeomagnetic information content of the studied rocks and the possibility of their more detailed study in order to analyze anomalous palaeomagnetic data in the ediacaran and study the evolution of the Earth's geomagnetic field.

The analysis of directions and poles indicates that the paleomagnetic results do not contradict the data on the extremely high variability of the geomagnetic field in the studied time interval.

The new paleomagnetic determinations correspond to the previous results obtained by other authors for different regions of the East European platform, thereby supplementing them.

How to cite: Poliachenko, I., Cherkes, S., and Hlavatskyi, D.: Paleomagnetism of terrigenous rocks of the Volynian series (Ediacaran) from Podolia (Ukraine), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1669, https://doi.org/10.5194/egusphere-egu21-1669, 2021.

EGU21-9035 | vPICO presentations | EMRP3.6

Synthesis of palaeomagnetic datasets suggests that the geomagnetic field was persistently non-uniformitarian in the Devonian

Annique van der Boon, Andy Biggin, Daniele Thallner, Mark Hounslow, Jerzy Nawrocki, Kristyan Wójcik, Mariusz Paszkowski, Peter Königshof, Tim de Backer, Pavel Kabanov, Sofie Gouwy, Richard Vandenberg, and Richard Bono

The Devonian has long been a problematic era for paleomagnetism. Devonian data are generally difficult to interpret and have complex partial or full overprints. These problems arise from paleomagnetic data obtained from both sedimentary and igneous rocks. As a result, the reconstruction of motions of tectonic plates is often troubling, as these rely on apparent polar wander paths constructed from Devonian paleomagnetic poles. Also the geomagnetic polarity time scale for this time period is poorly constrained. Paleointensity studies suggest that the field was much weaker than the field of today, and it has been hypothesised that this was accompanied by many polarity reversals (a hyperreversing field). We review studies on Devonian paleopoles, magnetostratigraphy and paleointensity. We tentatively suggest that the field during the Devonian might have been so weak and perhaps of a non-dipolar configuration, that obtaining reliable paleomagnetic data from Devonian rocks is extremely difficult.  In order to push forward the understanding of the Devonian field, we emphasise the need for studies to provide fully accessible data down to specimen level demagnetisation diagrams. Incorporating all data, no matter how complex or bad they might seem, is the only way to advance the understanding of the Devonian magnetic field. Recent paleointensity studies appear to suggest that the Devonian and Ediacaran were both extreme weak field intervals. For the Ediacaran, it has been hypothesised that the field had an impact on life on earth. A fundamentally weak and perhaps non-dipolar field during the Devonian might have had an influence on evolution and extinctions. As there is a large number of biological crises in the Devonian, we here pose the question whether the Earth’s magnetic field was a contributing factor to these crises. New independent evidence from the Devonian-Carboniferous boundary suggests that the Hangenberg event was caused by increased UV-B radiation, which is in line with a weak magnetic field.

How to cite: van der Boon, A., Biggin, A., Thallner, D., Hounslow, M., Nawrocki, J., Wójcik, K., Paszkowski, M., Königshof, P., de Backer, T., Kabanov, P., Gouwy, S., Vandenberg, R., and Bono, R.: Synthesis of palaeomagnetic datasets suggests that the geomagnetic field was persistently non-uniformitarian in the Devonian, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9035, https://doi.org/10.5194/egusphere-egu21-9035, 2021.

EGU21-1737 | vPICO presentations | EMRP3.6 | Highlight

The strength of the Earth’s magnetic field from Pre-Pottery to Pottery Neolithic, Jordan

Anita Di Chiara, Lisa Tauxe, Thomas Levy, Mohammand Najjar, Fabio Florindo, and Erez Ben-Yosef

Constraining the secular variations of the Earth’s magnetic field strength in the past is fundamental to understanding short term processes of the geodynamo. Such records constitute a powerful and independent dating tool for archaeological sites and geological formations. In this study, we present 10 new and robust archaeointensity results from Pre-Pottery to Pottery Neolithic and, for one of the first times, flint (burnt chert) from Jordan. Two of these results constitute the oldest archaeointensity data for the entire Levant, ancient Egypt and Mesopotamia extending the archaeomagnetic dating reference for the Holocene. Virtual Axial Dipole Moments (VADM)s show that the Earth’s magnetic field in the Southern Levant was weak (about half the present field) at around 7,600 years BCE, recovering its strength to greater than the present field around 7,100 BCE and gradually weakening again around 5,200 years BCE. In addition, successful results obtained from burnt flint demonstrate the potential of this rarely used material in archaeomagnetic research, in particular for prehistoric periods from the first use of fire to the invention of pottery.

How to cite: Di Chiara, A., Tauxe, L., Levy, T., Najjar, M., Florindo, F., and Ben-Yosef, E.: The strength of the Earth’s magnetic field from Pre-Pottery to Pottery Neolithic, Jordan, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1737, https://doi.org/10.5194/egusphere-egu21-1737, 2021.

EGU21-4232 | vPICO presentations | EMRP3.6 | Highlight

Reuse of medieval bricks as important limitation for construction of geomagnetic secular variation curves based on archeomagnetic studies of brick buildings in Poland. 

Jerzy Nawrocki, Karol Standzikowski, Tomasz Werner, Maria Łanczont, Jan Gancarski, and Zdzisław Gil

The bricks can be one of the best material for archeomagnetic studies. Their backing technique (i.e. horizontal location in the furnace) allow to determine also the value of inclination of geomagnetic field.  However, reuse of older bricks for the construction of newer objects can limit the utility of this material in archeomagnetic studies. A set of the brick samples from 26 historical buildings in SE Poland was taken for archeomagnetic investigations. As a result of this study, the secular variations of palaeointensity and inclination of the geomagnetic field from 1200 to 1800 AD were defined for this part of Poland. The paleointensity of geomagnetic field  was determined using the IZZI-Thellier-Thellier protocol. The course of the new regional palaeosecular curves is approximately the same as so far obtained in other parts of Europe. Data obtained from four brick buildings, however, do not fit substantially to the reference European curves. The remarkable difference  is a rapid and deeper drop of inclination and significantly higher than expected values of  palaeointensity. These features indicate that bricks used for the construction of these buildings (dated on XVI – XVII centuries) were taken from older brick constructions, most probably from the Gothic time (XIII/XIV c.). We compared our data with the earlier data obtained from brick buildings in N Poland. The regional archeomagnetic curves calculated for these two regions of Poland are completely different in their segments as old as the first half of the 18th century. This fact could be explained by the reuse of medieval bricks during the construction of studied objects from N Poland (dated on the first half of the 18th century) and applied for the construction of reference curve or by later secondary heating of original bricks.

This research was supported by  the National Science Centre of Poland (project no: UMO-2016/23/B/ST10/0129).

How to cite: Nawrocki, J., Standzikowski, K., Werner, T., Łanczont, M., Gancarski, J., and Gil, Z.: Reuse of medieval bricks as important limitation for construction of geomagnetic secular variation curves based on archeomagnetic studies of brick buildings in Poland. , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4232, https://doi.org/10.5194/egusphere-egu21-4232, 2021.

EGU21-4385 | vPICO presentations | EMRP3.6

Secular geomagnetic field variations in Bulgarian lands during Classical Age and Late Antiquity – new archaeomagnetic data

Maria Kostadinova-Avramova, Petar Dimitrov, Andrei Kosterov, and Mary Kovacheva

Numerous historical sources and archaeological monuments attest the age of Antiquity in Bulgaria – from both the early Roman period (I – III c.) and Late Antiquity (IV – VI c.). Owing to systematic archaeological excavations, lasting more than 100 years, plenty of information has been accumulated concerning not only all aspects and manifestations of its material culture, but also their evolution and chronology.  This in turn allows for interdisciplinary fields such as archaeomagnetism to progress.

