## 4 Discussion
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Site (Fig.)**} & \multirow{2}{*}{**H/V T\({}_{0}\) (s)**} & \multicolumn{2}{c}{**Seismic Classification (Table 1)**} & \multirow{2}{*}{\begin{tabular}{c} **V\({}_{\text{s}}\)** \\ **(m/s)** \\ \end{tabular} } & \multirow{2}{*}{\begin{tabular}{c} **H/V** \\ **Amplitude (A\({}_{\text{q}}\))** \\ \end{tabular} } & \multirow{2}{*}{\begin{tabular}{c} **Gravity** \\ **Residual (mGal)** \\ \end{tabular} } & \multirow{2}{*}{\begin{tabular}{c} **Seismic Basement** \\ **Depth (m)** \\ \end{tabular} } & \multirow{2}{*}{\begin{tabular}{c} **Gravity Basement** \\ **Depth (m)** \\ \end{tabular} } \\ \hline \begin{tabular}{c} Vina \\ (Figure 11) \\ \end{tabular} & 0.72 & D & 242 & 7.2 & \(-1.72\) & 48 & 55 \\ \hline \begin{tabular}{c} Mejillones \\ (Figure 12) \\ \end{tabular} & 5.9 & D & 390 & 1.2 & \(-29.2\) & 570 & 580 \\ \hline \
\begin{tabular}{c} Santiago \\ (Figure 13) \\ \end{tabular} & 0.68 & D & 288 & 5.5 & \(-1.15\) & 50 & 40 \\ \hline
\begin{tabular}{c} Iquique \\ (Figure 14) \\ \end{tabular} & 0.25 & C & 380 & 2.9 & \(-1.2\) & 29 & 40 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Summary of the comparison between seismic and gravity basement for the four sites studied. lithologies and from different ages and sources. These rocks can have densities from 2.3 to 2.9 ton/m\({}^{3}\)[7], implying the heterogeneous nature of the basement. Secondly, the deeper basement (300-400 m) can increase the dispersion in the correlation. The deeper the source is, the less contribution to the signal as a result of the 1/r\({}^{2}\) or 1/r amplitude dependence with the source-sensor distance (r) [1]. Thus, lower signals imply larger influence of noise over the observations. This can be seen in Santiago and Mejillones (see Figure 10) where the depth to the basement is much higher than in the other cities, and the dispersion is also higher. Finally, the infilling sediments present different facies over the whole basin, and thus most likely, affecting in a heterogeneous velocity and density sedimentary distribution. This can lead to local effects in gravity that can be misunderstood or to more than one peak in the H/V curve, making it difficult to select a predominant period. Even though the previous discussion on the gravity-H/V correlation shows a predominant trend, some differences are evident, and two first order aspects must be considered: (1) variability in the proportionality constant (or slope), and (2) the variance of the correlation. Both aspects are related to the degree of heterogeneity in the geological process that conform the basement and the infilling coverage. In small basins such as Vina del Mar and Mejillones, where a single basement and a unique sedimentary process took place, the variance in the correlation is minimal. However, the proportionality constant differs in a factor of two between them. This is most likely attributed to the very large density contrast between the basement and the sedimentary cover in the case of Mejillones as discussed in the previous paragraph. On the other hand, the case of Santiago is the opposite case, with a large basin evolving during the buildup of the Andes in the last 20 Ma [49,58], implying a heterogeneous basement and cover [7]. This geological heterogeneity mostly affects the gravimetric signal, amplifying the dispersion or variance in the correlation with the H/V data. The lower magnitude of the correlation factor in this case is probably the result of a mixture of fine and coarse sediments in the sedimentary column (superimpose sedimentary facies) that reduces the density contrast and consequently the amplitude of the gravity signal. In terms of the slope of the linear regressions obtained, it is possible to see that there is not a common value among the cities. The slope of the best linear regression in Vina, considering only soft soils, is around \(-2.5\) (Figure 7), while in Santiago it is around \(-1.4\) (Figure 9).