## 4 Methods
### Geophysical Surveys  
We used electrical resistivity tomography and seismic refraction techniques to evaluate the shallow subsurface formations, whereas the CSAMT and gravity techniques were performed to assess geologic formations at large depths. The joint geophysical approach of gravity and seismic reflection methods for the fast, large-scale characterization of hydrogeological potential was used in the Ain El Beidha plain (central Tunisia) [32]. The geophysical methods such as gravity and magnetic are usually implemented by the 2D grid because they use simple equipment and are the cheapest. The inversions of these methods were developed in 2D and 3D framework architecture [33]. ERT and CSAMT measure electrical resistivity with a unit of \(\Omega\)m. Electrical resistivity is often employed to differentiate subsurface rocks from one another because it is sensitive to changes in the composition, clay content, water content, porosity, and degree of fracturing in different rock samples. Furthermore, low resistivity values are closely associated with weathering troughs and fractured rocks, which may indicate the presence of significant groundwater resources, whereas high resistivity suggests fresh rock [34; 35; 36; 37; 38]. Compared to the traditional electrical methods, the ERT provides higher resolution results at shallower depths. The ERT was conducted using a McOHM 21 (produced by the OYO Company of Japan), and the data were processed with the Gauss-Newton least squares method. ERT was performed along a profile from south to northwest with a total of 381 electrodes, spread length of 1900 m, and electrode interval of 5 m (Figure 3a). ERT evaluated the subsurface layers with a maximum depth of 300 m. The seismic refraction provides insight into the seismic velocities (in m/s) of the rocks sampled by the seismic waves. The seismic velocity is inversely proportional to the degree of rock fracturing or weathering. Thus, high seismic velocity suggests the unweathered rock, whereas low seismic velocity reveals the rock mass of poor quality. The seismic refraction data were acquired using a 24-channel McSeis-F170 (OYO Company of Japan) with 1.024 sampling points, a sampling interval of 0.05 ms, a geophone interval of 5 m, and a shot point interval of 30 m. The seismic refraction survey was carried out along two profiles with a total length of 690 m and an investigation depth of 100-150 m (Figure 3a). Gravimetric data provide information about the rock densities (in g/cm\({}^{3}\)) in the gravity survey. The high precision gravity survey was performed along the ERT profile using the CG-3 gravity meter manufactured by Scintrex (Canada) to acquire gravitational data at seven repeated observation checkpoints with a data point distance of 10 m. The 2D gravity model of 1820 m exploration length provides investigation depth over 400 m (Figure 3a). The weathered/fractured rock is delineated by a low value of density, while the fresh rock is evaluated by high density. The electrical resistivity tomography data and the gravimetric data were collected along the center line of all tunnels with a horizontal position error of less than 3 m (mostly within 50 cm) and an elevation error of less than 5 cm. CSAMT is a frequency domain electromagnetic (EM) method that uses a grounded dipole or loops to generate repetitive signals. CSAMT data were obtained from electric and magnetic fields oriented along and perpendicular to the traverse line, respectively. Originally, we planned to use CSAMT to acquire data along the ERT profile; however, because of the sensitivity of the instrument to the high voltage power lines (present in the southern region of our study area), we only conducted the gravity survey along all of the tunnel lines. Therefore, the CSAMT profile was performed with a short length of 720 m on the north side of the ERT profile (Figure 3a). We assessed the surface and subsurface conditions using geophysical exploration techniques before borehole drilling.