There are many archaeomagnetically studied archaeological structures from the Antiquity. The results included in the Bulgarian database form 74 reference points. However, only 20 of them are full-vector determinations because 70 % of the investigated materials are bricks. Hence, the secular variation of declination is poorly constrained within the considered period. Moreover, the reuse of bricks in the constructions occurred quite often (especially in the Late Antiquity) providing for possible errors in archaeological dating. In addition, stronger effects of magnetic anisotropy and cooling rate are usually expected for bricks than for hearths, domestic ovens, production kilns or burnt dwelling remains (there are no results from pottery in the Bulgarian dataset) and both factors are not evaluated for most of the older results. All this can explain the contradictions observed between some of the experimental results juxtaposed over the absolute time scale. In an attempt to clarify these contradictions 13 baked clay structures from eight archaeological sites were archaeomagnetically studied producing seven new directional and eight new intensity data. The samples collected possess variable magnetic properties suggesting differences in clay sources and/or firing conditions. Magnetically soft minerals prevail in seven structures but in the remaining six, abundant HCSLT phase is detected. The success rate of archaeointensity determination experiments vary from 49 to 100 %. It appears that samples containing HCSLT phase always produces good araeointensity results unlike those with the dominant presence of soft carriers.

The new reference points are compared with the present compilation of Bulgarian archaeomagnetic dataset and with the data from the neighboring countries.

 

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., Dimitrov, P., Kosterov, A., and Kovacheva, M.: Secular geomagnetic field variations in Bulgarian lands during Classical Age and Late Antiquity – new archaeomagnetic data, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4385, https://doi.org/10.5194/egusphere-egu21-4385, 2021.

EGU21-13101 | vPICO presentations | EMRP3.6 | Highlight

Rapid intensity variations during the second half of the first millennium BCE in Central Asia and global implications.

Raquel Bonilla-Alba, Miriam Gómez-Paccard, Francisco Javier Pavón-Carrasco, Elisabet Beamud, Verónica Martínez-Ferreras, Josep Maria Gurt-Esparraguera, Enrique Ariño-Gil, Judit del Rio, Alicia Palencia-Ortas, Fátima Martín-Hernández, Annick Chauvin, and María Luisa Osete

Recent archeomagnetic studies performed in different regions of the world have revealed unusual periods of sharp changes in intensity during the first millennium. Here we focus on the study of intensity variations between 600 BCE and 600 CE in central Asia, where an important intensity decrease seems to be present during the second half of the 1st millennium BCE. For this purpose, we present a new paleosecular variation (PSV) curve obtained from 51 new archeointensities and the selected previous data located within a radius of 1000 km around Termez (Uzbekistan). The new curve shows an intensity maximum around 400 BCE followed by a rapid decrease. When the virtual axial dipole moment (VADM) values are compared with the Dipole Moment estimations derived from different global geomagnetic models key differences are observed, suggesting an important non-dipolar effect for this feature. Finally, in order to constrain the spatial behaviour of this phenomenon and its global implications, we investigate the PSV intensity and VADM trends from twelve regions distributed among Central America, Europe and Asia. A VADM maximum is observed in Western Europe (Iberia and Germany) around 450 BCE, associated to rates of change of about 9 µT/century. This feature is also observed eastwards, in the Caucasus and the Levant, but associated to lower rates of changes. In Central Asia (Uzbekistan) our new study suggests that maximum values of about 14 µT/century, between 400-300 BCE, were achieved. In other regions, as Eastern Asia and Central America, rapid variations of the intensity are not observed during the targeted period.

 

How to cite: Bonilla-Alba, R., Gómez-Paccard, M., Pavón-Carrasco, F. J., Beamud, E., Martínez-Ferreras, V., Gurt-Esparraguera, J. M., Ariño-Gil, E., del Rio, J., Palencia-Ortas, A., Martín-Hernández, F., Chauvin, A., and Osete, M. L.: Rapid intensity variations during the second half of the first millennium BCE in Central Asia and global implications., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13101, https://doi.org/10.5194/egusphere-egu21-13101, 2021.

EGU21-3409 | vPICO presentations | EMRP3.6

Comprehensive palaeomagnetic study of San Borja and Jaraguay monogenetic volcanic fields, Baja California (28–30°N): considerations on latitudinal corrections

Ahmed Nasser Mahgoub, Bernardo Ignacio García-Amador, and Luis Manuel Alva-Valdivia

We report 24 palaeomagnetic directions and 10 high-quality Thellier-derived palaeointensity (PI) values, obtained from 27 sites located in Baja California Peninsula, northwestern Mexico. Sampling was done in four rock units (magnesian andesites, calc-alkaline lavas, ignimbrites, adakites) belonging to San Borja and Jaraguay monogenetic volcanic fields. These units were erupted between ~ 15 and 2.6 Ma (previous K-Ar and 40Ar/39Ar data), hence results are presented in two consecutive periods: middle-late Miocene and Pliocene. Based on previous geological and geophysical records, the kinematic evolution of the region was carefully considered, allowing for the independent restoration of the palaeoposition of each sampled site. The identified main magnetic minerals are titanomagnetite, magnetite, and minor hematite, of variable grain size, present as intergrowths, which reflect varying oxidation/reduction conditions during emplacement of high-temperature magmas. We did not observe a clear relationship between the magnetic properties of the different sites and their success rate for PI experiments. This is with the exception of the FORC analysis which showed a fairly good correlation with PI success. Pliocene (Dec=359.2°; Inc= 47.4°; α95=7.6°; and k= 41.43) and Middle-late Miocene (Dec=353.9°; Inc= 38.5°; α95=9.2°; and k= 28.56) mean directions were calculated from 20 sites (10 sites per period), and PI mean values of 29.2 ± 9.1 μT and 23.2 ± 6.3 μT were determined for the two periods, respectively. Compiling global filtered PI data, together with our results, indicates that the strength of the geomagnetic field during middle-late Miocene was weak (virtual dipole moment = 5.0±2.2×1022 Am2) compared to Pliocene (6.4±2.8× 1022 Am2), and also relative to the present-day value (7.6 × 1022 Am2). This indicates the global nature of the low dipole moment during the middle-late Miocene, which is consistent with what was previously concluded that from the past 30 Ma to the present time the magnetic field strength has increased. However, issues related to the Spatio-temporal distribution of PI data still present an obstacle to validating these suggestions; therefore, more reliable data are still needed.

How to cite: Mahgoub, A. N., García-Amador, B. I., and Alva-Valdivia, L. M.: Comprehensive palaeomagnetic study of San Borja and Jaraguay monogenetic volcanic fields, Baja California (28–30°N): considerations on latitudinal corrections, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3409, https://doi.org/10.5194/egusphere-egu21-3409, 2021.

EGU21-8159 | vPICO presentations | EMRP3.6 | Highlight

Recurring magnetic field anomalies in the South Atlantic and the first palaeointensities from Saint Helena

Yael Engbers, Andy Biggin, and J. Michael Grappone

A long-lived hypothesis is that, if averaged over sufficient time (ca 10 million years), the Earth’s magnetic field approximates a geocentric axial dipole (GAD). Despite this common assumption, the question of how significant the non-GAD features are in the time-averaged field is an important and unresolved one. In the present-day field, the South Atlantic Anomaly (SAA) is the biggest irregularity in the field. We know that this anomaly has not always been a part of the field, but in Engbers et al., 2020, it was shown that the magnetic field shows irregular behaviour in this region on a million-year timescale. The irregular behaviour was demonstrated through a substantially high VGP dispersion (21.9º) for lava flows from Saint Helena that are between 8 and 11 million years old. The island of Saint Helena is located at the margin of the present-day SAA and has declination -16.6º, inclination -57.5º relative to expected GAD values of 0.0º/-7.8º (Dec/Inc). We have now commenced the measurements of absolute palaeointensity data from this location. So far, we have performed thermal and microwave IZZI-Thellier experiments on 2 localities from Saint Helena. The site mean results show variable but generally very low field intensities, although further work is required to make these sufficiently robust. Our low field estimates suggest a field in the South Atlantic that is not only unstable, but mainly weaker than expected. This could mean that recurring reversed flux patches (RFP) are responsible for the irregularities and weaknesses in the field in this region, stretching back up to 11 million years ago.

How to cite: Engbers, Y., Biggin, A., and Grappone, J. M.: Recurring magnetic field anomalies in the South Atlantic and the first palaeointensities from Saint Helena, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8159, https://doi.org/10.5194/egusphere-egu21-8159, 2021.

Studying of paleosecular variations (PSV) over geological time allows us to characterize not only the behavior and evolution of the geomagnetic field, but also to estimate the rate of formation of large igneous provinces (LIP). In order to use this paleomagnetic tool, the amplitude of paleosecular variations during the corresponding time interval has to be known, but for the end of the Cretaceous superchron, in particular for high latitudes, such the data sets are extremely small. Our study is aimed at obtaining a limit on the PSV amplitude for Late Cretaceous in order to use these data to estimate the rate of formation of the Okhotsk-Chukotka Volcanic Belt.

