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Karst cavity mapping is attracting great interest from engineering geologists because of its relation to the dangerous geohazards faced during engineering construction. Ground-based geophysical methods still face challenges in karst mapping, and concealed karst cavities potentially pose threats to tunnel construction in southwest China. Given the significant contrast in electrical resistivity between karst cavities and their host rocks, geoelectrical methods are widely used for mapping these cavities. We have developed a successful case history of mapping karst cavities on a planned railway route using controlled-source audio magnetotellurics (CSAMT). Scalar CSAMT, with frequencies ranging from 0.5 to 8192 Hz, was used for field data acquisition. A full-frequency domain apparent resistivity correction method was used for near-field corrections. Electromagnetic array profiling (EMAP) filtering was used for topographic and static shift corrections, and the Bostick conversion was used for data interpretation. Our study indicated that the results of the Bostick conversion with EMAP filtering were more acceptable than the results of rapid relaxation inversion and nonlinear conjugate gradient inversion in this case. The G Tunnel is a key tunnel along the Gui-Guang high-speed railway in southwest China. Initial geophysical and engineering geologic results suggest that the bedrock of the survey section of the G Tunnel route is sandstone. A CSAMT survey with three inline sections and three crossline sections over the tunnel route was conducted in two phases to verify the rock conditions of the tunnel route. A concealed karst cavity with a low-resistivity anomaly was found on the tunnel route and was verified by the borehole. Data from the CSAMT survey significantly refined our understanding of the subsurface engineering geologic conditions along the tunnel route.

GEM Beijing 2011: International Workshop on Gravity, Electrical & Magnetic Methods and Their Applications Beijing, China. October 10-13, 2011. Karst cavity mapping using CSAMT: A case history of tunnel investigation in China Lanfang He, Xiumian Hu, Nanjing University; Ligui Xu, BGP; Xuben Wang, Chengdu University of Technology; Jian Li, Xuyou Lei, Donghua Wei, China Railway Eryuan Engineering Group

Summary Two case histories of mapping the near surface for railway route investigation in SW China using high resolution resistivity imaging (ERI) are presented in the paper. A set of SuperSting® R8/IP (Advance Geosciences Inc.) was used for data acquisition. Four cables with 84 passive stainless steel electrodes spaced 5 m apart were used. Based on the testing result, Schlumberger with varied MN was used in data acquisition. The Schlumberger array with fixed MN as one trace space has poor data quality in while AB is move far from MN stakes. Case history in XM indicates that ERI could be successfully used for mapping the near surface in karst area, the underground river is characterized by lower risistivity, most karst caves features as low resistivity traps. Case history in DF indicate that ERI could be successfully used for mapping the coal bearing rocks and soft rocks, but it is hard to use ERI to shape and locate the coal goafs, even using the trace space as little as 0.5 m, locate and shape the coal goaf still proves difficulty.