**3. Geophysical investigations**

#### **3.1 Geophysical techniques and geotechnical model: the seismic tomography**

The most common procedures for recognizing unstable sites include drilling boreholes, shallow excavations, and engineering geology studies. However, more and more often, some geophysical techniques are associated to the above intervention, and usually they represent the first phase of assessment and allow optimizing the campaign of excavations and boreholes. Compared to direct surveys, the geophysical ones provide extensive and continuous information (usually along sections not necessarily vertical), are moderately invasive and have a remarkably advantageous information-to-cost ratio.

 Among the various geophysical methods, the seismic ones (e.g., [6]) are the most widely used for geotechnical purposes. The reason is that the velocity of propagation of the elastic (or seismic) waves depends on the density and the elastic properties of the medium in which they propagate. The seismic methods are of different types: first of all the classical seismic refraction, seismic reflection, and the seismic tomography methods. In recent decades, some more methods based on the analysis of the surface wave's dispersion, such as the MASW, multichannel analysis of surface waves; the SASW, spectral analysis of surface waves; and the REMI, refraction microtremor, have been added [7].

 Geophysical prospecting methods based on the refraction of seismic waves date back to the 1920s of the last century, mainly in the field of petroleum research. Over time, refraction techniques for oil and gas research have been progressively supplanted by reflection techniques, and their use has shifted to other prospecting fields with objectives falling within the first hundred meters of depth. Actually, the classical seismic refraction shooting based on the analysis and interpretation of the travel time curves has been widely used in the geotechnical field, especially in the study of foundation soils and slope stability. However, before the 1980s of the last century, the processing of seismic refraction data provided approximate models, except in cases of quite regularly layered subsoil. An important step was made with new processing techniques, first the GRM, generalized reciprocal method [8], but still the work was done in terms of seismic rays substantially

 conceived and represented as broken lines, and only the main refractors could be highlighted. With respect to geotechnical study field, the most important leap, leading to current refraction data processing, has been the operational advent of the seismic tomography [9]. Among the numerous scientific works concerning the topic, the one of White [10] is undoubtedly worth mentioning. As for the inversion algorithm, the most used were FBP, filtered back projection; ART, algebraic reconstruction technique; and SIRT, simultaneous iterations reconstruction technique. In the examples discussed here, the inversion algorithm named ASA—adaptive simulated annealing [11]—has been used. In the seismic tomography, data acquisition is carried out with ordinary energy sources, such as hammers, dropping masses, downhole energy sources, and small charges of dynamite, and with standard receivers such as electromagnetic geophones and piezoelectric hydrophones. **Figures 2**–**4** show the most common acquisition schemes in seismic tomography. The first one is the classical refraction tomography with both shots and receivers at the ground surface; the second, in **Figure 3**, is the cross-hole tomography where both shots and receivers are placed inside boreholes, better if filled with water; the third, in **Figure 4**, is the up-hole tomography. Referring to **Figure 4**, if the position of shots and detectors is inverted (i.e., the detectors are placed inside the borehole and the shots are placed at the ground surface), the acquisition system is properly named "downhole tomography." It must be underlined that though in the schematic sketch of both cross-hole and up-hole, the ray paths are rectilinear, and this can

**Figure 2.** 

*Acquisition system of the classical seismic refraction tomography.* 

**Figure 3.**  *Acquisition system of the seismic up-hole tomography.* 

*Application of Seismic Tomography and Geotechnical Modeling for the Solution of Two Complex… DOI: http://dx.doi.org/10.5772/intechopen.81876* 

**Figure 4.**  *Acquisition system of the seismic cross-hole tomography.* 

happen only when the subsoil is homogeneous and isotropic, otherwise, ray paths are curvilinear due to refraction.
