## 4 Geophysical Research and Applied Methodology  
### Geophysical Surveys  
Geophysical research has applied two different geophysical methods combined: distributed geoelectric and seismic, as shown in Figure 4. The designed campaign of surveys and applied techniques have been reduced to the available spaces in the studied area. They were the access road over the three levels where it cuts the slope (Figure 4): one at the center of the landslide and the other at both sides (upper and lower zones). It was because the slope of the area being studied is very steep and has thick material on the surface (see Figure 2). Little compact shallow sediments are prone to movements or punctual landslides, and the falling of loose material could interfere with data quality acquisition or prevent the correct application of the measuring devices (even personal security). That is also an obstacle to installing drill hole machines so that the results can provide better locations in further studies. Figure 4: Situation map of surveys (noted that the north is rotated counterclockwise) and observed landslide area (blue dashed line). Black dots: HVSR surveys; yellow triangles: VES surveys; and red lines: seismic refraction and MASW profiles, both made at the same position. White crosses refer to the coordinate points (Modified from [22]). Concerning the geoelectrical method, the VES technique has been applied in five test points (yellow triangles in Figure 4). These surveys use an electrical field to define the resistivity of materials into the ground. The electric current was injected through the terrain from two electrodes (A and B), whereas the potential was measured in the other two electrodes (M and N). The use of different geometries in the A-M-N-B disposition is called measurement arrays, and they can be seen in different ways (as Wenner, Schlumberger, Dipole, for example) [4]. VES surveys use a 400 m-length AB electrodes aperture applying a Schlumberger array, providing information on materials and stratigraphy about the first 60-80 m from the surface elevation application point. The performed interpretation could show the presence of water and altered materials over the basement. In addition, it can be the tool that clearly defines the basement position from the resistivity contrast between rock and shallow sediments and identifies the stratigraphic levels. The seismic method has been applied through two different techniques: active refraction and type MASW profiling (shown as red lines in Figure 4). Both techniques were performed over the same line to provide information about S-wave and P-wave velocities so they can be correlated. Using these seismic techniques can define the substratum interface as a high increase of velocities and complementary show variations in the compaction of materials [14]. The three refraction seismic profiles all have a 115 m length with a 24-channel array and five different shooting points. They have been executed at three levels on the road path of the access road from the upper intermediate to the lower part. The first one, named SR-1, was performed at the high elevation area of the landslide, whereas SR-2 was at the middle elevation and SR-3 on the lower part, near the supposed landslide toe. The final model can obtain more accuracy and resolution by combining with VES interpretation. Another technique was the punctual or single station passive-type HVSR, applied over 22 station points (represented in Figure 4 by black dots). This technique is easy to apply, and it was acquired by 3-direction geophones equipment with a 2.0 Hz frequency. It used a 20 min time record to have enough information from every single station. The results from data processing define the fundamental frequency of vibration (\(f_{o}\)) of the ground, which is related to thickness [15]. Complementarily, it can use the spectral ratio (\(H/V\)) or amplification (\(A_{o}\)) in further calculations.