The formation of a paleomagnetic record in volcanic flows occurs by acquiring a thermal remanent magnetization (TRM) during their cooling below the Curie temperature of the magnetic minerals. Direction of this TRM can be used for calculation of the virtual geomagnetic pole (VGP), which characterizes the direction of the geomagnetic field at a given time and place. The angular dispersion of virtual geomagnetic poles (VGP scatter, Sb) is generally accepted as a measure of the paleosecular variations and uses to assess the duration of volcanic section formation. If the volcanic section was formed for a long time (more than 10 000 years), then the amplitude of the recorded geomagnetic variations will correspond to the expected dispersion for a given latitude. In the case of significantly higher eruption rates, the amplitude of the recorded PSV will be lower than it is predicted by the model for a given latitude.

During the 2019-2020 field seasons paleomagnetic studies were carried out on a number of Late Cretaceous volcanic sections of the Okhotsk-Chukotka Volcanic Belt located in the Bilibinsky District of the Chukotka Region. VGPs and their scatter were calculated for 79 flows of the Kupol object. Preliminary results show that the amplitude of PVS in the Cretaceous for high latitudes of the northern hemisphere was close to that for the last 5 million years (Sb=21.4, [19.0; 23.9]).

The work is supported by the Russian Science Foundation grant N 19-47-04110.

How to cite: Bobrovnikova, E. and Lebedev, I.: Amplitude of paleosecular variations during the Cretaceous superchron: Okhotsk-Chukotka Volcanic Belt, high latitudes, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13171, https://doi.org/10.5194/egusphere-egu21-13171, 2021.

EGU21-13406 | vPICO presentations | EMRP3.6

Harmonic fluctuations in the relative paleointensity data?

Marcia Ernesto, Thamyris Britto, and George Caminha-Maciel

The existing relative paleointensity (RPI) database allowed the construction of reliable stacking curves for at least the last 1 Myr. Observed fluctuations in the RPI curves suggest both lithologic/climatic influence or geodynamo processes. Stacked power spectra for RPI data from ten North and South Atlantic cores revealed a spectral peak at ~5.3kyr for data covering the last 100 kyr. This signal exhibits a similar phase for most of the series. The observed spectral peak has no apparent correspondence in the benthic O18 spectra from the same cores, suggesting the RPI signal is free from the climatic influence. Therefore, it may be a real geodynamo feature.

How to cite: Ernesto, M., Britto, T., and Caminha-Maciel, G.: Harmonic fluctuations in the relative paleointensity data?, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13406, https://doi.org/10.5194/egusphere-egu21-13406, 2021.

EMRP3.7 – Paleomagnetism and magnetic fabrics: Recent advances and geological applications

EGU21-1686 | vPICO presentations | EMRP3.7

Magnetic fabric in carbonatic rocks from thrust shear zones

Sara Satolli, Claudio Robustelli Test, Elena Zanella, Dorota Staneczek, Fernando Calamita, and Evdokia Tema

 

The aim of this study is to investigate how structural deformation in shear zones is documented by the anisotropy of magnetic susceptibility (AMS). The study area is located in the Pliocene outer thrust of the Northern Apennines, which involved Cretaceous to Neogene calcareous and marly rocks. Here, brittle-ductile tectonites show different characteristics along two differently oriented thrust ramps: the NNE-SSW-trending oblique thrust ramp is characterized by the presence of S tectonites, while the NW-SE-trending frontal ramp is characterized by the presence of SC tectonites.

Samples for AMS fabric investigation were collected on shear zones from three sectors of the belt, at different distance from the main thrust to detect possible magnetic fabric variations. The three study area are characterized by different combinations of simple and pure shear, thus different degree of non-coaxiality, which has been quantified through the vorticity number Wk.

Specimens were measured with an AGICO KLY-3 Kappabridge at the CIMaN-ALP Laboratory (Italy) on 15 different directions mode. Only measurements with all three F-statistics of the anisotropy tests higher than 5 were accepted as reliable. Moreover, outliers characterized by ± 2σ difference with respect to the mean value of AMS scalar parameters were excluded from further analysis. In order to distinguish groups of specimens affected by different sedimentary or tectonic processes, we identified clusters of AMS scalar parameters; when clusters were not defined by these parameters, we applied a combination of contouring and cluster analysis on each principal axis to identify different subfabrics.

The magnetic fabric revealed straightforward correlations with structural data and specific changes of AMS axis orientation depending upon the increasing of deformation (lower vorticity number) and proximity to the main thrust. Similar evolution was detected in different deformation regimes. Overall, the magnetic fabric is more sensitive to the simple shear deformation, as the magnetic lineation tends to parallelize mostly with the computed slip vector; however in pure-shear dominated regimes, the magnetic lineation becomes parallel to the transport direction when the deformation is really intense (sites at less than 15-30 cm from the thrust plane).

The applied combination of density diagrams and cluster analysis on AMS data successfully allowed discriminating subfabrics related to different events, and shows a great potential to unravel mixed sedimentary and/or tectonic features in magnetic fabrics.

How to cite: Satolli, S., Robustelli Test, C., Zanella, E., Staneczek, D., Calamita, F., and Tema, E.: Magnetic fabric in carbonatic rocks from thrust shear zones, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1686, https://doi.org/10.5194/egusphere-egu21-1686, 2021.

EGU21-1481 | vPICO presentations | EMRP3.7

Unravelling heterogeneities in magnetic fabric record of the strain: a combined AMS and ApARM data analysis applied to intraplate shear zones.

Claudio Robustelli Test, Elena Zanella, Andrea Festa, and Francesca Remitti

Deciphering the stress and strain distribution across plate boundary shear zones is critical to understanding the physical processes involved in the nucleation of megathrust faults and its behaviour. Plate boundaries at shallow depth represent complex and highly deformed zones showing structures from both distributed and localized deformation.

As magnetic minerals are sensitive to stress regime, the investigation of the magnetic fabric has proven to be an effective tool in studying faulting processes at intraplate shear zones.

Anisotropy of magnetic susceptibility (AMS) provides insights into the preferred orientation of mineral grains and the qualitative relationships between petrofabrics and deformation intensity.

We present an approach of combined Contoured Diagram and Cluster Analysis to isolate the contribution of coexisting petrofabrics to the total AMS and evaluating the significance of magnetic fabric clusters.

Our results reveal distinct subfabrics with reasonably straightforward correlations with structural data. Specific AMS pattern may be associated to the intensity of the reworking related to tectonic shearing and the structural position within the shear zone (i.e., the proximity to the main thrust faults).

Close to the main thrust the magnetic fabric is dominantly oblate with magnetic foliation consistent to the S-C fabric and/or mélange foliation and the magnetic lineation parallel to the shear sense.

Away from the thrust faults the degree of anisotropy as well as the ellipsoids oblateness gradually diminishes. Thus, the presence of subfabrics related to previous tectonic events or less intense deformation (i.e. intersection lineation fabric) became dominant. The discrimination of subfabrics also allowed to unravel the presence of minor thrust plane and qualitatively evaluate the heterogeneous registration of strain (i.e. distributed versus localized deformation).

An abrupt change in magnetic ellipsoid shape and parameters is also observed below the basal décollements showing purely sedimentary magnetic fabric or previous deformation history with minor to absent evidences of shearing in the hanging wall.

Then, the integration with anisotropy of magnetic remanence experiments in different coercivity windows (ApARM) allow to separate the contribution of different ferromagnetic subpopulation of grains, constraining the significance of the different magnetic pattern/clusters detected through the AMS analysis.

In conclusion, our results show the potential of a combination of density diagrams and cluster analysis validated by ApARM experiments in distinguishing the superposition of deformation events, unravelling strain partitioning/concentration and thus to better understand the geodynamic evolution of subduction-accretion complexes.

How to cite: Robustelli Test, C., Zanella, E., Festa, A., and Remitti, F.: Unravelling heterogeneities in magnetic fabric record of the strain: a combined AMS and ApARM data analysis applied to intraplate shear zones., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1481, https://doi.org/10.5194/egusphere-egu21-1481, 2021.

