Geothermal prospecting through integrated CSAMT and soil radon methods in Baozigou village, Yuncheng Basin, Fenwei Rift, China

The Yuncheng Basin, located in the southern part of the Fenwei Rift, North China, exhibits obvious crust thinning (Moho uplift of 6–8 km) and shallow Curie point depth (less than 18 km) and hence holds great potential for geothermal resources. However, geothermal exploration within the Yuncheng Basin typically faces significant challenges due to civil and industrial noise from dense populations and industrial activities. To address these challenges, both Controlled-Source Audio-frequency Magnetotellurics (CSAMT) and radon measurements were employed in Baozigou village to investigate the geothermal structures and identify potential geothermal targets. The CSAMT method effectively delineated the structure of the subsurface hydrothermal system, identifying the reservoir as Paleogene sandstones and Ordovician and Cambrian limestones at elevations ranging from -800 m to -2500 m. In particular, two concealed normal faults (Fa and Fb) with different scales and opposing dips were newly revealed by the combination of CSAMT and radon profiling, which are likely responsible for controlling the convection of thermal water within the subsurface hydrothermal system. Moreover, this study developed a preliminary conceptual geothermal model for the Fen River Depression within the Yuncheng Basin, which encompasses geothermal heat sources, cap rocks, reservoirs, and fluid pathways, providing valuable insights for future geothermal exploration. In conjunction with the 3D geological model constructed from CSAMT resistivity structures beneath Baozigou village, test drilling is recommended in the northwestern region of the Baozigou area to intersect the potentially deep fractured carbonates that may contain temperature-elevated geothermal water. This study establishes a good set of guidelines for future geothermal exploration in this region, indicating high-permeability faults in the central segments of the Fen River Depression as promising targets.