## 4 Results and Discussion
### Electrical Resistivity Tomography 3D Modelling
#### 4.3.2 3D Model  
As direct measures for supporting the electrical resistivity tomography study, four boreholes were drilled on the ERT3 profile. These were distributed uniformly along the profile. Table 5 contains the summary of the study results. Samples at different depths of each borehole were employed to perform the analysis. A 3D model is a proven tool that contributes significantly to identifying different subsurface structures used in several studies [45, 46]. Figure 5: Regression models showing the relationships between resistivity and particle-shape variables. The 3D model computed with the electrical resistivity tomography data identified the same changes in sediment texture (patterns) as those found in the grain size study [47]. The colour bar of the model was calibrated with 1000 \(\Omega\).m as the step value. This value allowed for the distinguishing of the underground formations previously found in the 2D study, and the visualising of the extent of them simultaneously. Figure 6 shows the different patterns, which varied in accordance with the increase/decrease in the resistivity. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & \multicolumn{6}{c}{**Resistivity Values in \(\Omega\).m**} \\ \hline  & **Depth** & **Max** & **Min** & **Mean** & **SD** & **COV** \\ \hline  & **m** & & & & & **\%** \\ \hline BH1 & 0.0–1.1 & 2543.4 & 1590.3 & 2187.2 & 252.9 & 11.6 \\  & 1.1–3.0 & 1427.3 & 318.7 & 595.9 & 253.6 & 42.6 \\ \hline BH2 & 0.0–1.4 & 6923.6 & 3370.8 & 4695.5 & 1303.5 & 27.8 \\  & 1.4–2.3 & 4304.8 & 3558.7 & 4023.4 & 211.6 & 5.3 \\  & 2.3–3.0 & 3558.7 & 2919.3 & 3160.4 & 228.4 & 7.2 \\ \hline BH3 & 0.0–1.2 & 8599.2 & 3744.0 & 6623.5 & 1447.4 & 21.9 \\  & 1.2–1.6 & 3744.0 & 2694.3 & 3180.3 & 309.8 & 9.7 \\  & 1.6–2.5 & 2694.3 & 1716.1 & 1984.3 & 276.4 & 13.9 \\  & 2.5–3.0 & 1716.1 & 1706.3 & 1710.0 & 4.3 & 0.3 \\ \hline BH4 & 0.0–1.5 & 7183.5 & 1832.7 & 3843.7 & 1665.2 & 43.3 \\ \hline \hline \end{tabular} BH: Boreholes, SD: Standard deviation, COV: Coefficient of variation.  
\end{table} 
Table 5: Resistivity results from the ERT study. Figure 6: 3D model. (**a–f**) illustrates the different layers detected by the electrical resistivity tomography survey. Figure 6 shows the 3D model; this model has four vertical sections in the same place where the boreholes were drilled, aiming to facilitate the data interpreting process and visualising how the material evolves in that space. Figure 6a-f presents the different layers and body formations continuously. Notice that while the resistivity values rise, materials tend to group in the central site of the model, which corresponds to bar place. Figure 6a represents to the lowest resistivity values (0-1000 \(\Omega\).m) of the whole study. These values belong to the softer rocky substrate, which is in line with BH1 results, and show that the model is a realistic reproduction of the study area. Figure 6b presents a new layer featured with the green colour (1000-2000 \(\Omega\).m), which is spread in the whole model. This layer, representing medium gravel with little clay matrix, cut the borehole BH4 at 1.5 m deep. Since the samples collected from BH4 only reached to a depth of 1.5 m, this pattern was not contemplated in the granulometric study; however, the ERT detected its presence. Figure 6c-f, with values of resistivity >2000 \(\Omega\).m, represent the layer 1 to 1.5 m thick, which is common in all boreholes, except in BH2, composed mainly of medium and coarse gravels. The layer disposition in the 3D model adopts an onion-like shape, finding the least resistivity values in the external part and the most resistant in the core. Values of resistivity ranging from 2000 to 3000 \(\Omega\).m (Figure 6c) are linked to the green-yellow colour, forming a layer in the model that constrains in the lateral direction of the bar by 6-8 m, acquiring a shell form at the bottom of the model. The inner part of the onion-like structure is represented in Figure 6d-f, which is focused on BH2 (7.5 m).