# Uncertainty of the 2D Resistivity Survey on the Subsurface Cavities
## 3 Results and Discussion
### Model Accuracy, Resolution, and Sensitivity
#### 3.3.1 Model Accuracy
In karstic terrain, cavities are not usually aligned in the horizontal direction; thus, we examined the surface resistivity methods using a cavity target (0.5 \(\times\) 6 m) set at a different inclination (I), such as the horizontal (I = 0\({}^{\circ}\)), inclined (I = 45\({}^{\circ}\)), and vertical (I = 90\({}^{\circ}\)). As the horizontal cavity models are discussed in the earlier sections (Figure 14), we present the inclined (Figure 16) and vertical (Figure 17) cavity models. The result showed that the orientation and geometry of the inclined cavity were correctly resolved in the DD and PD arrays. The WS and PP arrays recovered the upper part of the inclined cavity, but they did not properly detect the deeper parts (Figure 16). As shown in Figure 17, the PD and DD arrays also accurately recovered the orientation and geometry of the vertical cavity, whereas these were poorly reproduced in the WS and PP arrays. All the arrays recovered the geometry of the top cavity boundary since the resistivity method detected a high signal near the electrode position. For increasing cavity depth, the inverted resistivity value decreased, and the cavity anomaly width considerably increased. Pronounced inversion artefacts were observed in the inclined and vertical cavity models. It is well-known that the DD array is highly sensitive to the lateral variation of the subsurface structures [21; 64]. However, our results show that the DD array can also resolve cavities oriented in the horizontal and inclined directions. The PD array could be more effective for vertical and inclined cavity structures compared to the horizontal structures. The WS array is known to be moderately sensitive to both lateral and vertical variations [22; 27]. However, in our results, it showed relatively better resolution for the horizontal cavity than the inclined and vertical ones. The PP array depicted low resolution for all the tested orientations of the cavity. Conductive Cavity Modeling  
The air-filled cavity showed a high anomaly in resistivity imaging, while a conductive cavity appeared as a low anomaly. For conductive cavity modeling, a cavity with a 1.5 \(\times\) 6 m dimension set at 4.2 m depth was filled with dry clay (90 \(\Omega\cdot\)m) and saturated clay (50 \(\Omega\cdot\)m). Figure 19 shows the inverted model results for a cavity filled with dry clay. The DD array accurately resolved the cavity; however, the anomaly size was higher than the actual cavity size. The PD and WS arrays also showed relatively high resolution, and their anomaly sizes were wider compared to the actual cavity. The recovered cavity orientation was tilted in the case of the PD array. The PP array showed the lowest resolution, and its anomaly geometry was unclear. The noticeable inversion artefact had a resistivity value above 1160 \(\Omega\cdot\)m that can lead to inappropriate model interpretation in resistivity studies. These artefacts were pronounced at shallow depth. Figure 5:1 The inverted resistivity model for the vertical (90\({}^{\circ}\)) cavity size using: (**a**) DD array, (**b**) PD array, (**c**) WS array, (**d**) PP array. The rectangular box indicates the actual cavity target. Figure 5:2 The inverted resistivity models for the cavity geometries using: (**a**) DD array, (**b**) PD array, (**c**) WS array, (**d**) PP array. The solid line indicates the actual cavity targets. Significantly low resistivity may be detected in resistivity imaging over a cavity filled with clay and water [65]. We consider the groundwater level (2 m) above the cavity depth for the saturated clay-filled condition, as shown in Figure 5:3 The model inversion result showed that the saturated clay-filled cavity was accurately resolved in the DD array. The PD and WS arrays obtained high resolution; however, the WS array showed a slightly lower resolution than the PD array. The PP array identified the cavity zone, but its resolution is lower than the other arrays. All the arrays exhibited noticeably wider anomaly sizes with regard to the actual cavity size.