Flexible WFEM Imaging for Deep Tunnel Investigation in Complex Terrain: A Case Study of the Sichuan–Tibet Railway

Geological investigation of deep tunnels in high-altitude regions with strong topographic relief remains challenging because conventional geophysical methods are often limited in both penetration depth and field operability. In this study, a flexible wide-field electromagnetic method (WFEM) based on arbitrary electric-field-component measurements was applied to the Deda Tunnel along the Sichuan–Tibet Railway. More than 800 stations were deployed along a 16 km survey line, providing high-quality electric field data. Seven conductive zones were delineated and interpreted as possible fault-related fracture zones based on their resistivity contrast, geometry, and geological context. Three resistivity-based rock-mass categories were adopted, while qualitative comparison with eight engineering boreholes provided local support for the interpretation of reduced rock-mass integrity. Deep geostructural features were inferred from the WFEM results to depths of up to approximately 2 km. The identified low-resistivity zones are interpreted as highly fractured and potentially water-bearing structures that may pose significant risks to tunnel construction. These results demonstrate that WFEM, when combined with an optimized acquisition and interpretation workflow, is effective for deep tunnel investigation in high-altitude regions. The proposed approach provides practical guidance for similar engineering projects in complex geological settings.