## 6 Conclusions  
The vehicle-mounted transient electromagnetic method is fully and perfectly proposed, and it aims to realize the rapid detection of internal defects of operational tunnels. The main results are summarized as follows.  
1. Based on the investigation of tunnel defects, the internal defects, especially the hidden and undiscovered water behind the tunnel lining, are the main origin of many tunnel defects, and directly affect the normal operation state of the tunnel;
2. Aimed at the rapid detection of tunnel defects, a new type of detection technology called the vehicle-mounted transient electromagnetic method is proposed. This detection technology is a promising application of a transient electromagnetic method in tunnel defect detection, and its principle is also based on the obvious resistivity difference between tunnel defects and surrounding rocks;
3. The rationality of the simulation solution of the secondary magnetic field of the homogeneous half-space medium is verified by the fast Hankel transform. Based on the typical whole-space case, the transient electromagnetic response characteristics of the inhomogeneous medium are obviously different from those of the homogeneous medium, especially in the middle stage of the electromagnetic response curve;
4. Based on the similarity criterion of the electromagnetic field, a set of saline water model experiments are designed. As a whole, the induced voltage amplitude of the bag with saline water is greater than that of the bag without saline water, so the attenuation trend of the electromagnetic response measured in the experiment is consistent with the results of the theoretical analysis and numerical simulation;
5. According to apparent resistivity inversion results, the low resistivity region revealed in front of the transmitter coil can reflect the existence of a potential water body with low resistivity to some extent. Meanwhile, the distance obtained from the inversion essentially conforms to the distance in the experiment, considering the existence of the inversion blind zone. Therefore, the inversion results can qualitatively verify the rationality of the vehicle-mounted transient electromagnetic method.  
Conceptualization, W.Q.; Formal analysis, H.L.; Funding acquisition, Z.G.; Investigation, J.Y.; Methodology, W.Q., H.L. and Z.G.; Resources, Z.G.; Software, J.Y.; Supervision, W.Q. All authors have read and agreed to the published version of the manuscript.  
This work was supported by The National Natural Science Foundation of China (grand No. 51808298), Jiangsu Province College Students' innovation and entrepreneurship training program (grand No. 202110304115Y), Natural Sciences Fund for Colleges and Universities in Jiangsu Province (grand Nos. 20 KJB560036 and 20 KJD560002), and Nantong Science and Technology Plan Project (grand Nos. JC2020122 and JC2020124).  
Not applicable.  
The data used to support the findings of this research are available from the corresponding author upon request.  
The authors would like to equally thank [PERSON] from the Xuzhou Economic Development Zone Industrial School, China, for linguistic assistance during the preparation of this manuscript. The insightful comments and significant suggestions from the anonymous reviewers of _Applied Sciences_ are sincerely appreciated.  
The authors declare that there are no conflicts of interest.