An application of CSAMT for detecting weak geological structures near the deeply buried long tunnel of the Shijiazhuang-Taiyuan passenger railway line in the Taihang Mountains

The Shijiazhuang-Taiyuan (ShiTai) high-speed railway line is designed to cross the Taihang Mountains. The line will include the construction of a 27.85 km long tunnel with a maximum depth of 450 m. The Taihang Mountain Tunnel is the key to the successful construction of the railway. The geological structure of the tunnel is highly complex and the lithology changes dramatically throughout. Fault zones, karst, and groundwater ingress are the main geological engineering problems for the tunnel construction. However, conventional geophysical methods are not effective for detecting these geological structures in a region with such a complex terrain. Therefore, the controlled source audiofrequency magnetotellurics (CSAMT) method was used to effectively characterize the resistivity distribution of the underground rock. This technique is highly sensitive to low resistivity geological structures, such as fault zones, and it is effective for studying the geological engineering conditions of the tunnel. Various corrections, designed denoising, and inversion methods are used in CSAMT data processing to obtain the resistivity profile of the entire tunnel. Based on the resistivity results and existing core information, the underground lithological distribution is inferred. The location of the fault zone is determined and the distribution of dangerous geological bodies is delineated. The interpretation of the CSAMT results is verified by boreholes and related geological information. These results are important for understanding the subsurface geological engineering conditions and have provided stability, safety, and cost evaluation support for the construction of a long deeply buried tunnel in Taihang Mountains.