## 3 Analysis of Geophysical Data
### Seismic and Gravimetric Basement Depth Inversion
\[\Delta g\ (\mathrm{mGal})=0.0419\times\Delta\rho\ \left(\mathrm{ton/m^{3}} \right)\times H\ (m) \tag{2}\]  
With \(\Delta g\) the gravity anomaly, \(\Delta\rho\) the density contrast between the soil and the rock and H the sedimentary layer thickness. The comparison between both basements is made using four different sites. Three sites (Figures 11-13) are located in soft sites with seismic classification of D, according to Table 1, while the fourth site is located in a site with dense soils (Figure 14). In dense soils most of the sites show an H/V curve with no peak or with a peak of low amplitude, where it is difficult to define a predominant period (some examples are in Figure 15). For that reason, in many of them it is not possible to perform an inversion using ellipticity or using the Equation (1). Figure 11 shows the results of the seismic inversion in one site located in the fluvial deposits of Vina del Mar (Figure 3). This site has a seismic classification of D, with an H/V predominant period of 0.72 s and a \(V_{s,30}\) of 242 m/s. The soils in this site are medium density sands and silts [46]. The panel (a) of Figure 11 shows the adjustment of the ellipticity between the observed curve (black line) and the best inversion models. Here, both flanks of the peak were used in the inversion. Panel (b) shows the same results for the dispersion curve, where active and passive curves were used as input to constrain the ellipticity curve inversion. Finally, panel (c) displays the shear-wave velocity models, where the colors are related with the misfit bar below. Here the black line corresponds to the best model with a misfit value slightly lower than 0.11. From this figure, it is possible to see a very good agreement in the dispersion and ellipticity curves between the observed data and the resulting models. The resulting inversion profile shows a deposit of soil of about 250 m/s of shear wave velocity and then, at 48 m depth, a stiff material appears with a shear wave velocity over 2000 m/s, associated with the seismic bedrock. This value of seismic bedrock velocity is only tentative, since no information is available on the dispersion curve at low frequencies, nor is the peak of the H/V curve being used. However, regardless of the exact value of the impedance contrast, the thickness of the soil layer is well defined. The Equation (1), using the values previously mentioned in this site, gives a depth to the basement of 44 m, which is very close to the inversion. In terms of gravity, the residual anomaly in this site is \(-\)1.72 mGal. [PERSON] et al. [24] estimates a density contrast between the soils and bedrock in the zone of 0.7-0.8 ton/m\({}^{3}\). Using a mean value of 0.75 and the Equation (2), the gravimetric basement results in 55 m.  
Figure 12 shows the same results of the seismic inversion in Figure 11 but in one site located in the Quaternary marine sediments in Mejillones (Figure 2). This site has a seismic classification of D, with an H/V predominant period of 5.9 s and a \(V_{s,30}\) of 390 m/s. The minimum misfit value in this inversion is around 0.12. From this figure, it is possible to see good agreement in the dispersion and ellipticity curves between the observed data and the resulting models. The resulting inversion profile shows a thick deposit with velocities increasing from 200 to 550 m/s and then, from 570 m appears a layer with a velocity around 1200 m/s, associated with the seismic bedrock. The Equation (1), using the values previously mentioned in this site, gives a depth to basement of 575 m, which is very similar to the inversion. In terms of gravity, the residual anomaly in this site is \(-\)29.2 mGal. [PERSON] et al. [8] estimates a density contrast between the soils and bedrock in the zone of 1.2 ton/m\({}^{3}\). Using this value and Equation (2), the gravimetric basement in this site results in 580 m.  
Figure 11: Inversion results in a point in Vina using ellipticity of the Rayleigh waves from the H/V curve.