EGU21-6377 | vPICO presentations | EMRP3.7

AMS of strained shales fragments: a fast way to quantify the matrix damage

Francho Gracia Puzo, Charles Aubourg, and Antonio Casas Sainz

With the objective of mapping strain on the footwall of a thrust in an orogenic context (Leyre thrust, South Pyrenean Range), more than 1500 unoriented shale fragments (0.7-6.2 g) have been collected. 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 such as shales. On the contrary, collecting rock fragments presents the net advantage to provide a much better statistical characterization of the site.

All samples belong to the Eocene shaly formations from the Jaca Basin. Rock fragments are mostly fractured according to the bedding and/or cleavage surfaces. We demonstrate that the anisotropy parameters P and T 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.

In the footwall of the Sierra de Leyre we have defined up to 7 parallel sampling sections, whose traces are perpendicular to the direction of the main thrust. On average, each section is made up of about 10 sampling sites and about 15 fragments are collected per site, covering a few square meters.

We are restricted by the dimensions of AGICO holders (8cm3 for cubes, or 10 cm3 for cylinders). It is possible to use an empty 10 cm3 cylinder, which can be filled with smaller fragments of rock. The automatic rotator allows a fast and precise description of the AMS tensor. We removed from analysis low susceptibility, carbonate-rich samples, that show a higher variety 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 the minimum AMS axis (K3) provides a sensitive proxy to characterize the competition between bedding and cleavage.

The comparison between the different sections allows to map the areas of damage linked to the propagation of faults associated with the folds. 5 stages of development of the magnetic fabric allows the detection of damage gradients. The mapping has allowed the identification of hidden faults.    

This new approach is very promising, and allows much more detailed samplings in difficult areas, providing more robust statistical description of scalar AMS data. This methodology could be useful for the study of outcrops that are difficult to access, and more interestingly, from borehole cuttings.

How to cite: Gracia Puzo, F., Aubourg, C., and Casas Sainz, A.: AMS of strained shales fragments: a fast way to quantify the matrix damage, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6377, https://doi.org/10.5194/egusphere-egu21-6377, 2021.

EGU21-1278 | vPICO presentations | EMRP3.7

Explaining the large variability in empirical relationships between magnetic pore fabrics and pore space properties

Andrea Regina Biedermann, Yi Zhou, and Michele Pugnetti

The magnetic pore fabric method was proposed in the 1990s as a fast and efficient way to characterize the preferred shape and alignment of pores in sedimentary rocks. Since then, magnetic pore fabrics have been interpreted based on empirical relationships, stating for example that the orientation of magnetic pore fabric relates to pore alignment, and that degree and shape of the magnetic anisotropy correlate with pore aspect ratio and pore shape. Unfortunately, these empirical relationships vary largely between studies, making it challenging to interpret the data quantitatively. Here, we are presenting numerical models and experiments that help explain the variability in published relationships. Magnetic pore fabrics are determined by first impregnating the samples with a suitable ferrofluid, followed by magnetic anisotropy measurements in various fields and at multiple frequencies. These data are compared to known pore properties in synthetic samples, or X-ray-computed-tomography-based models of the pore space in natural samples. Our data show how the magnetic properties of various ferrofluids vary with measurement conditions, in particular measurement frequency. Ultimately, the effective susceptibility of the ferrofluid at measurement conditions largely affects measured magnetic anisotropy, and thus the observed empirical relationships. These findings allow to re-interpret existing empirical relationships and their variability, and also provide recommendations for future magnetic pore fabric studies. 

How to cite: Biedermann, A. R., Zhou, Y., and Pugnetti, M.: Explaining the large variability in empirical relationships between magnetic pore fabrics and pore space properties, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1278, https://doi.org/10.5194/egusphere-egu21-1278, 2021.

EGU21-8720 | vPICO presentations | EMRP3.7

Magnetic fabric of Devonian and Precambrian dikes from the northeastern Kola Peninsula, Russia

Sofya Fursova and Roman Veselovskiy

Nowadays, anisotropy of magnetic susceptibility (AMS) is a very widespread method to investigate the magnetic fabric of the rocks. AMS studies provide the information on the magma flow during formation of intrusives (dikes and sills) based on the orientation of the AMS ellipsoid. In this work we present the results of the AMS measurements of Devonian, Paleoproterozoic and Neoarchean dikes, which are located on the northeastern part of the Kola Peninsula. We use these data to reconstruct the direction of magma flow with the final aim to reconstruct the spatial distribution of the magma sources.

Laboratory studies of AMS were carried out on 1282 samples representing 102 dikes. At this stage, all studied dikes were typified based on their geological age and type of AMS (normal, reversed, intermediate). It is shown that not all the studied intrusive bodies retained the primary magnetic fabric and only a few of them can be used to reconstruct the position of magmatic centers and are promising for obtaining reliable paleomagnetic data. We also present the first results of interpretation of the direction of the magma flow during dike swarms forming.

The study is supported by the grant of RSF #16-17-10260.

How to cite: Fursova, S. and Veselovskiy, R.: Magnetic fabric of Devonian and Precambrian dikes from the northeastern Kola Peninsula, Russia, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8720, https://doi.org/10.5194/egusphere-egu21-8720, 2021.

The Miocene Tejeda Complex on Gran Canaria (Canary Islands) is characterized by more than 500 trachytic and phonolitic conesheets, dikes, hypabyssal syenite stocks and subordinate radial dikes from a 20-km diameter intrusive complex in the volcaniclastic fill of the Miocene Tejeda caldera (20 x 35 km) on Gran Canaria, Canary Islands. The dikes intruded concentrically around a central axis or radial symmetry and dip uniformly an average of~41 degrees toward the center.We have conducted a pilot study of magnetic properties as well as Anisotropy of Magnetic Susceptibility (AMS) on a variety of dikes (trachytic and phonolitic and basaltic in composition) to investigate the possibility of obtaining petrofabrics results that would allow us to test the origin of the formation of the Tejeda conesheets that most likely resulted from deformation processes due to resurgent doming initiated by recurrent replenishment of a flat  laccolith-like magma chamber. The current ideas indicate that the formation of the cone-shaped fractures were originated by a magma supply exceeding the volume that could be compensated for by up-doming of the overlying caldera fill. Thus far, our AMS results indicate that all the ten intrusives studied despite their different lithologies are susceptible of carrying a measurable magnetic signal. Low-field magnetic susceptibility vs temperature (k-T 28-700oC) experiments have identified mainly one primary magnetic mineral phase namely stoichiometric magnetite, Curie temperature of 585oC,  SIRM, hysteresis loops and back-field were performed and yielded a series of secondary magnetic mineral present as well and corroborated by FORC’s results. The petrofabric in the intrusive bodies results show coherent flow azimuths regardless of their time of emplacement. Three main types of magnetic fabrics, (i.e. A to C) were found. Fabric type A (plane Kmax-Kint parallel to the dike plane) represents magma flow direction within the intrusives and is the dominant fabric (~60% of all the intrusives) studied thus far. The Kmax axis inclinations show that about 70% of the intrusives were fed by inclined vertical magma fluxes (inclinations greater than 30o), and the rest of them (~30%) by horizontal to sub-horizontal magma fluxes. Vertical magma flow means inclined magma injection inside fractures, and become more probable as the source is therefore located very close to the origin of the caldera

How to cite: Herrero-Bervera, E. and Calvo Rathert, M.: On the rock magnetic properties and petrofabric (AMS) analyses of trachytic, phonolitic, basaltic dikes and conesheets emplaced on the Miocene Tejeda caldera complex, on Gran Canary Island, Spain., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9019, https://doi.org/10.5194/egusphere-egu21-9019, 2021.

