Exploring The Geothermal Fluid Reservoir of Tangquan Geothermal System Via The Controlled Source Audio-Frequency Magnetotelluric Survey

The Tangquan geothermal field is located at about 120km east of Beijing. There were hot springs. But now they are disappeared due to man-drilled wells. The wells are about tens of meters to about 400m depth, with temperatures of pumped water between 30°C to 67°C. As no high-resolution and effective geophysical survey, such as EM exploration, was done before, the existing inference about the structure and the genesis of the geothermal system was inadequate and unreliable. To detect subsurface geophysical properties and the water-bearing fracture systems, we carried out an controlled source audio-frequency magnetotelluric(CSAMT) survey based on the geological knowledge. The survey included 5 lines with total length of 9km. After the field data acquisition, we carried out fine processing and derived reliable inversion results. The resistivity profiles show that there is a clear low-resistivity band between granite and metamorphic rock. We infer that the contact zone is the passageway along which the hot water flows from deep to shallow. According to the results, we believe that the hot water in the geothermal field is rich and has good connectivity, and it covers a much larger area than the known range. After our exploration, three deep wells were drilled with depth from about 600m to 1100m. They verified our results, and the scope of geothermal field has been greatly expanded.