Displays

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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