EMRP2.11

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.

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

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

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

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

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

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

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

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

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

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

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

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

How to cite: Weiss, M., Rulff, P., and Kalscheuer, T.: In-depth comparison of the spectral-element and finite-element method for 3D CSEM forward modelling, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2339, https://doi.org/10.5194/egusphere-egu21-2339, 2021.