## 6 Results and Discussions
### Shear-Wave 2D Velocity Profiles  
The inversion of the obtained HVSR curves allowed for the retrieval of the S-wave velocity models at each site. For sites where array measurements were also taken, the joint inversion of the obtained dispersion and HVSR curves was carried out. The number of analyzed layers, as well as the range of the respective Vs and thickness parameters, have been constrained according to previous information obtained from geological cross-sections, boreholes, and geotechnical studies (see Table 1). The aim was to obtain a result closer to the true model by minimizing the misfit value. Some examples are shown in Figure 12.  
For the first zone, the eight obtained 2D velocity profiles are mapped in Figure 13. The two uppermost layers in the plain correspond to the two Quaternary stages, the Holocene and Pleistocene, mainly composed of alluviums. The obtained Vs values for the Holocene alluviums vary between 210 and 350 m/s. This range is explained by the lateral variation between clays and silts. For the Pleistocene layer, the Vs value ranges between 405 and 630 m/s. This variation is due to the alternance between grave and clay facies. The high contrast in velocity between the two Quaternary stages is responsible for the second frequency peak in the area. The thickness of the Holocene formation does not exceed 10 m, while the Pleistocene deposits reach a maximum thickness of 65 m north of Oued-Fodda city. On the Sara-El-Maarouf anticline, in the profiles Pr6, Pr7, and Pr8 (Figure 13), the two topmost layers correspond to the Upper and Lower Pliocene. The Vs values obtained for the Upper Pliocene sands vary between 300 and 400 m/s, and between 610 and 780 m/s for the Lower Pliocene sandstones. As for the Quaternary stages down in the plain, the contrast in velocity between the two Pliocene layers is responsible for the second frequency peak in the HVSR curves obtained from recordings on the Sara-El-Maarouf anticline. These formations reach a maximum thickness of 100 m in the oriental part of the anticline. A thin  
Figure 11: Resistivity profiles. The shear-wave velocity models correspond to the HVSR points, P77 and P78. The top left panel represents the resistivity scale for the Middle-Chelif Basin [7]. The bottom left panel represents the lithological units identified in the resistivity profiles. Tortonian limestone layer lies under the Pliocene layers, for which the Vs value ranges between 650 and 900 m/s. This layer dips into the northwest and is not present in the Oued-Fodda Plain. The Quaternary layers in the plain are, therefore, lying directly over a thick layer composed of Serravallo-Tortonian marks and clays. This is the thickest sedimentary layer in the Middle-Chelif Basin. It reaches a maximum thickness of 250 m in this zone. The Vs value for this layer varies between 850 and 1290 m/s. This formation is in direct contact with the Cretaceous bedrock, which is mainly composed of hard marks. The obtained shear-wave velocity of these marks varies between 1700 and 2300 m/s in this zone. Figure 12: Examples of the inversion results. For each site, the left panels for each site represent the Vs models. The black line corresponds to the best fit model, the dark grey represents models with minimum misfit + 10%. All the tested models are in light grey. In the right panels, we show the computed fundamental mode of the Rayleigh wave ellipticity curve (dark grey curve), and the inverted part of the HVSR curve (black dotted curve). For sites AR1 and AR3, the bottom of the right panel represents the surface wave dispersion curve. Figure 13: 2D Shear-wave velocity profiles for Zone 1. Qt: Quaternary. Pl: Pliocene. Mi: Miocene. Cr: Cretaceous. In the profiles, Pr 6, Pr 7, and Pr 8 (Figure 13), we can see that the sediments of the plain are separated from the ones of the Sara-El-Maarouf anticline by a deep reverse fault that outcrops at the surface, the so-called \"El-Asnam fault\".