## 4 Results
### 2D ER Models  
Sequential 2D ER models are used to refine geometry of aquifer H. As described in Section 4.2, when porous geological media hardly contribute, ER changes can be attributed to transient GER changes produced by temporal water transferences and fluxes [29; 31]. The ER models showed in Figure 6 were interpreted recurring to (i) previous VS models (Figure 5) and geological data [40; 43; 47; 48] to infer geology, and (ii) natural processes and some human actions modifying GER to deduce transient water transferences and fluxes (Figure 4). Figure 5: MASW1 and MASW3 surveys at sites 1 (**a**) and 3 (**b**), respectively. A preliminary geological interpretation of VS values is included, showing projected (Pj) log of geotechnical sounding SI after [40], and water table recorded in September 2014 in wells W4 (site 1) and W1 (site 3) as in Figure 4. Profiles are topographically corrected and its vertical-to-horizontal scale ratio is 1:1. CS, Casalheira Stream; H, Holocene; Pe, Pleistocene; Pe-d, Pleistocene sand dunes; Pi, Pliocene; and F1 to F3, NW-SE strike-slip faults. The dotted-line rectangle is the area covered by time-lapse ERT surveys at sites 1 and 3. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Profile ID 1** & **AV VS 2** & **SD VS 3** & **CV VS** \\ \hline MASW1 & 273.1 & 161.4 & 0.59 \\ MASW3 & 215.1 & 126.8 & 0.59 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: VS models statistics. In sites 1 to 3 (Figure 1d), time-lapse ERT were in March, June, September, and December 2014 (Table 1) attending to the minimum frequency for optimal ERT surveying calculated in Section 4.1. For reliable comparisons in each site, the 2D ER models keep the same prospecting distance and depth (same grid). Prospecting depth was 13 m in ERT1 (Figure 6a), 15 m in ERT2 (Figure 6b), and 13 m in ERT3 (Figure 6c). In site 1, the December 2014 survey was cancelled because the SAL overflowed and inundated the ERT space after heavy rainfall events while the coastal sandbar was closed. Figure 6: At sites 1 (**a**), 2 (**b**), and 3 (**c**), time-lapse ERT1, ERT2, and ERT3 surveys in March, June, September, and December 2014 (rows 1 to 4), and relative difference, \(RD=(z-z^{*})/z^{*}\), of nodal ER data from March, June, and December lapses (\(z\)) regarding nodal ER data from the September lapse (\(z^{*}\)) (rows 5 to 7). The projected (Pj) logs of geotechnical soundings S2 and S3 after [40], and water table recorded in each lapse in sites 1 (well W4), 2 (well W2), and 3 (well W1) as in Figure 4 are shown; in the top left-hand corner of plots, green and red vertical arrows show, respectively, water-table raise and depletion relative to previous lapse. Preliminary geological and hydrogeological interpretations of ER models after integration of geological findings from VS models (Figure 5) and aquifer conceptualization (Figure 2) are included; red acronyms denotes groundwater types, and blue arrows and acronyms denote natural processes and human-induced actions determining transient water transferences and fluxes. Profiles are topographically corrected and its vertical-to-horizontal scale ratio is 1:1. CS, Cascalheira Stream; H, Holocene; Pe, Pleistocene; and F8, NE-SW strike-slip fault. For all time lapses in ERT1 to ERT3 surveys, the ER dataset is positively skewed in the 1.6-237.9 \(\Omega\) m range; the average value being 41.03 \(\Omega\) m (Figure 6). This experimental ER range agrees with the theoretical GER baseline deduced in Section 4.2. Thus, ER datasets in sites 1 to 3 are hardly influenced by subsurface geology. In general, ER is 32-49 \(\Omega\) m in ERT1, 21-44 \(\Omega\) m in ERT2, and 37-38 \(\Omega\) m in ERT3.