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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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 V_p/V_s 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.

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:

[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.

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.

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.

Superhot geothermal environments with temperatures of approximately 400-500^︒C 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 400^︒C 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.

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.

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.

 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.

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.

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.

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.

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.

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.

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.

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.

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.

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 VB_South 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 VB_South 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.

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.

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 1^st 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.

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.

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.

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.

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.

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.

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.

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.

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 (H_x, H_y, H_z) 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.

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.

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.

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.

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.

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.

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.

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.

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.

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.