## 4 Geophysical Research and Applied Methodology
### Applied Methodology  
The methodology proposed in this investigation consists of the correlation of the geophysical data obtained between the HVSR surveys and the techniques of refraction seismic, MASW seismic, and VES. That will define an empirical relationship between fundamental frequency and sediment thickness. Equation (1) can provide an easy way to delineate the sedimentary ground (related to mobilized landslide material) over the fixed rocky substratum, i.e., the rupture surface. The first step is to analyze the shallow material and define the sedimentary layer (even the altered material) overlaying the rock basement. Once the values of the thicknesses \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Geophysical** & **Survey** & **Dimension** & **Number of** & **Parameter** \\
**Method** & **Technique** & **Surveys** & **Parameter** \\ \hline REFRACTION & 2D profile & 115 m & 3 & \(V_{p}\) \\ MASW & 1D profile & 115 m & 3 & \(V_{s}\) \\ VES & Application point & 400 m (A-B) & 5 & Resistivity \\ HVSR & Single station & Point & 22 & \(f_{o},A_{o}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Resume of applied geophysical techniques and surveys. of the most superficial materials are obtained, they could be related to altered ones and, consequently, potentially moving layers. Thus, the position of the rocky substrate will be identified through changes in resistivity (by electrical method) and seismic impedance (by seismic method). That simple geological model (defining the soft layer thickness) was related to the natural frequency of vibration (\(f_{o}\)) obtained from the HVSR survey from a two-layer model [15], and it could be established as an empirical relationship between both values [12; 13]. From this relationship, \(f_{o}\), as a tool to delineate the depth of the basement, can be used [18; 19; 20; 21]. Firstly, the apparent resistivity field curves obtained in the VES surveys have been inverted, and a layers' vertical distribution (resistivity and thickness) was performed for every test. These results provide the first definition between the rocky basement and sedimentary overburden from a high contrast in resistivity values. The second part consisted of processing the seismic data of refraction and MASW surveys with obtaining distribution profiles of the values of the velocities \(V_{p}\) (two-dimension profile with the geometry of geophysical layers) and vs. (one-dimension distribution of velocity and depth). Compared with the resistivity changes mentioned above, these values allow for obtaining the position of the rocky substrate in-depth (more accurate), that is, the thickness of the sediments and differentiation of internal levels in the overburden. At every point where VES tests have been carried out, an HVSR test has been applied. These combinations of HVSR and VES results were used to establish the empirical correlation between sedimentary thickness with the fundamental frequency of the ground, and they are used as control points [18; 19; 20; 21]. Once establishing this relationship, its graphic representation can be adjusted to a potential type curve based on Equation (1), in which all the parameters that will be defined are considered. Therefore, the\(f_{o}\) values will be related to the thickness of sediments obtained at these control points [18; 19; 20]. This equation will make it possible to calculate the depth of the rocky substrate under the rest of the HVSR measurement stations and thereby draw an isopach map of sediment thickness and the definition of the slip rupture plane. From the geological point of view of this slide area, the separation surface between the shallow materials (sedimentary or altered) and the rocky substrate will be established as the position of the landslide rupture surface. It can be defined because of the geological materials' conditions and their direction and dip indicated in the previous paragraphs.