Vehicle-Borne Transient Electromagnetic Numerical Characteristic Parameter of Water-Bearing Body behind Tunnel Linings

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 μ s 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.