## 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
However, the apparent amplitude frequency amplitude of the two measurements is much lower than that at point 1. The horizontal axis coordinates corresponding to the raised portion of the apparent amplitude frequency curves at measurement points 2 and 3 can be used to better estimate the depths of the contaminated field at the top and bottom of the field. The advantage of point 2 over point 3 is that the anomaly contrast of the apparent amplitude-frequency curve obtained from point 2 is greater when the depth of migration of the portion with the largest contamination concentration increases. Anomaly contrast refers to the visibility of the abnormality relative to the background value, defined as the ratio of the abnormal maximum value to the normal field value. The apparent amplitude frequency curves obtained by a symmetrical quadrupole sounding device in undulating terrain are shown in Figure 13d-f. The overall trend of the apparent amplitude frequency \(F_{\mathrm{s}}\) curves is still similar to that of flat terrain. However, there are slight fluctuations due to the influence of terrain undulation. The apparent frequency \(F_{\mathrm{s}}\) excitation anomalies detected at point 1 in Figure 13d decrease with the increase of the number of days. However, the curves are all in a two-layer pattern, and it is not possible to judge the burial depth at the bottom of the contaminated field. The curves from point 2 in Figure 13e can more accurately estimate the depths at the top and bottom of the contaminated site, and its anomaly contrast is larger than that of point 3 in Figure 13d. Through the above analysis, measurement point 2 is the best detection among the three measurement points in both terrains. The apparent resistivity change curve corresponding to measurement point 2 (see Figure 14) is now made for further analysis. As shown in Figure 14, the topographic relief will cause fluctuations in the apparent resistivity value under a symmetrical quadrupole sounding device. However, it will not affect the valley of the apparent resistivity. This means that the apparent resistivity at the center of the source is constant (about 45 \(\Omega\cdot\)m), consistent with the results obtained from the intermediate gradient detection in Section 4.1.1. The migration of the pollution source downstream does not significantly affect the valley value of the apparent resistivity curve. If Figures 10 and 11 are one group for the Intermediate Gradient detection device and Figure 13b,e and Figure 14 for the symmetrical quadrupole sounding device are another group, then a comparison of the two groups shows that: the peak of the apparent amplitude frequency and the valley of the apparent resistivity in the symmetrical quadrupole sounding device have a poorer correspondence with each other at the same point of measurement than that in the Intermediate Gradient detection device. Figure 13: Variation of apparent amplitude frequency of pollution field sounding in different days. (**a**) Flat terrain (Point 1). (**b**) Flat terrain (Point 2). (**c**) Flat terrain (Point 3). (**d**) Undulating terrain (Point 1). (**e**) Undulating terrain (Point 2). (**f**) Undulating terrain (Point 3).