## 3 Materials and Methods  
### Electrical Resistivity Tomography (ERT)  
Electrical resistivity tomography method is a non-invasive technique. This method is based on a combination of electrical vertical sounding and profiling techniques for gathering subsurface information. ERT uses a basis of four electrodes for acquiring apparent resistivity values in the field. Electrical currents injected into the subsurface employ two electrodes, named current electrodes, which are commonly labelled as A and B. Simultaneously, potential measurements are addressed using another pair of electrodes, named potential electrodes, which are labelled as M and N [12; 13]. The four-electrode-based measurement unit principle applies all along the electrical profile, resulting in an over-lapping of layers, allowing for the generation of 2D electrical sections. For 3D models, parallel profiles were conducted, enabling the creation of 3D-format files, which were processed through a next stage by means of an inversion algorithm [14; 22]. By using the ERT method, the collection of information from the subsurface will depend on several parameters that are affected by the electrical contrast that defines the subsurface materials. One of these parameters is the investigation depth, where the length of the profile will have a considerable influence on the investigation depth since the more distance is covered by the profile, the deeper the obtained data is [15]. Another key parameter is the type of measurement array, which determines how the measurements will be conducted. Among the different available arrays, this study employed a dipole-dipole array, which offers higher resolution and more detailed imaging. A dipole-dipole  
Figure 1: Study area. ERT profiles distribution, red marker determines the beginning of the electrical profile. array allows for gathering a wider range of measuring points with the same quantity of electrodes [23]. The field data acquisition was conducted with a SuperSting resistivimeter from AGI (Austin, TX, USA) and a gasoline-based power generator from Honda (Valencia, Spain) as a continuous and suitable power supply. The SuperStingTM Wi-Fi R4/IP multielectrode resistivimeter from AGI (Austin, TX, USA) is equipped with 4 channels and 56 stainless steel electrodes, a 56 switch box and measurement resolution for max. 30 nV, which vary according to the voltage level [24]. Figure 1 shows the situation of the ERT profiles and their distributions and covered area. In this case study, a total of four electrical tomography profiles were conducted, where three of which were laid out parallel with a separation of 1.5 m. Those profiles are referred to as ERT1, ERT2, and ERT3, respectively. On the other hand, the ERT profile, named as ERT4, was laid out perpendicular to the others. ERT4 profile used 56 stainless steel electrodes with a spacing of 0.55 m and a length of 30.25 m. ERT1, ERT2, and ERT3 profiles used 28 stainless steel electrodes with a spacing of 0.55 m and a length of 14.85 m. The investigation depth was \(\approx\)6 and 3 m, respectively. Additionally, each electrode was suitably georeferenced using a Leica Zeno 20 GPS equipment (accuracy of \(\pm\)1 cm) from Leica (Barcelona, Spain). Before the inversion process, the whole dataset was submitted to pre-processing, consisting of a filtering stage for removing anomalous values and static correction, normalising the variations in resistivity due to the difference in height between electrodes. Then, the terrain topography was incorporated, since any change in the morphology of the terrain affects ERT measurements considerably; and here, the subsequent data inversion was processed [15]. Finally, the dataset was inverted with AGIUSA's EarthImager2D y 3D software (ver. 2.4.4, 1.5.5) which uses the least squares approach [25]. Not only does the software use the finite-element method to solve the inversion, but it also considers the terrain topography. The inversion process computes a model that fits with the observed data.