## 2 Proposal of Vehicle-Mounted Transient Electromagnetic Method  
### Investigation of Operational Tunnel Defects  
Tunnel structural defects are usually manifested on the lining surface, such as lining cracking, water leakage, concrete peeling, and convergence deformation [1; 3; 10; 11; 12]. The occurrence of these defects is usually a superficial characteristic, but it is closely related to the internal conditions of surrounding rocks. These internal conditions mainly include insufficient lining thickness, voids, water behind linings, etc., which are also known as tunnel internal defects. Therefore, the tunnel surface defects in most cases are directly related to tunnel internal defects. Essentially, tunnel internal defects are the origin of many tunnel surface defects to the same extent.  
According to the investigation on the tunnel defects of high-speed railway tunnels in the mountainous area of southwest China, the lining thickness of many operational tunnels is insufficient, which further leads to the obvious dry cavity behind the lining, as shown in Figure 1. When the dry cavity is filled with water under the influence of continuous rainfall, the serious water-spraying phenomenon appears in a different position of the tunnel lining surface, such as the arch foot, vault, and sidewall, as shown in Figure 2. The above phenomenon shows that large amounts of hidden and undiscovered water exist behind the tunnel lining, and will seriously affect the normal operation of the tunnel structure.  
In general, this outflow of water contains a large amount of fissure residue, as shown in Figure 2. The accumulation of fissure residue blocks the tunnel drainage system, and the original water body is undischarged smoothly. Meanwhile, many limestone tunnels are rich in calcium carbonate in southwest China, and the drainage outlets of such tunnels are  
Figure 1: Cavity phenomenon behind tunnel lining.  
Figure 2: Water leakage in different position of tunnel structure.  
prone to crystallization. Under the dual action of the crystallization of calcium carbonate and accumulation of fissure residue, the drainage system of tunnel lining will be partially or completely blocked during the long-term operation of tunnels, as shown in Figure 3. Due to the limitation of the drainage function, the normally discharged water will gradually accumulate behind the tunnel lining. This phenomenon has a long time-dependent effect. The hidden and undiscovered water gathered behind the tunnel lining cannot be found without involving the normal operation and structural safety of the tunnel lining.  
With further damage to the drainage system of the tunnel lining, a large amount of water accumulates behind the tunnel lining. Once the accumulated water reaches a certain level, different degrees of water leakage will occur to the surface of the tunnel lining, as shown in Figure 2. This hidden and undiscovered water behind the tunnel lining is applied to the tunnel lining structure in the form of water pressure. As for non-drainage tunnels, the stress state of the lining structure is usually changed, and the bearing capacity of the tunnel lining is seriously reduced. When the maximum bearing capacity of the non-drainage tunnel is not enough to support the hidden and undiscovered water behind the tunnel lining, the lining structure will suffer local or overall instability, which is manifested as the partial or large-scale disruption of the secondary lining, as shown in Figure 4. This hidden and undiscovered water behind the tunnel lining can only be discovered and revealed when the serious phenomena of water leakage and lining disruption are found. Therefore, it is an urgent problem for tunnel maintenance engineers to detect and find the hidden and undiscovered water behind the tunnel lining.  
Figure 4: Disruption phenomenon of secondary lining.  
Figure 3: Blocking phenomenon of tunnel drainage system.