EGU21-2385 | vPICO presentations | EMRP3.7

Anisotropy of out-of-phase magnetic susceptibility due to eddy currents in graphite ore and its structural implications

František Hrouda, Jan Franěk, Martin Chadima, Josef Ježek, Štěpánka Mrázová, and Michal Poňavič

Magnetostatic susceptibility of single crystals of graphite is negative (the mineral is diamagnetic) and strongly anisotropic. The in-phase component of dynamic susceptibility (measured in alternating magnetic field) is also negative, but an order-of-magnitude stronger than the magnetostatic susceptibility. The out-of-phase component, which is no doubt due to electrical eddy currents, is positive and strong. Consequently, if the graphite crystals in graphite ore are oriented preferentially by crystal lattice (LPO), one would expect strong anisotropy of magnetic susceptibility (AMS) of graphite ore in both in-phase (ipAMS) and out-of-phase (opAMS) components. The ipAMS is controlled not only by the LPO of graphite, but also by the preferred orientation of paramagnetic and ferromagnetic minerals of the barren rock, while the opAMS indicates only the LPO of graphite. In graphite ores occurring in the Moldanubian Unit of Southern Bohemia, the in-phase susceptibility ranges from negative values in the order of 10-5 [SI units] to positive values in the order of 10-4. This probably indicates simultaneous 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 ipAMS is moderate, that of opAMS is truly high. The ipAMS foliation is roughly parallel to the metamorphic foliation in ores and wall rocks and the ipAMS lineation is parallel to the mesoscopic lineation. The opAMS is inverse to the ipAMS with the opAMS lineation being perpendicular to the metamorphic foliation. All this indicates a conspicuous LPO of graphite in the ore that was probably created during Variscan regional metamorphism and associated ductile deformation. The opAMS has therefore shown an effective tool for the investigation of the LPO of graphite in graphite ore or graphite-bearing rocks provided that the opAMS is strong enough to be determined with sufficient precision and graphite is the only conductive mineral in the samples investigated.

How to cite: Hrouda, F., Franěk, J., Chadima, M., Ježek, J., Mrázová, Š., and Poňavič, M.: Anisotropy of out-of-phase magnetic susceptibility due to eddy currents in graphite ore and its structural implications, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2385, https://doi.org/10.5194/egusphere-egu21-2385, 2021.

In order to better understand the anisotropy of out-of-phase magnetic susceptibility (opAMS) due to eddy currents of rocks and minerals, we investigated the ipAMS and opAMS of copper cylinders, because copper is diamagnetic, internally isotropic magnetically, and its opMS is no doubt due to eddy currents. Results of laboratory measurements were compared to theoretical models.  The degree of AMS (P=k1/k3, k1>k2>k3) of internally isotropic substance is controlled by the internal susceptibility and demagnetizing factors solely controlled by the substance shape. If the internal susceptibility is very low and this is the case of magnetostatic susceptibility of most diamagnetic substances, there is virtually no shape anisotropy of such material. The magnetostatic susceptibility of copper is about -8 x 10-6 [SI units], which gives rise to undetectably low degree of magnetostatic AMS. The ipMS is also negative, but several orders lower depending on grain size, grain shape, and operating frequency. The opMS is on the other hand positive, relatively high and also dependent on operating frequency. Calculated from signed (negative) k1, k2, k3 values, the degree of ipAMS, P<1, and decreases with increasing elongation of copper cylinders. It also depends on grain size and operating frequency. The degree of opAMS, P>1 and increases with increasing cylinder elongation. It also depends on grain size and operating frequency. Parallel to the axes of elongated cylinders are the ipk1 and opk3 directions, while in flattened cylinders are parallel to the cylinder axes the ipk3 and opk1 directions. The above results are useful in the structural interpretation of the opAMS of rocks and ores whose opAMS is due eddy currents (in electrically conductive materials).

How to cite: Ježek, J. and Hrouda, F.: Unexpectedly strong shape anisotropy of out-of-phase magnetic susceptibility due to eddy currents of internally isotropic copper cylinders, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3076, https://doi.org/10.5194/egusphere-egu21-3076, 2021.

EGU21-2835 | vPICO presentations | EMRP3.7

Anisotropy of out-of-phase magnetic susceptibility: an approach for magnetic subfabrics determination

Cláudia Cruz, Helena Sant'Ovaia, William McCarthy, and Fernando Noronha

The Anisotropy of Magnetic Susceptibility (AMS) represents the contribution of all minerals in rock samples (paramagnetic, diamagnetic, and/or ferromagnetic minerals). An intermediate AMS tensor may be recorded in rocks where a composite fabric is present, due to the presence of both paramagnetic and ferromagnetic minerals, being possible to be resolved into two distinct subfabrics using techniques as out-of-phase AMS (opAMS). The magnetic susceptibility measured in alternating field can be resolved into in-phase and out-of-phase components. In-phase AMS (ipAMS) measures the bulk response of all minerals in a sample however, opAMS is only sensitive to selected ferromagnetic minerals such as hematite, titanomagnetite, and ultrafine magnetite. The opAMS can be harnessed as a tool for direct determination of magnetic subfabrics defined by ferromagnetic minerals. This work focuses on three Portuguese plutons: Lamas de Olo, Lavadores-Madalena, and Santa Eulália. The preliminary results show that magnetic susceptibility is lower in opAMS, the degree of magnetic anisotropy is much higher in opAMS and the ellipsoid shape parameter has no significant differences in opAMS or ipAMS. The ipAMS and opAMS tensors are in general coaxial, pointing out that standard AMS fabric is parallel to the subfabric of minerals like hematite, titanomagnetite, and ultrafine magnetite. Two sites from Lamas de Olo Pluton with low in-phase magnetic susceptibility (ipKm) values were also measured, showing two different scenarios: (i) the coaxially is present in one site, pointing out the presence of minerals like hematite (after magnetite) but with the same orientation as the matrix; (ii) different orientation of K1 and K3 in ipAMS and opAMS suggesting the presence of a ferromagnetic oxide like hematite (after magnetite) but with a different orientation from the paramagnetic minerals. Nevertheless, it should be noted that in samples with low Km values, the presence of ferromagnetic minerals is scarce (or absent) and the opAMS has minor accuracy (the associated error is greater). The opAMS findings attain similar results to the anisotropy of anhysteretic remanent magnetization (AARM) studies, once both are related to the presence of ferromagnetic minerals, and their magnetic properties. However, the opAMS does not require the permanent magnetization of samples and is measured simultaneously with the ipAMS. With further works, a larger number of samples will be measured to accomplish more information, and AARM measurements will be performed on the same samples to compare the ipAMS, opAMS, and AARM tensors.

Acknowledgements: This work was funded by the Fundação para a Ciência e a Tecnologia (FCT) under UIDB/04683/2020 project.

How to cite: Cruz, C., Sant'Ovaia, H., McCarthy, W., and Noronha, F.: Anisotropy of out-of-phase magnetic susceptibility: an approach for magnetic subfabrics determination, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2835, https://doi.org/10.5194/egusphere-egu21-2835, 2021.

EGU21-11885 | vPICO presentations | EMRP3.7

Reduced- or ilmenite-type granites versus oxidized- or magnetite-type granites: occurrence in Iberian Variscan Belt

Helena Sant Ovaia, Cláudia Cruz, Ana Gonçalves, and Fernando Noronha

The magnetic susceptibility (Km) of granites is an important characteristic and it is mainly controlled by the presence of certain oxide minerals like magnetite and/or ilmenite, as well as ferromagnesian phyllosilicates such as biotite. The abundance of magnetite or ilmenite can be explained by different redox conditions in the magma chamber and distinct magma sources. The presence of magnetite or ilmenite as accessory minerals represents oxidized- or magnetite-type granites and reduced- or ilmenite-type granites, respectively.

This work focuses on the Km of 20 Variscan granitic massifs from northern and central Portugal and considers the results obtained in about 750 sampling sites, in order to deduce the redox conditions in the magma system. These granites are essentially two mica mesocrustal and biotite-rich basicrustal/infracrustal in origin and their emplacement was related to Variscan orogeny. In the northern and central Portugal, three main ductile deformation Variscan phases were recognized and described: D1, D2 and D3. The studied granites were subdivided in three main groups according to U-Pb dating, field observations and emplacement relative to the D3 phase. Therefore, the studied granites are subdivided as following: (1) syn-D3 two-mica (mosc=biot) granites, ca. 311 Ma; (2) late-D3 monzogranites, biotite-rich and two-mica granites (biot>mosc), ca. 300 Ma; (3) post-D3 monzogranites and biotite-rich granites, < 299 Ma.

The evaluation of the Km variation of the different granite groups shows that, as granites become progressively younger, the Km parameter tends to increase as a result of the increasing in the mantellic contribution to the genesis of the magmas. Syn-D3 granites display Km between 17.8 μSI and 186 μSI. This variation is due to the high textural and compositional diversity, including two-mica granites with different relative proportions of muscovite and biotite. Late-D3 granites are represented by two-mica and biotite-rich granites with calcium plagioclase, with several degrees of post-magmatic alteration, implying iron leaching processes. These processes also promote the crystallization of secondary muscovite, which implied a decrease in the Km values, and also a wide dispersion of Km values ranging between 7.3 μSI and 276 μSI. Post-D3 granites are mostly represented by biotite-rich granites with calcium plagioclase close to I-type granites. This granite group is divided into two subgroups: (i) post-D3 ilmenite-type granites with Km values of ca. 113 μSI, typical of biotite-rich granites; and (ii) post-D3 magnetite-type granites with Km values between 2078 μSI and 11676 μSI representing magnetite-type granites. In N and Central Portugal, these magnetite-type granites can occur in homogeneous plutons or in composite plutons constituted by ferromagnetic and paramagnetic facies.

