## 5 Discussion  
### The MASW Technique for Geological Definition  
The MASW technique has widely been used in seismic hazard research, especially in urban areas [29; 30; 31; 32; 53; 69]. The experience in shallow groundwater research is incipient [26; 27; 28] and has mostly focused to disambiguate geological structures when other \begin{table}
\begin{tabular}{l l l l l} \hline \hline
**Recharge Component** & **Area Covered 1** & **Surface, km2** & **Recharge, Mm3 year\({}^{-1}\)** & **Reference** \\ \hline Precipitation & PUA, NSUS & 3.41 & 0.03 & [15] \\ Urban return & NSUS & 0.77 & 0.10 & [46] \\ Lrigation return & SPUS & 1.41 & 0.17 & [47] \\ \hline \hline \end{tabular}  
* PUA = peri-urban area; NSUS = northern steep urban sector; SPUS = southern peri-urban area devoted to irrigation agriculture in greenhouses.  
\end{table} 
Table 4: Groundwater recharge in the NSUS. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
\begin{tabular}{c} \(<\)0.01–0.14 \\ (0.01) \\ \end{tabular} \\ \hline \hline \end{tabular}  
* In Table 2, age, lithological description, and PGS of each hydrogeological formation. 2\(i\) = average dimensionless hydraulic gradient; \(K\) = permeability in m day\({}^{-1}\) (Table 2); \(b\) = aquifer saturated thickness in m; \(l\) = aquifer discharge section in m; \(D\) = NSUS groundwater discharge in Mm\({}^{3}\) year\({}^{-1}\); and \(AGS\) = actual groundwater storage in Mm\({}^{3}\) as the product of \(b\), surface (Table 2), and effective porosity (Table 2). \(K\), \(b\), \(D\), and \(AGS\) include range and average value into parenthesis.  
\end{table} 
Table 3: NSUS groundwater discharge after the Darcy’s Law formulation. near-surface geophysical techniques fail to obtain a reliable interpretation [26; 27; 62]. This paper widens the experience in shallow groundwater research to define the geometry of geological structures once reference VS values are available. In Adra town, the confident use of the MASW technique had the VS values assigned to each geological formation in previous research [19; 53; 67], e.g., VS > 1200 m s\({}^{-1}\) for Paleozoic bedrock, 800-1200 m s\({}^{-1}\) for WB, 500-900 m s\({}^{-1}\) for Pliocene deltaic facies, 350-600 m s\({}^{-1}\) for Pleistocene formations, and <350 m s\({}^{-1}\) for Holocene formations. However, the MASW technique must be used together with other near-surface geophysical techniques relying on other subsurface properties other than VS in order to solve potential constraints imposed by the possible similar VS response of different geological structures. For instance, g1 and g2 show similar VS values than t1 and t2 (Figure 3), meaning a handicap at defining the geometry of the confined NSUS aquifer formed by WB, t1, and t2 (Figure 7). Adra town is a low-resistivity coastal area having high environmental salinity and clay-rich formations such as g1 and g2, so the geophysical electrical techniques do not seem to be suitable for disambiguating the pairs g1-t1 and g2-t2. The GPR technique was chosen to disambiguate these structures having similar VS values, as well as to identify key hydrogeological features, such as piezometric level, capillary fringe, and seawater-freshwater interface (Figure 5). MASW provides higher exploration depths than GPR, but less detailed resolution, whereas GPR is highly responsive to detailed subsurface electrical and magnetic changes related to natural and human-induced geological and hydrological heterogeneities [33; 55; 58; 59; 60]. This paper introduces a novelty in VS data post-treatment. Several vertical-equispaced MASW maps from -35-m to 30-m elevation regarding the sea level were created to identify continuity of the geological structures. This VS data post-treatment has enabled us to display where and why the boundary between the bedrock and the Pliocene to Quaternary sedimentary formations changes at different depths, for instance, abruptly due to normal and strike-slip fault systems or smoothly due to sedimentary processes (Figure 4).