Deep-rooted Geothermal System Imaged by Magnetotelluric Measurements Under the Tsenkher Hot Spring Area, Mongolian Hangai
Batmagnai ERDENECHIMEG, Friedemann SAMROCK, Alexander V. GRAYVER, Alexey KUVSHINOV, Martin O. SAAR, Demberel SODNOMSAMBUU, Tsegmed BATTUULAI, Tserendug SHOOVDOR, Purevsuren DORJ, Dolgorjaw OYUNTSETSEG
[ETH Zurich, Switzerland]
The Mongolian Hangai dome is an intra-continental mountain range, located far from tectonic plate boundaries and it is characterized by dispersed low-volume intraplate volcanism. More than 40 hot springs are found in the Hangai dome with temperatures of up to 90°C and flow rate up to 25l/s. One of the most prominent springs is the Tsenkher hot spring 25km south of Tsetserleg city, which is the capital of Arkhangai province. The Tsenkher hot spring has high temperatures of 87°C at the surface, at a water flow rate of 10 l/s. 10km south of Tsenkher are two other hot springs Gylgar and Bortal, with temperatures of 52°C and 46°C, respectively. Being one of the largest hot springs, and due to its proximity to Tsetserleg, the Tsenkher geothermal site is the place, where geothermal energy utilization and studies, regarding the realization of a combined heat and power (CHP) geothermal plant, are most advanced. However, to date the hot spring is only used locally for greenhouse heating and in a spa. Previous geophysical studies to find a geothermal reservoir at Tsenkher focused on shallow-depth (~150 m) structures and could not image the deeper geothermal reservoir and source region. Exploration wells drilled around Tsenkher and Gyalgar, to find the geothermal reservoir, did not succeed and reached a maximum of 450m depth. A regional magnetotelluric study of the Hangai and Gobi-Altai mountains revealed that hot springs are typically associated with electrically conductive crustal anomalies, that can be interpreted in terms of small fractions of melt, indicating deep-rooted geothermal heat sources. With this new project, we establish and deploy methods and tools for geothermal exploration in Mongolia. The studies are supported by the Swiss Research for Development Grant (SNF r4d) and include magnetotelluric, geomagnetic, seismic and gravity measurements in the Tsenkher hot spring area. Here, we present the first results of the 3-D magnetotelluric (MT) study. We conducted MT measurements in an area spanning 30km by 40km encompassing the hot springs of Tsenkher, Gyalgar and Bortal, with an average site spacing of 2km. During the field survey, we measured the magnetic and electric fields at 77 MT sites, and electric fields at 97 telluric sites to make use of the telluric-magnetotelluric (TMT) technique. TMT measurements enable to increase the amount of acquired data, while keeping equipment costs and survey duration relatively low. We calculated the 3-D electrical conductivity model using an adaptive finite element code, GoFEM, with locally refined unstructured hexahedral meshes to account for topography and the irregular distribution of sites. The obtained best-fitting model is characterized by a prominent crustal electrical conductor that follows the hot spring locations and rises from a depth of more than 15km to the surface. This orientation of the conductor follows major fault zones, that were identified in the study region. The crustal conductor likely represents a cooled volcanic intrusion, where enhanced electrical conductivities are caused by magma-derived fluids. This conductor probably plays a key role in the formation of the hot springs. Despite the fact that volcanic activity is dormant, the intrusive rock might still be hot enough to heat meteoric water that rises up along permeable faults and feeds the hot springs at the surface.
|        Topic: Geophysics||Paper Number: 13076|