As a conclusion, mostly of the granites from northern and central Portugal exhibit average Km values below 1000 μSI and are characterized by a paramagnetic behavior corresponding to reduced- or ilmenite-type granites. Among the all studied granites, only one pluton showed to be a truly oxidized- or magnetite-type granite, with Km of the order of 11676 μSI.

Acknowledgements: This work was funded by the Fundação para a Ciência e a Tecnologia (FCT) under UIDB/04683/2020 project.

How to cite: Sant Ovaia, H., Cruz, C., Gonçalves, A., and Noronha, F.: Reduced- or ilmenite-type granites versus oxidized- or magnetite-type granites: occurrence in Iberian Variscan Belt, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11885, https://doi.org/10.5194/egusphere-egu21-11885, 2021.

EGU21-15561 | vPICO presentations | EMRP3.7

Unravelling the dynamics of magma emplacement through palaeomagnetic backstripping of intrusion-induced host rock deformation: Analysis from the Sandfell Laccolith, SE Iceland

William McCarthy, Vincent Twomey, Craig Magee, Michael Petronis, and Tobias Mattsson

Injection and inflation of magma in the shallow crust is commonly accommodated by uplift of the surrounding host rock, producing intrusion-induced forced folding that mimics the geometry of the underlying intrusion. 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. Analysis of ancient examples where magmatism has long-since ceased typically 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 a new hypothesis suggesting that thermoremanent magnetization records progressive deflection of the host rock during laccolith construction where these measurements can be used to measure the rate and dynamics of the magma emplacement of. Our test site is located within the basaltic lava pile of the ~800 m wide structural aureole surrounding the rhyolitic Sandfell Laccolith in SE Iceland, which intruded <1 Km below the palaeosurface at ~11.7 Ma. Our results show heat from the laccolith resets the remanence from samples within 50 m of the contact. Several variations in thermoremanent vectors observed further outward along the structural aureole reflect stepwise folding from incremental injection of magma suggesting as and the laccolith develops, different sections of the host rock are incrementally tilted and possibly reheated. This procedure could be tested in other ancient structure aureoles to investigate whether single or multiple thermal wm37@st-andrews.ac.ukoverprints coupled with structural observations could be used a proxy for ground deformation patterns in volcanic hazard assessment.

How to cite: McCarthy, W., Twomey, V., Magee, C., Petronis, M., and Mattsson, T.: Unravelling the dynamics of magma emplacement through palaeomagnetic backstripping of intrusion-induced host rock deformation: Analysis from the Sandfell Laccolith, SE Iceland, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15561, https://doi.org/10.5194/egusphere-egu21-15561, 2021.

EGU21-10918 | vPICO presentations | EMRP3.7

Paleomagnetic properties of laboratory TRM of natural titanomagnetite during it`s low-temperature oxidation

Grachev Roman and Valery Maksimochkin

It is well known that low-temperature oxidation of rocks in the magnetic field could lead to the formation of secondary magnetization of chemical nature (CRM). This case is a common reason for errors in determining the strength and direction of the ancient magnetic field related to primarily thermoremanent magnetization (TRM). The absence of explicit criteria for separating these types of magnetization in rocks is a fundamental problem of paleomagnetic studies.

To try to recognize CRM and TRM components of magnetization in the basalt we have carried out a laboratory simulation of the low-temperature oxidation of the Red Sea basalt P72/4 by annealing samples in the magnetic field. The main magnetic mineral of carried basalt is homogeneous titanomagnetite with a median Curie temperature Tc=260°C and NRM=55 A/m. The Ti content in titanomagnetite grains varies in the range of 6.8-7.5 wt. %, lattice constant a=8.455A. Titanomagnetite magnetic grains are mostly in the PSD state and have a dendritic structure. Laboratory TRM was created when demagnetized samples were cooled from 400 °C to room T in an argon atmosphere and a magnetic field B=50 µT. The TRM value is about 85-90% of the NRM (TRMmean=49.2 A/m). 

Annealing of samples in the air was carried out at Tan=260 °C in a magnetic field with induction B=50 µT parallel and perpendicular to the previously created TRM. As a result of annealing during 12.5-1300 hours, Tc increased by 40-170 °C, Is increased from 2400 A/m to 3050 A/m, Hc decreased from 17.3 mT to 14.3 mT, Hcr – from 22.1 c to 19.2 mT. The oxidation state (Z) after annealing for 12.5, 100, 400, and 1300 hours were found to be 0.16, 0.31, 0.58, and 0.67, respectively.

Regardless of the field B direction, magnetization co-directed with the initial TRM and blocking temperatures above 400 °C occurs. The effect increases with the annealing time: particularly t=12.5 h, the proportion of magnetization with unblocking temperatures T>400 °C is about 15-20% of TRM (260-400 °C), while for t=400 hours it is already 35-40% of TRM (260-400 °C). Then again the magnetization obtained as a result of annealing in B⊥TRM, two components are detected by thermal cleaning: one co-directed with B (CRM component), the other in the direction of the original TRM. The thermal stability of the TRM component is significantly higher than that of the CRM: most (75-80%) of the blocking temperatures of the CRM are confined to a narrow range near Tan (260-340 °C), while the TRM is destroyed after heating to 340 °C only by 35-40%. Also, CRM and TRM components differ in their resistance to the influence of an alternating magnetic field. In the case of B∥TRM, only one component is diagnosed with magnetic and thermal cleaning. In this case, it is not possible to detect the presence of two superimposed magnetization components of different genesis.

This work was supported by the Russian Foundation for Basic Research, project 20-05-00573.

How to cite: Roman, G. and Maksimochkin, V.: Paleomagnetic properties of laboratory TRM of natural titanomagnetite during it`s low-temperature oxidation, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10918, https://doi.org/10.5194/egusphere-egu21-10918, 2021.

South American Jurassic/Cretaceous rocks has been troubled by elongated virtual geomagnetic pole (VGP) distributions, while many discordant poles from the Carboniferous to the Triassic have also been recognized, rendering the South American apparent polar wonder path (APWP) problematic. 

We have conducted a paleomagnetic study of the sedimentary Permo-Carboniferous Itararé Group rocks within the state of São Paulo, Brazil, including three intruding mafic sills that are attributed to Early Cretaceus magmatic activity. The site-mean VGP distributions obtained from the sedimentary rocks define elongations that include the VGPs of the mafic intrusions, and are interpreted as remagnetization paths toward the directions characteristic of the sills. These interpretations are supported by extensive rock-magnetic data that provide a viable mechanism for the secondary magnetizations. Careful analysis of the paleomagnetic data of the sedimentary rocks enables isolation of a primary VGP distribution that is consistent with the reference Carboniferous pole position.

Analysis of other Carboniferous to Triassic South American paleomagnetic VGPs reveals that the majority of these data are also elongated: regardless of the age of the rocks, the elongations dominantly intersect at the location of the Late Cretaceous reference pole, and a second location similar to the intersection of the VGP elongations of some Jurassic/Cretaceous rock formations, and also coincides with the cusp of the debated loop in the Carboniferous-Triassic APWP. Based on multiple lines of evidence, we interpret the elongations and their intersections to reflect remagnetizations that occurred as a result of the widespread magmatism associated with the opening of the South Atlantic. We suggest that the extent of the remagnetizations is formation-specific, and that other rock-formations should be carefully re-evaluated.

How to cite: Bilardello, D., Callebert, W., and Davis, J.: Great circle analysis of South American VGP-distributions suggests widespread Cretaceous remagnetizations: case study of the Itararé Group Rocks from Brazil, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6270, https://doi.org/10.5194/egusphere-egu21-6270, 2021.

EGU21-16483 | vPICO presentations | EMRP3.7

The Paraná magmatism (Brazil) as the villain regarding the remagnetization of the Paleozoic sediments in the basin – half-truth!

