The Tangquan Geothermal System Inferred From the Controlled Source Audio‐Frequency Magnetotelluric Survey

ABSTRACT The Tangquan geothermal field in Zunhua County, Hebei Province, China, has been renowned since the Tang dynasty (618–907 AD) for its ancient hot spring, which disappeared due to the proliferation of anthropogenic wells. The geothermal wells in this area range in depth from tens of metres to 400 m, with water temperatures between 30°C and 67°C. Prior to this study, no effective high‐resolution geophysical surveys, such as electromagnetic (EM) exploration, had been conducted, resulting in limited constraints on the subsurface structure and conceptual understanding of the geothermal system. In this study, controlled‐source audio‐frequency magnetotelluric (CSAMT) surveys were conducted along five profiles with a total length of 9 km and 180 soundings to characterize the electrical structure and identify potential fluid‐conducting pathways. The inverted resistivity profiles reveal a main fault controlling the geothermal field and detect a low‐resistivity zone in the shallow subsurface, which is interpreted as the fracture zone filled with thermal water. We infer that groundwater gathers in the fault and gets heated. The resulting lower density hot water then ascends along the fault to shallower depths, where it is stored in the mixed contact zone of fractured granite and metamorphic rocks, forming the Tangquan geothermal field. An updated conceptual model of the field is therefore proposed. Subsequently, three deep wells were drilled, with depths ranging from approximately 600 to 1100 m. The geological conditions encountered during drilling were in excellent agreement with the predictions, and the water temperature and yield are satisfactory. One well produces a flow rate exceeding 100 m 3 /h at a temperature above 70°C, making it the most productive geothermal well in the area. The other two wells yield flow rates of approximately 40–50 m 3 /h at a temperature of about 50–60°C. The results demonstrate the effectiveness of CSAMT for imaging deep fracture‐controlled geothermal reservoirs and provide a transferable exploration model for geothermal systems hosted in granitic terrains.