## 4 Results
### Hydrogeophysical Basis for VS and ER Models Interpretation  
When MASW and ERT are aimed at defining geological structures in porous saturated media, a preliminary conceptualization of properties governing the magnitude of variables VS (MASW) and ER (ERT) is needed. Some basic interpretative criteria are described below. Figure 3: For natural years 1997–2019 in the SAL area, (**a**) normalized North Atlantic Oscillation (NAO) index (purple bars) [NAO website: [[http://www.cpc.ncep.noaa.gov/](http://www.cpc.ncep.noaa.gov/)]([http://www.cpc.ncep.noaa.gov/](http://www.cpc.ncep.noaa.gov/))]; (**b**) annual precipitation (P) time series from Sines meteorological station and cumulative deviation (CD) from average annual P, mm year\({}^{-1}\); and (**c**) annual actual evapotranspiration (E) time series from Sines meteorological station and CD from average annual E, mm year\({}^{-1}\). For (**b**,**c**), the average (0.5 percentile) of yearly data series is indicated. Vertical dotted line indicates the study year 2014. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Site** & **ID 1** & **Elevation, m a.s.l.** & **Aquifer and Flow Zone** & **Variable 2** & **GEC 3** & **GER 4** \\ \hline Upstream & W6 & 35.18 & Pliocene, recharge & PL, GEC & 200 & 50 \\ \hline
1 & W5 & 14.06 & Pleistocene, transit & PL, GEC & 500 & 20 \\  & W3 & 9.00 & Pleistocene, discharge & PL & & \\  & W4 & 10.07 & Holocene, recharge & PL & & \\ \hline
2 & W2 & 8.82 & Holocene, transit & PL & \\ \hline
3 & W1 & 4.57 & Pliocene, discharge & PL, GEC & 393 & 25 \\ \hline \hline \end{tabular}  
* ID and location as in Figure 1d. 2 PL is piezometric level as in Figure 4. 3 GEC is groundwater electrical conductivity in \(\mu\)S cm\({}^{-1}\) measured on 20 August 2014 when the multi-parameter probe was available; wells W3 and W4 were not accessible. 4 GER is groundwater electrical resistivity in \(\Omega\) m after GEC reversion using Equation (2).  
\end{table} 
Table 3: Description of monitored variables in open wells. Figure 4: For year 2014 in the SAL area, (**a**) 24-h P and T distribution, after P (mm) and T (\({}^{\circ}\)C) records from Sines meteorological station (Figure 1a); (**b**) monthly distribution of average precipitation-weighed EC after EC (\(\mu\)S cm\({}^{-1}\)) and P (mm) records from Monte Velho meteorological station (Figure 1a), and some Cascalheira stream water EC (\(\mu\)S cm\({}^{-1}\)) measurements at site 2 after the SWQM Programme (Figure 1d); and (**c**) piezometry (m a.s.l.) in handmade open wells W1 to W6 (Table 3) ordered according to the monitored geological formation (Pliocene, Pleistocene, Holocene) and groundwater flow zone (recharge, transit, discharge). Vertical black dotted lines indicate geophysical surveying (Table 2) and groundwater monitoring (Table 3) as ERT (ER) and MASW (MA) surveys, and piezometry (PI). Vertical green dotted lines indicate human actions modifying water dynamics as SAL aperture (SA) and groundwater pumping for irrigation (IR). In soft coarse-grained sediments, VS propagation is considered a site-specific steady property determined by effective compaction, and as such, it is dependent on the age and depth of each geological material piled on vertical [57; 58]. Table 4 summarizes some reference VS values for soft and stiff geological materials equivalent to those described in the SAL area [34; 59]. Different relationships between VS and age and depth for other similar lithologies can be found in the scientific literature [60; 61; 62; 63; 64], which can be used to enlarge the ranges reported in Table 4. For the sedimentary formations in the study area, VS values in the 50-900 m s\({}^{-1}\) range are expected. In porous saturated media, ER corresponds to EC inferred from certain intrinsic geological features and groundwater salinity; the former remains unvaried, whereas the latter varies over time. In pristine coastal areas, groundwater electrical resistivity (GE) determines ER when salinity contribution from subsurface geology is negligible.