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Assessing the risk posed by Geomagnetically Induced Currents (GICs) to the regular operation of power grids requires a hazard specification in terms of the induced geoelectric field (GEF). In order to specify the GIC hazard posed to the German high-voltage transmission grid, we model the GEF within Germany for geomagnetically disturbed conditions using magnetic field measurements from INTERMAGNET observatories and a predefined electrical conductivity model of the subsurface as input. This contribution focuses on a comparison of GEFs modeled assuming electromagnetic (EM) induction scenarios of different complexity, which are imposed through the specification of the inducing EM field on the one hand and the dimensionality of the conductivity model on the other. The former is specified either as a plane wave or a laterally varying field obtained by fitting Spherical Elementary Current Systems basis functions to the magnetic field observations. The conductivity distribution is given either by the “EURHOM” model or our new dedicated 3D model for Germany obtained from legacy magnetotelluric data. The latter also includes realistic 3D conductivity variations due to the ocean/sea and marine sediments, which are known to affect magnetotelluric impedance tensors at GIC-relevant frequencies near coastlines. Based on the modeled GEFs, we calculate GICs exemplarily at selected substations and geomagnetic disturbance events. The results indicate the level of complexity needed in the GEF modeling in order to derive meaningful GIC hazard maps for Germany down the line., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

A major source of time-varying magnetic fields are external ionosphere-magnetosphere currents and associated fields induced in subsurface and oceans. Induced fields may bias external and core field models if not accounted for in a physically consistent manner. In addition, induced fields contain unique information on the subsurface conductivity distribution. Yet, induced fields are often considered noise (and neglected) or at best treated peripherally as a nuisance parameter. The outstanding accuracy of Swarm magnetic field data and increasing demand for fast-track magnetic field products has pushed the frontier of the field to a point where sophisticated core and external magnetic field models are hindered by improper modelling of induced fields. We propose a versatile and efficient way of modelling induced fields in ground and satellite measurements, which also enables modelling in near real-time settings as is required by many present Space Weather products. The proposed approach also offers a more consistent way to assimilate geomagnetic data from ground observatories and satellites into physics-based magnetic field models., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

Understanding the geological setting of nonvolcanic geothermal systems is vital to explaining the formation of geothermal reservoirs and their observable surface manifestations. The magnetotelluric method, used to determine the subsurface electrical conductivity distribution, is a common tool in geothermal exploration. In this study, we present an integrated interpretation of an electrical conductivity subsurface model, together with geological analyses and geochemical probes from a nonvolcanic, intermediate-temperature geothermal system in Mongolia. We conducted magnetotelluric (MT) and telluric-magnetotelluric (TMT) measurements at the Tsenkher geothermal area in the Mongolian Khangai dome during the summers of 2019 and 2020. The 20km*20km large area is characterized by three major hot springs with water temperatures up to 87°C. From a total of 196 MT and TMT stations, we obtained a 3-D electrical conductivity model of the subsurface. To interpret the data, we used a high-order finite-element electromagnetic modelling code (GOFEM) with locally refined hexahedral meshes that allows including accurate topography while ensuring high numerical accuracy with a sufficiently fine discretization of the inversion domain. The best-fitting model provides essential insights into the subsurface structure of the Tsenkher geothermal area. The model is characterized by a strong vertical crustal conductor that appears south of the hot springs area and rises from depths of more than 10 km to the surface. We interpret this conductor as a remnant of past local volcanism and a zone of former magma ascent, indicating the potential source for the observed enhanced surface heat flow in the hot springs area around Tsenkher. Additionally, the model includes a prominent striking conductor beneath the hot springs at depths down to more than 3 km below the surface. The conductor is spatially aligned with a major fault that intersects the survey area, and is accompanied by several basaltic dyke intrusions. We interpret the fault-aligned conductor as the major area of deep fluid circulation and an accumulation zone for heated fluids. The interpretation agrees with theoretical concepts of topography-driven deep fluid circulation and local fault zones playing a major role in the transport of hot water from a reservoir to the surface. Inferred reservoir temperatures from geochemical fluid analyses are inagreement with interpretations of the maximum depth of fluid circulation inferred from the MT model. Our MT subsurface model serves to better understand the formation of the Tsenkher hot springs in particular and intermediate-temperature geothermal systems in general, 25th EM Induction Workshop