107 5. Geophysical methods 5.5 Electrical methods Electrical methods (Chapellier, 2001a) are extensively employed in near-surface geophysical investigations due to their sensitivity to subsurface resistivity contrasts. Surface-based techniques primarily include vertical electrical soundings (VES) and electrical profiling, implemented with various electrode configurations such as Wenner, Schlumberger, dipole-dipole, and pole-pole arrays. These configurations enable the investigation of both lateral and vertical resistivity variations (Loke & Barker, 1996a; Chapellier, 2001a). Advanced 2D and 3D electrical resistivity imaging techniques have been developed to obtain spatially distributed resistivity models of the subsurface. These methods rely on the deployment of dense arrays of electrodes (often 48 to 256), connected via multicore cables, and positioned along linear or areal profiles. A computercontrolled multichannel resistivity meter sequentially selects electrode pairs for current injection and for measuring the resulting potential differences (ΔV), allowing for a large number of independent measurements in a single acquisition sequence. Apparent resistivity values are computed for each measurement based on the known electrode geometry and are spatially referenced in terms of pseudo-depth (Z) and lateral position (X, Y). The resulting dataset is then processed using non-linear, iterative inversion algorithms that solve the governing equations of electrical conduction, typically using finite-element or finite-difference approaches (Loke & Barker, 1996b). The inversion reconstructs the true resistivity distribution within a 2D or 3D volume, subject to regularization constraints and starting from an initial a priori model. This methodology, commonly referred to as Electrical Resistivity Tomography (ERT), is particularly effective for imaging complex geological structures, such as stratigraphic interfaces, fault zones, and karst features (Daily et al., 2004; Dahlin & Zhou, 2004; Torrese, 2020). In addition to the 3D seismic survey, the HES was also analyzed using pseudo three-dimensional (3-D) Electrical Resistivity Tomography (ERT, https://doi. org/10.1016/j.jhydrol.2019.124257), where a 3-D model was constructed through the joint inversion of resistivity data acquired along parallel profiles (Torrese, 2020). The inverse resistivity model successfully identified the primary hydrogeological units, albeit with lower resolution compared to the 3D seismic survey. Modeling of synthetic datasets demonstrated the detectability of karst features through the pseudo 3-D ERT method. Furthermore, synthetic dataset modeling enabled the estimation of the experimental and inversion setup responses to varying levels of aquifer heterogeneity, utilizing well-log data and the 3D seismic block as geological prior information (Torrese, 2020). In the context of borehole geophysics, conventional electric logging tools (Chapellier, 2001b) include laterolog and induction probes, which measure formation resistivity and conductivity over lateral investigation depths ranging from decimetric to
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