6.1. Data Integration and Calculation
The information obtained in the seismic refraction and passive MASW surveys, together with the interpretation of the VES surveys, has defined a zone of low compaction surface materials with velocities below 1000 m.s-1 (Vp) and 600 m.s-1 (Vs), which are correlated with resistivities lower than 1500 Ohm.m. These surface materials would correspond to vegetative and transported soils and colluvial ones with a high percentage of coarse material (pebbles and blocks) embedded in a clay-silt matrix. The geophysical levels below these materials’ present characteristics, both in seismic and electrical techniques, of compact to very compact materials, which would correspond to a competent and cemented substrate.
From this considered model, the correlation between the fundamental frequencies fo, obtained at the control points in the HVSR tests and the thicknesses of surface materials (sediments) in said areas obtained from the data of the SEV tests has been proposed and executed at the same points.
So, the results of the thickness of the sedimentary materials over the metamorphized rocky substrate are presented in
Table 3, together with the values obtained in the tests of the HVSR technique. The depths shown for each of the five VES surveys carried out also correspond to the values of the position of the said basement in the refraction and MASW-type tests of the seismic method.
This relation will be the essential one in the whole area to establish the empirical correlation concerning the fundamental frequencies obtained in HVSR surveys, which respond to Equation 1.
Figure 8 shows the graphic correlation established between both groups of data in
Table 3 and the value of the adjustment of the curve. In this case, said value (R
2) has been established in 0.932, being a result that can be considered a good correlation value, even using the few available data, with which we can consider an estimation error of 10% in subsequent calculations.
Therefore, Equation 1 would be transformed according to the following values:
If we consider that H is the value of the thickness of surface sediments expressed in meters, from that Equation 3, it can be calculated for each point of the HVSR test station measured and based on the value obtained from the fundamental frequency (fo, in hertz), the value of the thickness of the sediments found over the rocky substratum.
Table 4 presents the calculations established for each of the measured HVSR station points, including the five corresponding to the control points (in the position of the VES tests and indicated with the prefix S), applying the formula of Equation 3. To calculate the
Kg value, also shown in
Table 4, the results obtained in
Table 2 were used and applied Equation 2. As [
46] considers, the values over 5 to 10 in
Kg value (it depends on area and materials conditions) are prone to show instability or are capable of showing it. In this case, it can be correlated with a susceptibility to the present capability of sliding or a movement downhill, so that they can mark areas of potential movements in a landslide.
The methodology proposed in this research has made it possible to establish the thickness of less compact materials (soft sediments not compacted o cemented), not only from direct investigations such as VES, also in refraction or MASW seismic profiles. So, also from the relationship established in Equation 3 have defined the values of said thicknesses under each point of the HVSR point station.
The use of relationships between the thicknesses obtained from VES surveys with HVSR tests has very few publications references. Only [
21] uses this correlation between sediment thickness investigation and VES-type tests, while other authors, such as [
47] use correlations obtained from electrical tomography tests but also combine and use results from mechanical drilling. In both cases, they concluded that the application of the geoelectric method techniques in combination with the HVSR measurements offers reliable results in determining the thickness of surface sediments.
From the results obtained and presented in
Table 4, these have been represented and analyzed by constructing a map of isolines of thickness values (isopach) that is presented in
Figure 9. A special shape is located below HS5 and H9 points, which is the existence of a zone of great thickness (more than 55 m) in this central area and in relation to the position of the greatest size of movement (just where the reinforced earth wall of
Figure 2 has been built).
To complete the analysis, it was drawn over the isopach map, several sections.
Figure 9 shows the position of the sections that have been made to analyze these results obtained (black dashed lines).
Figure 10 shows the longitudinal sections of the landslide (according to the direction of movement), and variations can be observed in the bottom and the position of the rocky substrate (
Section 6) with a section (central
Section 1) in which a typical distribution of a landslide with a thickening of the materials in the lower part due to deposition.
In said longitudinal sections, it is also possible to observe the presence of a projection in the area of the lowest elevation (a peak from the basement), which may be one of the factors preventing the whole slide of all mass over the basement from continuing towards lower elevations of the terrain, which could be observed in the field. Lastly, in
Section 7, traced by the zone where less movement is observed and the rock outcrops at some nearby point, the thickness of the sediment is less than 15 m.
