3-D magnetotelluric investigation of the mid-enthalpy geothermal region near Tsetserleg city in Mongolian Arkhangai province

The Hangai Dome is an intra-continental mountain range in central Mongolia, characterized by dispersed low-volume, intraplate volcanism. The existence of more than 40 hot springs with a high temperature in the Hangai region is indicative of abundant geothermal energy resources.

As an example, using hot springs surrounding Tsetserleg city could dramatically reduce air pollution due to coal burning for heating during long and cold winters. To understand the nature of the geothermal reservoir feeding the hot springs, we conducted a magnetotelluric (MT) survey in the Tsenkher hot spring region south of Tsetserleg. In summer 2019 we measured MT data at 184 sites over an area of 20x30 km, which is characterized by several hot springs with temperatures up to 87°C. For the MT survey, we used the so-called telluric-magnetotelluric approach, which allowed us to increase the amount of acquired data, while keeping equipment costs and survey duration low.

For 3-D modelling and inversion of the MT data we use the high order finite element code GoFEM. Locally refined unstructured meshes are used to ensure numerical accuracy with a sufficiently fine discretization of the inversion domain, while keeping the computational cost feasible, and allow to compensate for static shift. We obtained the best-fitting model by using a 1-D starting model derived from 1-D inversion of the regional average of the impedance tensor. The model is characterized by a prominent crustal conductor that appears under the hot spring areas and rises from depths of more than 10 km to the surface. We interpret the conductor as being related to past volcanic activity and as a zone rich in partial melt and magmatic fluids. The conductor therefore might act as the heat source for meteoric fluids that feed the hot springs, providing important insights into the nature of the geothermal system in the Tsenkher hot spring area.