# Uncertainty of the 2D Resistivity Survey on the Subsurface Cavities
## 3 Results and Discussion
### Model Accuracy, Resolution, and Sensitivity
#### 3.3.1 Model Accuracy
The boundary between unsaturated and saturated limestone was precisely resolved in the DD, WS, and PP arrays, while the recovered boundary was slightly deeper in the PD array. The cavity filled with conductive materials showed a slight improvement in the resolution of cavity anomaly compared to the air-filled cavity model, particularly in the PP array (Figure 20). In line with our study, the DD array field study shows the highest resolution for the conductive cavity [21, 66]. The numerical study for sinkhole modeling showed lower resolution in the DD array than the PD array, contradicting our study [33]. The field study for cavity detection displayed relatively high resolution in the PD array, while the PP array showed low resolution [67], consistent with our results. Figure 5:4 The inverted resistivity model for a cavity filled with dry clay using: (**a**) DD array, (**b**) PD array, (**c**) WS array, (**d**) PP array. The rectangular box indicates the actual cavity target. Figure 5:5 The inverted resistivity model for the saturated clay-filled cavity using: (**a**) DD array, (**b**) PD array, (**c**) WS array, (**d**) PP array. The rectangular box indicates the actual cavity target. The broken line represents the actual groundwater level. Experimental Cavity Study  
A field experiment was also conducted to validate the numerical modeling results. Our numerical model cavities were embedded in a limestone unit; however, in the field experiment, we used moderately compacted sandy clay as the host unit around an empty plastic box which acted as the cavity (Figure 21). In our experiment, the one-tenth scale of the cavity target T\({}_{2}\) was used. Thus, the plastic box's center with 0.15 \(\times\) 0.15 \(\times\) 0.6 m size was situated at 0.42 m depth. The experimental resistivity datasets were measured using 89 electrodes of a 4-point light 10W (LGM Lippmann) resistivity meter with 0.1 m spacing (Figure 21). Figure 22 shows the experimental resistivity survey results for the DD, PD, WS, and PP arrays. The DD and PD arrays showed a high resistivity anomaly at the cavity zone, whereas the lower anomaly amplitude was obtained by WS and PP arrays. The experimental cavity geometry was more adequately reconstructed in the DD and WS arrays; however, they obtained wider and shallower anomalies. The geometry of the cavity in the PD array was not properly restored--its left side anomaly was elongated to the greater depth. The PP array exhibited a triangular anomaly instead of providing the rectangular cavity shape. Compared to the numerical results, the experimental cavity anomaly in all the tested arrays showed considerable underestimation of anomaly depth. The depth underestimation in cavity studies can lead to inappropriate interpretation, similar to other field studies [26; 68]. Since our study was conducted in shallow sandy clay, the patchily distributed high and low resistive features were inferred as the inversion artefacts. The DD, PD, and PP arrays artefact agreed with synthetic model results, whereas a relatively greater artefact was observed in the WS arrays. As the experimental cavity was set in low-resistive sandy clay, its obtained resistivity values were considerably underestimated compared to the numerical cavity, which was embedded in limestone rock. Based on the experimental result, the WS and DD arrays are more appropriate for the subsurface cavity study, whereas the PD array is reasonable. The PP array is the least suitable, particularly in obtaining the cavity geometry and position. Figure 5:6 The experimental data collection procedure for resistivity imaging. Our numerical and experimental study can help to decide proper arrays for subsurface target (e.g., cavity) studies. In line with our results, the field study and numerical modeling have shown that the DD array can accurately resolve the subsurface structures such as cavity [38, 67], thin dike [25], buried wall [29, 32], and sinkhole [21]. These structures were moderately recovered by the WS and PD arrays [24, 26, 69].