M. Irene B. Raposo, Marcia Ernesto, Daniele Brandt, and Daniel Ribeiro Franco

The sedimentation in the Paraná Basin started during the Ordovician and ended with the Early Cretaceous magmatic event. Thick lava pilescovered the entire basin, and voluminous dikes and sills occur in the sedimentary sequences, mainly in the northeastern border of the basin. Despite the thermal effect, some Paleozoic sedimentary formations preserved their primary magnetization. The paleomagnetic results from the glacial Aquidauana Formation, an equivalent to the Itararé group in the north-western portion of the basin, indicated that the magnetization is compatible with the Middle-Late Permian age assigned in literature. A detailed investigation of the magnetic mineralogy and the magnetization of other Paleozoic sedimentary rocks led to the conclusion that the intrusive rocks were more effective than the lavas in disturbing the primary magnetization, especially in the low-clay content rocks. The secondary magnetizations identified in the different areas of the basin are not always compatible with the Early Cretaceous magnetization imprinted by the Paraná magmatism. This component prevails in the northeastern area, while a Permo-Triassic magnetization was identified in other areas. The results obtained so far are coherent with the geomagnetic reversal scale for the considered time interval, and the paleomagnetic poles agree with the APWP for South America.

How to cite: B. Raposo, M. I., Ernesto, M., Brandt, D., and Franco, D. R.: The Paraná magmatism (Brazil) as the villain regarding the remagnetization of the Paleozoic sediments in the basin – half-truth!, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16483, https://doi.org/10.5194/egusphere-egu21-16483, 2021.

EGU21-14264 | vPICO presentations | EMRP3.7

Relative movements of the Bashkirian megazone blocks(the South Urals) according to paleomagnetic data.

Maiia Anosova, Anton Latyshev, and Alexey Khotylev

     The research object is magmatic bodies from the southern, central and northern parts of the Bashkirian megazone (the Southern Urals, meridian length of the Bashkirian megazone - 300 km). Most of the study intrusions have the Riphean age. In the Riphean the Bashkirian megazone was part of the East European craton. And in the Late Paleozoic rocks of the Bashkirian megazone were involved in the collision process. The formation of most studies bodies is associated with the Mashak magmatic event (the Riphean), which marks the collapse of the super-continent Nuna.

     The Middle Paleozoic component was isolated in 28 bodies. Probably it is the secondary component, that is widespread on the Southern Urals and has been repeatedly identified by other researchers. Directions comparison from different districts showed that there was a rotation of the southern, northern and central blocks of Bashkirian megazone relative to each other during the Late Paleozoic collision. At the same time, paleomagnetic directions from the northern regions (which are about 40-50 km apart from each other) statistically coincide or differ not so much. Which means that they were stable or relatively stable.

     Besides, the Riphean component was isolated and the paleomagnetic pole for the boundary of the Lower and Middle Riphean of the East European Craton (1349+/-11 Ma) is calculated from 8 thin sheet intrusions. Plat=8.4; Plong=162.4; A95=4.1.

How to cite: Anosova, M., Latyshev, A., and Khotylev, A.: Relative movements of the Bashkirian megazone blocks(the South Urals) according to paleomagnetic data., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14264, https://doi.org/10.5194/egusphere-egu21-14264, 2021.

EGU21-9596 | vPICO presentations | EMRP3.7

Cretaceous paleomagnetic directions in different volcanics in Chukotka (NE Russia)

Ivan Lebedev, Elizaveta Bobrovnikova, Artem Moiseev, and Bagdasarian Tatiana

The Cretaceous Okhotsk-Chukotka volcanic belt (OChVB) is one of the largest provinces of continental marginal magmatism with length more than 3000 km along the Pacific edge of Asia. In the field studies of 2019 and 2020 we sampled 21 sections in the northern part of the OChVB and 3 sections from basement of OChVB. These sections are represented by basalts and andesites; their tuffs, ignibrites and other volcanic rocks are much less common. The age of these volcanics is estimated based on U-Pb and Ar-Ar published data and our new Ar-Ar dates.

Based on the obtained data, a new paleomagnetic pole for the Chukotka part of the OChVB was calculated. The latitude of this paleomagnetic pole differs from the expected one when compared with that calculated for Chukotka from published data from Besse and Courtillot, 2003; Torsvik et al., 2012. These results are inconsistent with most of the existing geological data. Only a few works admit younger displacements in the southern part of the Verkhoyansk fold belt or in modern diffuse boundary of the Eurasian and North American plates. Moreover, we compare our OChVB pole with results from basaltic complexes from the basement, which has been likely remagnetized when OChVB was formed.

Acknowledgements: study of cretaceous volcanics is supported by RSF grant № 19-47-04110 and jurassic by RSF grant №18-77-10073.

How to cite: Lebedev, I., Bobrovnikova, E., Moiseev, A., and Tatiana, B.: Cretaceous paleomagnetic directions in different volcanics in Chukotka (NE Russia), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9596, https://doi.org/10.5194/egusphere-egu21-9596, 2021.

EGU21-12596 | vPICO presentations | EMRP3.7

First paleomagnetic and magnetic anisotropy results from Montenegro – the coastal area

Emö Márton, Vesna Cvetkov, Martin Đaković, Vesna Lesić, and Slobodan Radusinović

In the Southern part of Montenegro three main tectonic units are distinguished. The Dalmatian (South-Adriatic) zone is in the lowermost tectonic position and comprises shallow water Cretaceous carbonates, bauxites, Middle Eocene nummulitic limestones, transitional marls, and Upper Eocene flysch. It is thrusted over by the Budva zone characterized by deep water sediments of Triassic through Paleogene ages with tuffitic layers in the Ladinian. The uppermost unit is the High Karst zone developed in a carbonate platform environment from the Middle Triassic till the end of the Cretaceous. In this unit flysch sedimentation started after the K/T boundary.
From the above units we drilled and oriented in situ a total of 248 samples representing nine localities from the Dalmatian, six from the Budva and five from the High Karst zone, respectively. The harder rocks were drilled with portable gasoline powered, the softer ones with an electric drill. The laboratory measurements, demagnetizations, statistical evaluations of the measurements were carried out using standard methods in the Paleomagnetic Laboratory of the Mining and Geological Survey of Hungary. 
From the Dalmatian zone, the Upper Cretacous limestones have diamagnetic susceptibilities, very weak NRM, which either failed to provide statistically acceptable results (two localities) or obtained their remanence quite recently. The five flysch and transitional marl localities have well-defined AMS fabrics as well as paleomagnetic directions. The AMS fabrics must have been imprinted in a NE-SW oriented compressional strain field, which resulted in moderately strong fabrics with NW-SE oriented AMS lineations, perfectly following the Dinaridic trend recognized in Croatia. One locality, the only one with moderate tilt and rather poor statistical characteristics suggest post–Eocene CCW, while those with steep dips have well-defined paleomagnetic directions, which are aligned with the intermediate AMS direction. The positive tilt test, however, provides an overall mean direction with absurdly shallow inclinations, which suggest that we are not dealing with “real” paleomagnetic directions, but maybe with magnetic anisotropy governed NRMs.
The rocks in the Budva zone show a large variation in lithological sense as well as in their magnetic properties. Except a Pietra Verde localitiy, the magnetic susceptibilities are weak, we can not define AMS fabrics, yet most of the localities yielded fairly good paleomagnetic results, which suggest pre-folding age of the acquisition followed by a large CW rotation of unit.
From the High Karst, only a single Lower Jurassic pelagic limestone provided paleomagnetic results which exhibit CW rotation. The others had weak magnetizations, diamagnetic susceptibilities and no or scattered magnetic signals.
This work was financially supported by the National Development and Innovation Office of Hungary project K 128625, by the Geological Survey of Montenegro and Ministry of Science and Education of Serbia project No. 176016.

How to cite: Márton, E., Cvetkov, V., Đaković, M., Lesić, V., and Radusinović, S.: First paleomagnetic and magnetic anisotropy results from Montenegro – the coastal area, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12596, https://doi.org/10.5194/egusphere-egu21-12596, 2021.

