## 3 Field Test Analysis
### Analysis of Detection Results  
The inversion process of high-density resistivity testing involves converting measured apparent resistivity data through format transformation, data preprocessing, forward modeling, and inversion calculations, ultimately generating apparent resistivity tomographic images. The formatted apparent resistivity data undergo preprocessing to eliminate outliers while retaining points with higher consistency. Using the optimal fitting method, an initial geoelectric section is defined to calculate theoretical apparent resistivity curves. These theoretical curves are then compared with measured curves, and parameters are iteratively adjusted to achieve optimal fitting results, producing the final inversion-derived resistivity tomographic images.  
(1) Survey Line 2-1  
Survey Line 2-1 spans 60 m in length and is aligned colinearly with monitoring wells. The subsurface in this area contains a substantial gravel layer beneath the soil stratum, with a surface soil moisture content higher than that of Line 1-1. This survey line traverses two concrete steps at its midpoint. An inversion analysis of the acquired data yielded the geoelectric cross-section presented in Figure 4.  
The forward modeling results demonstrate a minimal discrepancy of 1.37% compared to the measured apparent resistivity values, indicating robust inversion performance. The subsurface exhibits distinct layered stratigraphy with resistivity values uniformly below 20 \(\Omega\)-m, consistent with soil geotechnical reports and literature-derived resistivity values, showing no significant anomalies. As revealed by the inversion profile, localized high-resistivity zones occur at 22-26 m and 42-46 m along the survey line, corresponding to two reinforced concrete steps traversed by the measurement transect.  
(2) Survey Line 2-2  
Survey Line 2-2 extends 60 m in length, collinear with Survey Line 2-1 and the monitoring wells, traversing two concrete steps along its path. An inversion analysis of the acquired data yielded the geoelectric cross-section presented in Figure 5.  
The forward modeling results show a 5.8% discrepancy compared to the measured apparent resistivity values, demonstrating good inversion performance. Distinct layered stratigraphy is observed beneath the surface. Due to the survey line crossing two reinforced concrete steps, localized high-resistivity zones are present at 18-22 m and 40-42 m along the line.  
(3) Survey Line a  
Survey Line a spans 20 m in length, positioned between Survey Lines 1-2 and 1-3. As it runs parallel to the water flow direction, this transverse line was designed to determine the horizontal distribution of subsurface contaminants. By collecting electrical resistivity data along the transverse direction, the horizontal extent and distribution characteristics of underground pollutants can be determined. The inversion analysis results of the acquired data are presented in Figure 6.  
The forward modeling results demonstrate a minimal discrepancy of 3.3% compared to the measured apparent resistivity values, indicating satisfactory inversion performance.  
Figure 4: Inversion modeling resistivity contour map of Survey Line 2-1.  
Figure 5: Inversion modeling resistivity contour map of Survey Line 2-2.  
Figure 6: Inversion modeling resistivity contour map of Survey Line a.  
At a detection depth of 4 m, the imaging results do not exhibit strictly layered stratigraphy, with the majority of the subsurface composed of silty soil. This image reveals approximately circular high-resistivity zones at a burial depth of 1.5 m, necessitating verification of potential subsurface structures.