## 4 Field Test  
A field test was performed on a calcarenite quarry located inside the \"Parco delle Cave\" (Park of the Quarries) near the city of Marsala in Sicily (Figure 9). From the time of the Phoenicians until today, this area has been exploited for the extraction of Marsala Calcarenite (Middle-Inf. Pleistocene). This calcarenite is light yellow, homogeneous, well cemented, scarcely fossilierous and present in very thick banks with calcareous granules and carbonate cement. The rock is vacuolar, highly porous and poorly fractured. The Fodera quarry (Figure 9) is currently used for tourist and cultural activities. The quarries in this area are mainly open-cast quarries, with almost regular shapes and sub-vertical excavation fronts. Most of these quarries were filled, albeit partially, with waste materials from excavation fronts of the same quarry or nearby. In the park, there are also several underground cavities, located at different depths concerning the ground level, made by excavating tunnels supported by pillars [53]. The quarry is developed in a series of tunnels and connected rooms, most of which have been subjected to underground topographic surveys with tools and software used for speleological activities. In particular, the Leica X310 laser distance measurer was used. The cavities of this quarry are almost all accessible (there are at least three entrances), and the hypogeal survey carried out allowed determining the shape and dimensions of the voids, allowing one to draw the planimetric map (Figure 10, left) and four reference sections (Figure 10, right) of the quarry. Figure 8: Trend of the inversion effectiveness parameters \(\mathbf{P}_{1}\) and \(\mathbf{P}_{3}\) referring to the 2D ERT (white squares and circles) and central sections of the 3D ERT (black squares and circles) crossing over prism-shaped models shown in Figures 2 and 3 as the \(s/l\) ratio between the orthogonal width \(s\) of the prism and the side of its square section \(l\) increased. Based on the speleological survey, a simplified 3D model of subsoil resistivity was realized (Figure 11). Using RES3 DMOD, we generated a discrete model of 120 m \(\times\) 40 m \(\times\) 50 m with cells having horizontal dimensions equal to one meter (i.e., half the electrode spacing) and a vertical dimension increasing with the depth. The predicted data were calculated considering 60 electrodes along the \(x\) direction and 21 electrodes along the \(y\) direction. We used the same array set as for the previously discussed models (electrode spacing equal to 2 m, dipole length \(a\) ranging from 1 to 12 times the electrode spacing and dipole-dipole distance \(n\) ranging from 1 to 12 times the electrode spacing). Figure 10: 3D speleological model of the Fodera Quarry: (**left**) planimetric map, where the thick black lines show the traces of the speleological sections (**right**) and the red dashed lines are the traces of the carried out electrical resistivity tomographies. Figure 9: The Fodera Quarry in Marsala, Sicily. View of the entrances of the main tunnels. The modeled cave presents four main tunnels (indicated by the yellow letters \(H\), \(J\), \(K\) and \(L\) in Figure 11) and several short galleries and rooms (letters from \(M\) to \(U\)) that connect the main tunnels. For this model, we calculated the predicted apparent resistivity measurements relating to the same previously discussed array data set. The predicted data were inverted by RES3 DINV, using the same inversion parameters as for the previous models and obtaining a data misfit equal to 3.95%. In particular, we considered four parallel sections A\(-\)A\({}^{\prime}\), B\(-\)B\({}^{\prime}\), C\(-\)C\({}^{\prime}\) and D\(-\)D\({}^{\prime}\) in correspondence with the speleological sections (Figure 11) and extracted from the aforementioned sections of the 3D inverse model.