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The correct interpretation of full-space transient electromagnetic data has always constituted a critical safety problem during tunnel excavation projects. Targeting the interpretation of water-filled caves under narrow tunnel conditions, we have developed an iterative inversion method based on 3D finite-difference time-domain (FDTD) forward calculations and a direction algorithm. In total, 125 groups of 3D FDTD forward calculation results are analyzed to identify the correlations between the response data and the geometric conditions of the cave. A direction algorithm is established based on the correlations, thereby increasing the iterative inversion convergence speed. Using the proposed iterative inversion method, the location and volume of the water-filled cave in front of the tunnel face are successfully inverted. Through an iterative program, the inversion results of simulations involving the detection of water-filled caves under tunnel conditions are accurately analyzed, and the relative error is less than 10%. The application of the iterative inversion method to the Mingyue Mountain Tunnel project suggests that this method is capable of interpreting the size of water-filled caves and it is valid for a narrow tunnel face with only a single available measurement point. The proposed iterative inversion method can be used alone or in combination with other detection techniques, thereby providing engineers with a better early warning system for detecting water-filled caves in tunnels.

The water-bearing body (WBB) behind tunnel linings has been some of the main causes of damage in operational tunnels. The WBB directly affects the serviceability state of tunnel linings; thus, determining a method to detect this problem is a widely studied issue regarding tunnel maintenance. In this paper, a vehicle-borne transient electromagnetic method (VBTEM) is put forward for the first time to detect WBB behind tunnel linings, and the aim is to fully investigate the transient electromagnetic response and numerical characteristics of the WBB behind tunnel linings. Firstly, the transient electromagnetic response curves of the WBB and surrounding rock are obtained and compared in detail by using the finite element method. Then, taking the distance, thickness, radius, and resistivity of the WBB as variable parameters, the parametric sensitivity rule of the response curve of the WBB is analyzed. Finally, a dimensionless response curve is proposed, a mathematical extraction equation is established, and seven numerical characteristic parameters are proposed and extracted. Based on seven numerical characteristic parameters, the technical parameters of the VBTEM equipment are put forward. The results illustrate that the transient electromagnetic response of the WBB is obviously different from that of surrounding rock. The seven numerical characteristic parameters of the dimensionless curve of the WBB can represent the entire response curve of the WBB. The results reveal that the first time gate of the VBTEM equipment needs to be less than 0.05 μs for short-distance problem, and the time span needs to reach four time spans at least. The research results can provide valuable technical references for the application of VBTEM.

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