The research on minimizing the induction between the transmitting and receiving coils in close range transient electromagnetic inspection of groundwater-related defects in the operating tunnels

The striking dominance of groundwater-related defects in the operational high-speed railway tunnels in China calls for swift and accurate detection and identification. Thus, it is a new attempt to detect the water-bearing defects at 5 to 10 meters via train-borne transient electromagnetic method in operating tunnels. Due to the short detection distance, the interaction between transmitting and receiving coils is more important than those normally used coils. Thus, numerical and experimental methods are combined to investigate the mutual induction. The influence of turns, current and coil size on the mutual induction and the impact of damping coefficient on the receiving system are manifested. To further verify these findings, full-scale model experiments are conducted. During these physical experiments, the detection results of different coil parameters including coil size, number of turns, and emission current are compared and analyzed. Then, a special effort to minimize the induction between transmitting and receiving coils is expended to acquire the suitable coils for close range detection in the tunnel context. Finally, in order to verify the availability of the detection system, different detection distances are conducted. It turns out that different detection distances have slight difference at the detection results, but they are still within the measuring range of the detection instrument. Obviously, these findings can provide theoretical support for the detection of water-bearing anomalies in operating tunnels and it also has reference significance for the detection of anomalies at close distance. Keywords: Transient electromagnetic method (TEM); close-range detection; coil induction; tunnel defects; water-bearing anomaly