Innovative Cross Hole Grounded Wire Source Transient Electromagnetic Method for Sensing Geological Disasters in Urban Underground Spaces

In the process of tunnel construction and urban underground space development, karst caves and cavities often cause casualties and economic losses. However, due to the influence of complex terrain and buildings on the ground, conventional geophysical detection methods cannot obtain accurate information on hidden geological bodies under buildings and in front of tunnels. The grounded-wire source transient electromagnetic method (TEM) has a relatively simple transmission and reception system and is widely used in the detection of coal mine goafs and karst caves. Therefore, we apply this method to bore-holes and propose a cross-hole grounded-wire source TEM detection method. This method involves deploying grounded-wire transmitting sources and receiving electrodes in different bore-holes to accurately detect concealed geological hazards in urban underground construction processes. In this study, we use the research methods of the TEM field diffusion mechanism of a vertical grounded-wire source, a numerical simulation of a cross-hole and grounded-wire source TEM of different electrical models and an indoor physical experiment in a strong interference environment to analyze the detection ability of a cross-hole grounded-wire source TEM. Differential processing was applied to both the numerical simulation and experimental data. The research results indicated that, owing to the influence of the ground–air boundary, extreme values of the z component of the electric field moved toward deeper regions away from the grounded-wire source. Differential processing effectively increased the detection accuracy of this method, and a higher differential order provided clearer characterization of the boundaries of the target body. Spatial averaging filtering can effectively remove the influence of random noise, enhancing the ability of the cross-hole grounded-wire source TEM to identify anomalous bodies. After difference processing, the cross-hole grounded-wire source TEM data could more precisely capture the boundaries of concealed geological bodies in urban underground spaces.