## 3 Comparative Analysis of Numerical Forward Simulation and Soil Trench Modeling Experiments
### Analysis of Experimental Results
#### 4.1.1 Numerical Simulation of Intermediate Gradient Detection Device Detection  
Taking the migration model of the pollution field on the 100 th day as an example, the intermediate gradient detection model of the pollution field in different terrains was constructed as shown in Figure 9a,b. Where the height of each valley bump in Figure 9b was 2 m, and the width of each valley bump was 50 m. The migration detection model of the pollution field for other days was the same as that in Figure 9. In order to get more accurate excitation anomaly data, the AB electrodes were set at the two ends of the wiring, and the measurement electrode MN pole spacing was set to 5 m. The spacing of each point number in Figure 9a,b was 5 m (the horizontal distance of each measurement point in the case of undulating terrain was 5 m). The total number of measurement points was 50. The variation rules of apparent amplitude frequency and apparent resistivity at different topographies of the polluted field from 10 to 400 days were obtained, as shown in Figures 10 and 11, respectively. Figure 11: Change pattern of apparent resistivity of pollution field in different days. (**a**) Flat terrain. (**b**) Undulating terrain. Figure 10: Change pattern of apparent amplitude frequency of pollution field in different days. (**a**) Flat terrain. (**b**) Undulating terrain. Figure 9: Intermediate gradient detection model in different topographic pollution fields (Day 100). (**a**) Flat terrain. (**b**) Undulating terrain. As shown in Figure 10a, the apparent amplitude frequency on the 10 th day of contaminated field migration jumps from 1.5% at point No.8 to 13.6% at point No.11 when the terrain is leveled. This is because there is a large concentration of Cr contaminants retained on the ground on the 10 th day, which makes the surface contaminated area and the seepage into the subsurface portion produce excitation anomalies at the same time, resulting in the jumps of apparent amplitude frequency at the boundaries of the source of the contamination. The depths of the center of the heavily polluted area were 2.2 m, 3.9 m, 5.2 m, and 5.5 m at the 100 th, 200 th, 300 th, and 400 th days, respectively. The corresponding peak values of the apparent amplitude frequency were 10.27%, 8.50%, 7.58%, and 6.30%, which indicated that the peak value of the apparent amplitude frequency decreased gradually with time when the pollution field spread downward. This is because as the depth of the pollution field migrates, the value of the secondary potential that can be detected gradually decreases, and the excitation effect anomaly has become less obvious. The projected orientation of the center of the heavily contaminated area on the ground at days 100, 200, 300, and 400 was 75 m, 130 m, 153 m, and 178 m, respectively. The corresponding point numbers of the peaks of the apparent amplitude frequency were 15, 25, 30, and 35, respectively, which indicated that the point numbers of the peaks of the apparent amplitude frequency gradually increased when the contaminated field migrated to the right. As the time increases, the angle of the apparent amplitude frequency curve also increases, indicating that the contaminated area of the detected soil area is increasing. On the 200 th day, due to the good permeability of the fine sand, under the action of convective dispersion, the contaminated area spreads significantly in the transverse direction. The part that moves out of the fine sand area is subjected to the action of the aquifer and the pressure head in the longitudinal direction, which leads to the appearance of double peaks in the apparent amplitude frequency at the point number of 15-25 at this time. The peak on the left side represents the part of the heavy contamination that is retained in the fine sand area (the low-resistivity part corresponds to the dark blue color in Figure 2). The peak on the right side represents the part of the heavily contaminated area (the black-blue semi-ellipse part in Figure 2c).