# Uncertainty of the 2D Resistivity Survey on the Subsurface Cavities
## 3 Results and Discussion
### Model Accuracy, Resolution, and Sensitivity
#### 3.3.3 Sensitivity to Resistivity Contrasts
Figure 28: Wenner-Schlumberger (WS) array sensitivity to resistivity perturbations. The inverted model for: (**a**) \(\pm\)20% perturbation, (**b**) \(\pm\)40% perturbation, (**c**) \(\pm\)60% perturbation, (**d**) \(\pm\)80% perturbation. Figure 27: Pole-dipole (PD) array sensitivity to resistivity perturbations. The inverted model for: (**a**) \(\pm\)20% perturbation, (**b**) \(\pm\)40% perturbation, (**c**) \(\pm\)60% perturbation, (**d**) \(\pm\)80% perturbation. In this study, we observed that a specific resistivity imaging method might provide varying model accuracy, resolution, and anomaly responses for inversion of a single cavity model situated at different survey depths. Such ambiguity in subsurface cavity studies can substantially be reduced by applying two or more geophysical methods. Various methods have different capabilities to resolve the subsurface structures; thus, they complement each other and obtain detailed subsurface information. The joint inversion of two separate datasets may improve the overall resolution and match the inverted and the actual model [78]. Moreover, the joint interpretation of different outputs from two or more geophysical methods can minimize model ambiguity [77]. For example, a joint interpretation of results from resistivity tomography and ground penetration radar survey can reduce the \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Adequacy of Arrays for Cavity** & **DD** & **PD** & **WS** & **PP** \\ \hline Shallow depth (\(\lesssim\)4 m) 1 & xxx & xxx & xxx & xx \\ Intermediate depth (\(\lesssim\)8 m and \(>\)4 m) 1 & xx & x & x & x \\ Deeper depth (\(>\)8 m) 1 & x & x & x & - \\ Small size (\(\lesssim\)0.5 m) 2 & x & - & - & - \\ Intermediate size (\(\lesssim\)3 m and \(>\)0.5 m) 2 & xxx & xx & xx & x \\ Large size (\(>\)3 m) 2 & xxx & xxx & xxx & xx \\ Horizontal cavity 1,2 & xxx & xx & xx & x \\ Vertical cavity 1 & xx & xx & x & x \\ Inclined cavity 1 & xxx & xxx & x & x \\ Dry clay-filled cavity 1,2 & xxx & x & xx & x \\ Saturated clay-filled cavity 1,2 & xxx & xxx & xxx & xx \\ \hline \hline \end{tabular}  
* Considering a 1.5 \(\times\) 6 m cavity size. 2 Considering 4.2 m depth. Except for dry and saturated clay, all the scenarios considered an air-filled cavity. The host medium is considered as homogenous limestone. For the heterogeneous background, the checkerboard cell size scenarios could be used.  
\end{table} 
Table 1: Adequacy of the resistivity imaging types for cavity study: highly effective = xxx, moderately effective = xx, and less effective = x.  
Figure 29: Pole-pole (PP) array sensitivity to resistivity perturbations. The inverted model for: (**a**) \(\pm\)20% perturbation, (**b**) \(\pm\)40% perturbation, (**c**) \(\pm\)60% perturbation, (**d**) \(\pm\)80% perturbation. non-uniqueness of the inverted resistivity model. Particularly in cavity study, the upper and lower boundaries of the cavity will be clearly determined by high-resolution ground penetration radar; however, it usually fails to provide information about the host medium and the cavity-filling materials. Instead, the resistivity imaging quite well detects the cavity-filling materials and the surrounding formation, but it cannot resolve the accurate geometry of the cavity [19]. Hence, using a joint interpretation of the model results and performing a joint inversion of different methods can reduce those inherited uncertainties of the resistivity imaging.