The analysis of the early electromagnetic response of the receiving coil and its application at close-range TEM detection

The frequent occurrence of groundwater-related damage in operational high-speed railway tunnels in China calls for swift and accurate diagnosis. Thus, this paper describes a way to detect water-bearing defects at 5–10 m using the train-borne transient electromagnetic method in operational tunnels. Due to the short detection distance, the early electromagnetic response of the receiving coil is crucial for determining the size and location of defects. First, the turn-off current in the transmitting coil is elaborated by the equivalent electronic circuit of the transmitting loop. Then the impact of changes in the relative position between the transmitting coil and the receiving coil on the early electromagnetic response are analyzed by a numerical simulation, which shows that a square transmitting coil is suitable for ultra-close detection due to the stability of the electromagnetic response at early times in the receiving coil. Finally a laboratory experiment of close-range TEM detection is carried out to clarify the necessity and feasibility of taking the early electromagnetic response of the receiving coil into consideration at this ultra-close range (5–10 m). This research provides an important theoretical basis for the development and application of train-borne transient electromagnetic method for automatic detection of tunnel defects. •The early EM characters in the transmitting coil plane are affected by loop shape.•The early EM characters in receiver coils are affected by loop relative position.•The square loop has the most stable EM response in the receiving coil at early times.•The test shows early EM response in the coil is essential for close-range detection.