## 2 Materials and Methods
### The Test Site  
A field test was carried out to validate the choice of the FRG array in combination with electrode arrangements on concentric quadrilaterals. The test site was a landfill of the Bellolampo dump site in the municipal area of Palermo, Sicily (Italy). For the purposes of regular monitoring of the environmental impact of a landfill, minimizing the risk of contamination, it was important to precisely locate and characterize the sources of contamination, in terms of lateral extension, depth, leakage of contaminants, and type of waste (metal, hypersaline fluids, leachate, debris, etc.). Unfortunately, the landfill area of Bellolampo (Figure 4) is at high environmental risk since it is quite close to urban areas and was built on a substatum of fractured limestones. The site is located northwest of the city of Palermo and is characterized by the slopes of a mountain relief in which a natural watershed is found, the Celona Valley, which has extremely steep slopes, relict river karst valleys and canyons, sub-horizontal surfaces, and karst depression [30]. In the area, calcareous and dolomitic limestones outcrops are characterized mainly by cracking-type permeability. The infiltration waters in the rock can reach the saturated zones on the slopes of the hills surrounding the Palermo plain [31]. Landfill no. 6, which is currently in operation, is divided into four sectors. The sealing of the bottom of the landfill was performed with two HDPE sheets with the intercalation of a clayey layer and another layer of drainage material with pipes for the leachate collection network. In waste landfills, the resistivity and chargeability variations are very abrupt and can be correlated with the characteristics of the waste. These physical properties depend on the presence of the leachate and its mobility and saturation [32; 33]. Furthermore, the gas content, the compaction density of the waste, and its variability affect the geoelectrical parameters. Consequently, the 2D ERT and IPT are frequently used tools for landfill  
Figure 3: For each of the nine combinations of electrode areal arrangement and array configuration of Table 1, the pseudospatial distribution (\(x,y,z\)) of apparent resistivity data is shown, where \(x\) and \(y\) are the coordinates of the surface center of each electrode array and \(z\) is the pseudo-depth calculated according to [PERSON] [29]. The black points show the electrode positions. surveys [34; 35; 36; 37; 38; 39; 40]. The high heterogeneity of waste repositories leads to a preference for 3D acquisitions [41; 42; 43; 44] and the combined use of ERT and IPT [45]. The 3D electrical tomography was acquired using the MAE X612-EM resistivity meter. The instrumentation can use up to 96 electrodes simultaneously, allowing a high measurement speed when optimized sequences are used, such as the multiple gradient array [25] or, even better, the FRG array [24]. The features of the landfill and the steep slopes did not allow the electrodes to be arranged according to a regular grid. Due to the considerations made in Section 2.2, an electrode areal arrangement similar to B was therefore opted for, arranging the electrodes on a series of approximately concentric perimeters along the access routes for waste disposal in the various sectors of the landfill with an electrode spacing equal to 5 m. In this way, an irregular layout of 180 electrodes was obtained (Figure 5). The electrode locations were geo-referenced, using as a topographical basis a Digital Elevation Model with a resolution of 0.5 m provided by the managers of the wasteland and obtained thanks to a detailed topographical survey. The \(\gamma\) electrode array configuration was used, involving the use of the FRG array [24] which was optimized for multi-channel instrumentation, to allow a high number of potential dipoles for each current dipole. In this way, enough measurements to guarantee a good resolution of the inverse model can be acquired in a short time.