Abstract: An experimental study is performed using the semiairborne transient electromagnetic method with ground excitation and airborne reception for exploring tunnels under complex terrain conditions in southwest China. Furthermore, the optimization of the route design and data acquisition process is achieved by implementing an airborne transient electromagnetic method, which utilizes a 〖WTBX〗α〖WT５《TNR》〗trimmed mean filter to remove astroelectric and motion noise based on the polynomial fitting method. Moreover, the data superposition method and time window integration parameters are determined, full domain apparent resistivity is calculated, threedimensional(3D) imaging of the probing area is achieved, the apparent resistivity of the tunnelfilled karst development area is characterized, and probing results are comprehensively interpreted. The results are validated through field application of the proposed method in the Fenghuan #2 tunnel. The main conclusions obtained from the study are as follows. (1) The semiairborne transient electromagnetic method can accurately detect the scope and depth of waterbearing fracture zones, karst development zones, and other undesirable geological structures; furthermore, it can detect the geological risk areas of tunnels in advance and acquire the distribution characteristics of underground electrical conductivity, demonstrating stronger terrain adaptability and higher detection efficiency in the tunnel survey of karst mountainous areas compared with the traditional groundbased physical exploration method. (2) The geological anomalies deduced from the 3D visual resistivity imaging and key location slicing outcomes strongly concur with the anomalies exposed through tunnel excavation. Thus, the proposed method has the potential to offer technical support for designing tunnel karst disposal plans to ensure safe construction, which can effectively avoid casualties and property losses due to hidden karst hazards during tunnel construction.

Key words: semiairborne transient electromagnetic method, complex terrain, tunnel, detection, karst development area