## 2 Background and Scope
### Geophysical Background and Scope  
The use of geophysical research in landslide studies or the excavation of slopes is widely recognized, with seismic and electrical methods and their techniques being the more commonly used in these investigations [4; 5; 6; 24]. The sliding mass and its movement generate changes in measured geophysical parameters, so it can be used this kind of surveys  
Figure 1: The situation of the investigation area. The yellow circle locates the sliding area (Modified from [22; 23]). Figure 2: A view of the middle part of the landslide area where the road was affected (cut) and reconstructed, applying observed earth-walls. In this location, the movement is prone to continue at a low displacement tax. The high slope on the landscape can be observed. to analyze this internal ground modifications and recognize the landslide body or even study its dynamic motion [5]. The HVSR passive seismic technique has been defined and developed by [PERSON] [15] in order to establish the fundamental ground frequency (\(f_{o}\)) values for a two-layer model (soil and compact material or basement). In this proposed model, the fundamental frequency is related to the soft or sedimentary layer shear-waver velocity and its thickness [15,16]. Therefore, if that shear wave velocity (\(V_{s}\)) can be estimated or measured, it can be calculated and defined as the soft overburden material that lies over the basement. That assumption is valid on homogeneous media such as a sedimentary basin or a broad soil-rock system where vs. changes are not so significant. However, it can produce some errors if the soil or sediments have lateral changes in composition and/or compaction. There, this relationship causes irregularities in the definition of the depth of the basement [17]. HVSR surveys are based on the knowledge that seismic noise in the indicated model is composed of surface waves and media with enough impedance contrast (related to the density and seismic velocity). Therefore, complex phenomena can be produced where the soft sediments have changes in their density and/or vs. (heterogeneities), and results have no accuracy [25,26]. In some landslide material conditions, where a high contrast impedance could exist (soft soils or sediments directly over a rocky substrate, as in this investigation area), shear and compressional waves can predict its position [27]. Nevertheless, compacity or density variations could still affect the relationship, and more than the HVSR technique is needed [28,29]. HVSR is a recognized technique to define \(f_{o}\) from a sedimentary layer over a compact material [30]. However, it has limitations for other purposes such as vs. determination [31]. Recently, the possibility of its use for other purposes and different methodologies has been investigated. The research of authors such as [18,32] in establishing empirical correlations that relate the thickness of the surface materials (soils and soft sediments) with the value of said fundamental frequency \(f_{o}\) was considered This relationship is expressed from a potential equation of the type:  
\[Z=a\;f_{o}{}^{-b} \tag{1}\]  
where Z would be the depth at which the rocky substrate is found (or what is the same, the thickness of the sediments that underlie it), and \(f_{o}\) would be the value of the fundamental frequency of the ground measured in the HVSR survey. Parameters \"\(a\)\" and \"\(b\)\" would be factors to be defined experimentally, which are related to the nature of the materials and the research area [33]. Most of the investigations about this formulation have been based on the use of mechanical drill holes to define the thickness of materials that lie over a rocky basement [19,20,34,35]. This type of correlation is considered to have a high accuracy in establishing the relationship when using a direct prospecting method.