## 4 Field Test
The section of the 3D inverse model (Figure 14b) from the simulated data figured out the pattern of the tunnels intersected by the section very well, although the resistivity  
Figure 13: Line B\(-\)B\({}^{\prime}\) comparison between simulated inversion of 3D and 2D resistivity surveys. (**a**) Vertical section of the 3D resistivity model. (**b**) Corresponding section of the inverse model obtained by simulating a 3D survey on the geophysical model. (**c**) Inverse model obtained by simulating a 2D survey on the geophysical model. values were not always representative of the model values. The 2D inverse model from the simulated data (Figure 14c) also had a similar data misfit in this case, was less clear and showed more smoothed patterns, especially regarding the base of the cavities. Gallery \(S\), which was close to the line but not crossed by it, generated a clear anomaly joining \(H\) and \(J\). The depth of investigation achieved by the field survey was lower than the simulated one because all the data with a pseudo-depth greater than 16 m were discarded, as they were too noisy. Probably due to this, the resulting misfit was greater (\(L_{1N}=7.34\%\)). However, the result was similar, even if it was more approximate and with greater heterogeneity, especially in the superficial part of the section. Figure 14: Line C\(-\)C’ comparison between simulated inversion of 3D and 2D resistivity surveys and  
2D inversion of field data. (**a**) Vertical section of the 3D resistivity model. (**b**) Corresponding section of the inverse model obtained by simulating a 3D survey on the geophysical model. (**c**) Inverse model obtained by simulating a 2D survey on the geophysical model. (**d**) Inverse model obtained by 2D ERT field data. Finally, section D\(-\)D\({}^{\prime}\) intersected the main tunnels \(H\), \(J\), \(K\)_and_\(L\) and gallery \(S\) connecting \(H\) to \(J\). It was also close to gallery \(T\), connecting \(H\) with \(J\), and gallery \(U\), connecting \(J\) with \(K\) (Figure 15a). In addition, in this case, although the data misfit values were comparable to each other, the section obtained from the simulated 3D inverse model (Figure 15b) was much closer to the geophysical model than the 2D inverse model (Figure 15c). Indeed, the latter clearly showed the effect of the neighboring gallery U. Furthermore, the base of the tunnels was not as sharply delimited. Figure 15: Line D\(-\)D\({}^{\prime}\) comparison between simulated inversion of 3D and 2D resistivity surveys. (**a**) Vertical section of the 3D resistivity model. (**b**) Corresponding section of the inverse model obtained by simulating a 3D survey on the geophysical model. (**c**) Inverse model obtained by simulating a 2D survey on the geophysical model. In summary, considering the simulated models, although the data misfit values of the 2D inverse models were comparable to those of the 3D inverse model, the latter had a greater likelihood. The sections of the 3D inverse model accurately represented the tunnels and rooms in the corresponding section, and the complexity of the shapes of the tunnels and chambers of the Fodera Quarry strongly affected the 2D inversions. These were influenced by phantom anomalies generated by the three-dimensionality of the intercepted structures and by the presence of other structures close to the investigation alignment. Moreover, the results from the 2D ERT carried out in situ were also disturbed by variations in the resistivity of the rock and by the detritus, which were not foreseen by the theoretical model. Unfortunately, not having been able to acquire all four sections in the field prevented us from performing a 3D inversion, or, more simply, from interpolating the 2D ERTs to obtain a 3D model [54; 55]. Therefore, we were limited to comparing the results relating to sections A\(-\)A' and C\(-\)C' with the 2D and 3D simulations.