## 3 Analysis of Geophysical Data  
The objective of this research is to determine possible relations between gravimetric and H/V method, and to understand the geological conditions in which these connections are valid. To achieve this objective, a long dataset of both methods in many Chilean cities (references in Section 2) is considered. Both methodologies are briefly explained here; a detailed description is beyond the scope of this research and can be found in specific literature for each method, e.g., [9; 10] for H/V and [1] for gravity. The gravimetric method is a passive technique that measures the gravity acceleration in different points of the earth. The density contrast between basement and coverage produces measurable anomalies in the gravimetric field. The infilling sediments have lower densities compared to bedrock, producing a negative gravity anomaly. To isolate the effect of the shallow soils in the gravity anomaly, it is necessary to subtract the rest of instrumental and geological sources that contribute to the results. In general, the data reduction in all the cities followed the standard procedure described in [PERSON] et al. [1]. This includes the subtraction of tidal variations and instrumental drift, and topographic, latitude, altitude and Bouguer corrections. Additionally, a regional effect produced by deep geological structures was eliminated specifically for each site. More details can be found in the respective references mentioned in Section 2. Once isolated the effect of the infilling sediments (residual gravity anomaly) by performing all the previous corrections, the result can be inverted to obtain the depth to basement through estimating the density of the different units, where complex settings are most likely the rule (i.e., heterogeneous basement and sedimentary facies varying laterally and with depth). This complexityimplies a particular treatment of each locality; for the sake of simplicity, the use of residual gravity anomaly is preferred to find correlations with the H/V method instead of the thickness resulting from the inversion process. The residual gravity anomaly is still a first order predictor of the depth to the geological basement, where larger negative values indicate higher depths to basement. On the other hand, the H/V method consists in determining the spectral ratio between the horizontal and vertical components of the ambient vibrations [10]. The period of the peak of this spectrum can be associated with the predominant period of vibration of a soil deposit. However, this correlation depends strongly on the impedance contrast between the soils and the bedrock; it is possible to have curves with no clear peak or with more than one peak, e.g., [25]. We defined the predominant period as the one associated to the highest amplitude related to the high impedance contrast. Depending on the geology, this may or may not correspond to the peak associated with the longest period. Additionally, the resulting predominant period (\(T_{0}\)) can be used to estimate the thickness of the soil deposit (H) combined with the harmonic mean shear-wave velocity (\(V_{s}\)) of the site using the Equation (1) (e.g., [26]). This relation assumes 1-D wave propagation, and just as in the case of gravity inversion, it involves a particular treatment in each city. \[H=\frac{T_{0}\,\times V_{s}}{4} \tag{1}\]  
The detailed description of the geophysical campaigns for each city can be found in the references mentioned in Section 2. However, all these campaigns were made with the current acquisition standards. In the case of the gravimetric survey, a Scintrex CG5 gravimeter was used together with a differential GPS Trimble C5 to have a net error lower than 0.05 mGal and altitude errors lower than 30 cm. Furthermore, all regional effects were removed by applying regional and low-pass filter techniques.