EGU21-15885 | vPICO presentations | EMRP3.7

New paleomagnetic results obtained in Upper Cretaceous-Paleocene detritic deposits of the North Iberian Meseta

Juan José Villalaín, Pablo Calvín, Puy Ayarza, Ruth Soto, and Manuel Calvo

The Iberian microplate and its evolution during the Mesozoic have been in-vogue topics in the field of Geodynamics, because of its location between two of the major tectonic plates, its interaction with both of them, and its significance in relation with the evolution of the western Tethys domain. Geodynamic models of Mesozoic Iberian evolution are based upon the knowledge of the kinematics of the microplate obtained from the ocean floor magnetic anomalies and particularly its apparent polar wander path (APWP) defined by existing paleomagnetic data. In this sense, the most important feature is the anticlockwise 30º-40º rotation that Iberia underwent during the Cretaceous. Nevertheless, there are still uncertainties about the chronology of this movement due to the poor definition of oceanic magnetic anomalies and the scarcity of high-quality paleomagnetic data in the continent. According to recent works, existing paleomagnetic poles are contradictory and inconsistent with the global apparent wander path (GAPWP) and ocean floor anomalies. This is due to the widespread presence of remagnetizations in the Mesozoic basins within Iberia.

To address this question we are starting to develop a new project that aims to obtain new paleomagnetic data from unexplored geological units meeting the necessary condition to obtain new Cretaceous paleomagnetic poles representative of the Iberian plate. In this talk we show new paleomagnetic data from detritic deposits with siliceous cement located at the Duero basin (North Iberian Meseta) ascribed to the upper Cretaceous-Paleocene (Areniscas de Salamanca Formation). A stable paleomagnetic component carried by hematite, showing normal and reversed polarities has been isolated. A stable paleomagnetic component carried by hematite, showing normal and reversed polarities has been isolated. We discuss the primary character of this magnetization in terms of inferring the age of this unit in the frame of the kinematic evolution of the plate.  

How to cite: Villalaín, J. J., Calvín, P., Ayarza, P., Soto, R., and Calvo, M.: New paleomagnetic results obtained in Upper Cretaceous-Paleocene detritic deposits of the North Iberian Meseta, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15885, https://doi.org/10.5194/egusphere-egu21-15885, 2021.

EGU21-3474 | vPICO presentations | EMRP3.7

PMTools – a new program for paleomagnetic data analysis and visualization

Ivan Efremov and Roman Veselovskiy

There are many programs for the analysis and visualization of paleomagnetic data, but each of them is good only in a certain use case and does not allow to perform a full cycle of paleomagnetic operations. Therefore, one has to resort to using a number of programs to complete the full path of processing paleomagnetic data. You often have to convert data from one format to another, manually vectorize charts, and generally spend more time and effort than could theoretically be spent. Thus, there is a long overdue need for a universal program capable of fast, convenient and high-quality performance of a full cycle of paleomagnetic operations. A set of programs written by Randy Enkin (Enkin, 1996) for DOS was taken as a time-tested example of such a program. The choice fell on them, since these programs (although they are very outdated) allow performing a full cycle of paleomagnetic operations and do it as conveniently and efficiently as possible for that time.

Our goal is to create a program devoid of all of the above disadvantages and capable of developing indefinitely as modular opensource software by the efforts of all people interested in this.

The result of our work is PMTools – a cross-platform software for statistical analysis and visualization of paleomagnetic data. PMTools supports all widely used paleomagnetic data formats and allows you to work with them simultaneously. All charts created in PMTools are vector, adapted for direct using in publications and presentations, and can be exported in both vector and raster formats. At the same time, PMTools implements a full cycle of routine paleomagnetic operations: from finding the best-fit directions to calculating the mean paleomagnetic poles. Moreover, all operations can be performed both with a mouse through a graphical user interface and with hotkeys, which significantly speeds up the data analysis process. 

In the near future, PMTools will become a modular open source application, so that each user will be able to add its own modules, thereby expanding the program's functionality.

References

Enkin, R.J., 1996. A Computer Program Package for Analysis and Presentation of Paleomagnetic Data, Pacific Geoscience Center, Geological Survey of Canada, http://www.pgc.nrcan.gc.ca/tectonic/enkin.htm.

How to cite: Efremov, I. and Veselovskiy, R.: PMTools – a new program for paleomagnetic data analysis and visualization, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3474, https://doi.org/10.5194/egusphere-egu21-3474, 2021.

EGU21-1782 | vPICO presentations | EMRP3.7

High-temperature three-axis IRM Lowrie test

Leonid Surovitskii, Andrei Kosterov, Mary Kovacheva, Maria Kostadinova-Avramova, Natalya Salnaya, and Aleksey Smirnov

The three-axis isothermal remanent magnetization (IRM) test (the Lowrie test; Lowrie, 1990, Geophys. Res. Lett., 17, 159-162) is a useful tool to identify ferromagnetic minerals by their coercivity and unblocking temperature spectra. In this study, we explore a variant of the Lowrie test in which measurements are conducted directly at elevated temperatures, and compare its performance with the results of the conventional stepwise procedure. IRM acquisition fields applied along three orthogonal axes were 1 T, 200 mT and 40 mT, respectively. The field value for the soft component was chosen so as to include ca. 90% of its coercivity spectrum. For the hard component the maximum available field was used. The test is applied to characterize the magnetic mineralogy of archaeological baked clays and bricks from Bulgaria and Russia. Bulgarian samples are baked clays from various Neolithic (5700-5300 BCE) archaeological sites and several bricks of the Roman epoch (III-IV c. AD). Samples from Russia are bricks originating from several regions with ages from XIII to early XIX c. AD.

The low- and intermediate-coercivity components of IRM in the studied samples are typically demagnetized by 520-550°C, compatible with substituted or cation-deficient magnetite or, possibly, maghemite. This is supported by the absence of the Verwey transition in studied samples (Kosterov et al., 2021, Geophys. J. Int., 224(2), 1256-1271). The high-coercivity component appears to be carried by two mineral phases with very distinct unblocking temperatures, 120-200°C and 500 to 640°C. The first phase is similar to the high coercivity, low unblocking temperature (HCSLT) phase described by McIntosh et al., 2007 (Geophys. Res. Lett., 34, L21302, doi: 10.1029/22007GL031168), and the second one appears to be hematite with variable degree of substitution.

Performance of the high-temperature variant of the Lowrie test compares favorably with the classical procedure, while the former is also significantly faster and yields a superior temperature resolution.

This study is supported by Russian Foundation of the Basic Research, grant 19-55-18006, and by Bulgarian National Science Fund, grant KP-06-Russia-10.

How to cite: Surovitskii, L., Kosterov, A., Kovacheva, M., Kostadinova-Avramova, M., Salnaya, N., and Smirnov, A.: High-temperature three-axis IRM Lowrie test, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1782, https://doi.org/10.5194/egusphere-egu21-1782, 2021.

EGU21-1155 | vPICO presentations | EMRP3.7

A toolbox for convenient calculation of anisotropy of magnetic remanence

Martin Chadima and Josef Jezek

The growing interest in ferromagnetic fabric, i.e. magnetic fabric carried solely by ferromagnetic (sensu lato) grains, creates the need for a simple way to obtain anisotropy of magnetic remanence (AMR) tensors from a set of laboratory imparted, spatially distributed vectors of remanent magnetization (RM). Here, we present a simple, user-friendly toolbox embedded into Anisoft software which facilitates AMR tensor fitting from an array of RM vectors acquired according the prescribed data acquisition protocol (6, 9, 12, 15, 18 magnetizing directions). As these protocols are usually quite laborious involving a series of demagnetizations and directional magnetizations, this toolbox provides a graphical visualization of the measured RM vectors in terms of their directional comparison with the respective magnetizing directions and their intensity in comparison with the intensity of demagnetized state (background magnetization). This visualization provides a convenient way to check whether there are no evident missteps and the measured vectors correspond to the acquisition protocol. Prior to tensor fitting, the correction for the background magnetization is optionally done by a direct subtraction of the measured background or by mutual subtraction of antipodally magnetized RM vectors, if available. The AMR tensors are fitted by the least-square algorithm using two slightly different approaches involving either (1) all three orthogonal components of magnetized RM vectors (Vector Method) or (2) their projections to the respective magnetizing directions (Projection Method). The differences between these two approaches are discussed. The calculated AMR tensors, including their confidence limits, are stored in the same file format as AMS data and can be thus instantly visualized and further processed using Anisoft software.

How to cite: Chadima, M. and Jezek, J.: A toolbox for convenient calculation of anisotropy of magnetic remanence, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1155, https://doi.org/10.5194/egusphere-egu21-1155, 2021.

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