## 3 Comparative Analysis of Numerical Forward Simulation and Soil Trench Modeling Experiments
### Analysis of Experimental Results
#### 4.1.2 Numerical Simulation of Symmetrical Quadrupole Sounding Device Detection  
The symmetrical quadrupole sounding detection model of the pollution field with different topography is constructed as shown in Figure 12a,b. The migration detection model of the pollution field for other days is the same as that in Figure 12. In order to have enough area to arrange the A electrodes to the left, the model range was increased on the left side of the pollution field, which was increased from 250 m to 300 m. In order to make the model still reflect the excitation level at a deep enough position, the bottom of the model was set to be grounded. There are 601 points on the two types of terrain, each with a horizontal spacing of 0.5 m. The horizontal spacing of the measuring electrodes MN is 1 m, and O is the midpoint of MN. When a certain measuring point is used for the sounding work on the two types of terrain, the A and B electrodes are gradually increased towards the two ends. In the paper, under both methods, 75 sets of AB positions were taken and then 75 sets of potential values were measured separately. Then, the horizontal spacing of AB is gradually increased from 2 m to 76 m. In the simulation detection, three orientations of measurement points were arranged under the conditions of flat terrain and undulating terrain:  
Measurement point O\({}_{1}\) was arranged above the center of the contaminated field. Measurement point O\({}_{2}\) was arranged in the center of the left edge of the contaminated field. Measurement point O\({}_{3}\) was arranged outside the projected range of the contaminated field. The apparent amplitude frequency in Figure 13 and the apparent resistivity graph in Figure 14 both adopt logarithmic coordinates with 1/2 of the horizontal net distance of AB electrodes as the horizontal axis, which can better reflect the anomalies of the polluted field buried in the middle and deep. When AB/2 is taken as the horizontal axis, the measured depth tends to be the same as the actual depth within the 0-100 m sounding section [30; 31]. The apparent amplitude frequency curves obtained from the sounding when leveling the terrain are shown in Figure 13a-c. From which it can be seen that, on the 10 th day of the migration of the contaminated field, the peaks of the apparent amplitude frequency of the extended line measured at the measurement points 1-3 are 7.82%, 4.88%, and 1.97%, respectively. This indicates that a larger excitation anomaly can be detected by arranging the measurement points right above the center of the contaminated field. The more by  
Figure 12: Symmetrical quadrupole sounding model in different topographic pollution fields (Day 100). (**a**) Flat terrain. (**b**) Undulating terrain. the edge of the arrangement of the measurement points, the more the excitation anomaly is not obvious. On the 10 th day of the migration, the depth of the contaminated field was very shallow, and the curves under all the measurement points showed a two-layer pattern. The apparent amplitude frequency \(F_{\mathrm{s}}\) decreased with the increase of the pole distance AB, according to which the depth of the contaminated field could be estimated. When the contaminated field migrates downward to leave the surface (100-400 days), the apparent amplitude frequency curve obtained at the measurement point 1 still shows a two-layer pattern. The \(F_{\mathrm{s}}\) increases with the increase of the pole distance AB and the depth of the contaminated field at the top of the field can be estimated according to the curve information. As shown in Figure 13b,c, the apparent amplitude frequency curves detected at measurement points 2 and 3 show a three-layer pattern from 100 to 400 days, regardless of the distance of the contaminated field from the ground. Taking 100 days as an example, at this time, the amplitude of the apparent amplitude frequency excitation anomaly at measurement point 2 is about 90% of that at measurement point 1, and that at measurement point 3 is about 34% of that at measurement point 1.