The landslide surface traced in this section from the available data seems to respond to a translational landslide possible typology since the thickness of moving materials along said section is maintained. However, the landslide can also have a rotational component or be considered a rotational-translational combination. In addition, by analyzing the morphology suggested by this research, it could be that the sliding area may respond to an accumulation of previous landslides (paleo-slides) as observed in nearby zones [
44].
A second depressed zone would be the outline of the shape of the landslide, in its central area, with a significant depression in the position of test H9 (the test that is also present in the longitudinal section) and with slopes to the north and south that present a steep slope.
The raised area that is observed more clearly in the cross section also seems to be evident, more gently in the central area of the longitudinal section, approximately where tests HS5 and H9 have been carried out (see
Figure 4 and
Figure 9 for reference).
Four cross sections of the landslide (considering the movement direction) were traced on the area, and others two with the oblique North to South direction (see
Figure 9 to identify its position).
Figure 11 shows, from left to right, upper sections to lower ones. They show an important deepening at the center of the landslide area, and at
Section 3 (over the intermediate position of the access road), it can be seen a V-shaped area deeper than the other ones. The same deepening zone can be observed in oblique sections (see
Figure 12) where the V-shaped sediment geometry is wider, but in all cross-sections, one of the flanks of that deepening area is sharply steep.
It can be in relation to the possible presence of a fault structure in the basement corroborated by low values in resistivity observed in VES-5. Now, this depressed area can correspond to a ravine or temporary torrential channel (marked by the low values in sediments resistivity).
The cross-sections 2, 3, and 5 obtained are concordant with seismic refraction profiles interpreted (considering that seismic profiles were applied in different directions).
The graphical distribution of the value of the Vulnerability Index (
Kg) is presented in
Figure 13. It shows peak values in the area around VES-5 and H9 survey points and coincides with the greatest movement and the area of action carried out by the company E.P. CELEC with the construction of the reinforced earth wall. In addition, in this area, according to communication from the company’s technicians, the movement continues to occur today.
This parameter is related to the ground shear strain or effective stress [
46] and can be correlated with the potential ability of materials to move (sliding susceptibility), so it can be recognized as a tool for assessing said capacity in landslides. In this study, it can be observed, in
Figure 13, that appears extremely high values under the actual moving area (remarked by a dark blue continuous-line oval) and high values towards the upper area from the last one described (remarked by a dark blue dashed-line oval). The rest of the points can be considered with low capabilities to sliding or are stable right now.
6.2. Data Correlation and Reliability Discussion
The results obtained in this landslide area are related to geophysical surveys alone, and no other direct data (as perforation bore-holes) is provided as verification. Using geophysical data, considered indirect surveys, could be an early investigation tool to delineate the characteristics of a sliding mass. It must be related to accurate direct information, but combining methods and techniques can improve the geophysical models [3-6].
In this area, the VES information shows a clear separation between shallow materials and the rocky substrate, with resistivity changing values up to four times (> 5000 Ohm.m for the rocky substrate). So, the obtained models could be used as a definition of the thickness of the soft material (sedimentary) and altered one (eluvium) with some grade of accuracy, thus defining the surface rupture of the landslide. In the metamorphic studied area, the materials overlaying the rocky substrate are those involved in landslides [
44].
Besides, seismic surveys provide similar distribution in shallow sediment layers using seismic velocity parameters, i.e., they have corroborated the VES geoelectrical interpretation. Also, the models offer a contrast in impedance values between the sediments and the basement that is up to 2.2, which is a piece of important information in applying the HVSR two-layer model [
10,
15,
16,
17,
21].
The use of few contrast points could be a limitation too. Of course, the more that can be used could reach better accuracy. Nevertheless, the most important thing is to get a variety of thicknesses to adjust the empirical curve (Equations 1 and 3), as can be seen in other investigations (related in [
20,
21]). That accuracy and reliability in the final results are related to the precision of geophysical models that were employed [
21,
47].
The
a and
b factors in Equation 3 differed from those obtained by other authors (applied in basin geological areas) as [
18] or [
47]. Considering [
33,
48], they indicated that
a factor is related to local geological characteristics (ground materials, impedance contrast, and humidity). In contrast, the
b factor is related to sediment thickness and the geometrical shape of the basement. The obtained factors here are close to those obtained in [
20], an investigation also made in Ecuador, where geological conditions are more similar to the present one.