Friedemann Samrock, Alexander Grayver , Batmagnai Erdenechime, Luise Dambly, Alexey Kuvshinov and Martin O. Saar:
The symphony of tectonics, magmatism and geothermal reservoirs unveiled by multi-scale magnetotelluric imaging
The formation of conventional geothermal reservoirs is controlled by the emplacement of magma in the crust and tectonic settings that allow deep fluid circulation and transport of heat to the surface. In geophysical exploration for geothermal resources the magnetotelluric (MT) method has established itself as eminently suitable for subsurface reservoir characterization. A reason for the prevalence of MT in geothermal exploration is that the bulk electrical conductivity of the subsurface, as recovered by MT, can be used as a proxy for key geothermal parameters such as melt properties, fluid saturation and the presence of hydrothermal alteration products. The interpretation of MT data is improving continuously with the development of powerful numerical modelling tools. Whilst industrial applications of MT commonly concentrate on the investigation of depths relevant for drilling, MT data provide the ability to image the subsurface across multiple scales from shallow near surface structure down to the asthenosphere. Modern numerical tools with flexible meshes that adapt to topography, survey layout and varying data resolution allow one to characterize geothermal systems from their surface manifestations down to their deep roots and sources of heat in the lower crust. We present a case study from the Ethiopian Rift, where we jointly interpret a localscale dataset from Aluto volcano together with a regional dataset collected along a 120km long profile across the rift. The resulting model unveils in great detail how high-temperature volcanic geothermal reservoirs form in the context of continental rifting in a close interplay of asthenospheric decompression melting, focused up-flow of melt along faults and melt storage in shallow magmatic reservoirs that drive convection of hydrothermal fluids. Another case study takes place in the Mongolian Hangai Dome, which is an uplifted intracontinental plateau. The Hangai is characterized by dispersed volcanism and numerous hot springs. Here large regional-scale MT surveys shed light into uplift processes and found evidence for localized zones of melting in the asthenosphere that are spatially correlated with regions of surface volcanism and geothermal activity. A local-MT survey in the Tsenkher hot spring area shows that main regional faults and dyke intrusions of past volcanic activity act as present-day geological controls for fluid circulation and pathways for hydrothermal fluids to reach the surface. The resulting MT models provide subsurface images that show the interplay of geological processes on multiple scales thereby providing a better understanding of the formation of geothermal resources.