## 4 Laboratory Experiment
### Experiment Result and Inversion  
During the experiment, the vertical distance between the plastic fixed plate and customized plastic bag was 3.0 m, and the electromagnetic field responses of two conditions were measured. The first condition is that the height of saline water in the bag is 0.3 m, which indicates that there is a water body with low resistivity in the indoor electromagnetic background field. The second condition is that there is no saline water in the bag, which manifests that the response of the indoor electromagnetic background field is measured. In the laboratory experiment, it is difficult to reproduce field conditions; namely the hidden water behind tunnel linings. Thus, the equivalent method is adopted in order to implement the field simulation. There may be some difference in the numerical accuracy between the results of the laboratory experiment and the actual field situation, but the actual field situation can also be qualitatively verified by the laboratory experiment to some extent. The normalized transient electromagnetic response results under the two conditions are illustrated in Figure 15.  
As shown in Figure 15, there are obvious differences in the transient electromagnetic response curves under the two conditions. The response curve of the bag without saline water presents linear attenuation in the double logarithmic coordinate, while the response curve of the bag with saline water illustrates nonlinear attenuation. On the whole, the induced voltage amplitude of the bag with saline water is greater than that of the bag without saline water. Meanwhile, the difference between the two response curves increased first and then decreased. Therefore, the response results of the laboratory experiment are close to the results of the theoretical analysis and numerical simulation as a whole.  
Based on the measurement results of the laboratory experiment, selecting the single point measurement as the analysis object, the transient electromagnetic response signal is inverted by the inversion software of EMIT MAXWELL, and the inversion result can be obtained as shown in Figure 16.  
Figure 14: The final layout of laboratory experiment.  
By comparing the apparent resistivity in the two conditions, as shown in Figure 16, it can be determined that there is a region with low resistivity in front of the transmitter coil. Therefore, it can be preliminarily determined that there is a potential water body with low resistivity in front of the transmitter coil, and it also conforms to the position relation of the bag with saline water in the laboratory experiment. Due to the interference of the two coils and the performance of the measurement instrument, the early measurement data cannot be directly used for the transient electromagnetics inversion, and the time gate that can be used to invert the measurement data is later than the time gate requirements for theoretical analysis and numerical simulation. Therefore, the results of the apparent resistivity in the vicinity of the transmitter coil cannot be used to evaluate the position of the water body with low resistivity; that is, there is a certain inversion blind zone, and the blind range is used to evaluate the position of the water body with low resistivity.  
Figure 16: Apparent resistivity map of laboratory experiment measure results.  
Figure 15: Normalized transient electromagnetic response results under the two conditions.  
approximately 4 m to 5 m. In the laboratory experiment, the vertical distance between the bag with saline water and the transmitter coil is 3.0 m, while the inversion apparent resistivity map illustrates that there is a water body with low resistivity in the range of 5 m to 8 m in front of the transmitter coil. Therefore, the inversion results can qualitatively verify the theoretical research and numerical simulation results of the vehicle-mounted transient electromagnetics method.