161 6. Hybrid seismic imaging The S-wave velocity model is characterized by velocities ranging between 50 and 600 m/s, with higher shallow velocity below the heat-flow anomaly observed between 50 and 120 m. Although the S-wave velocity model has a lower investigation depth than the VP model, it provides more information regarding the lateral variations of the velocities of shallow layers due to the intrinsic smoothing of tomographic inversion and the substantial horizontal component of P-wave travel paths. SWIP also calculates Poisson’s ratio, if P-wave velocity is available. The results, as shown in Figure 6.14, reveal values in the range of 0.3–0.5, typical of non-saturated and saturated media, respectively. For most of the subsurface, Poisson’s ratio values are between 0.45 and 0.5 indicating high water content, except in the highest part of the hill observed at depths below the degassing area visible at the surface, which presents a low Poisson’s ratio. Figure 6.14 Poisson’s ratio computed from P-wave velocity provided by tomography and S-wave velocity from surface wave dispersion inversion and masked below the depth of investigation estimated from S-wave velocity standard deviation. The topography extracted from airborne LiDAR data is represented with a solid black line. From Pasquet and Bodet (2017). Finally, this example indicates that the estimated Poisson’s ratio is a valuable parameter to clearly highlight gas pathways in the subsurface consistent with degassing observed at the surface. 6.3 Conclusion This chapter, which describes the handling of different types of waves present within the same dataset, underlines some of the advantages of hybrid seismic imaging strategies to provide efficient, accurate and reliable subsurface models, in terms of geometry and mechanical properties. In the first field example, the hybrid seismic imaging tool showed that seismic data derived from traditional refraction acquisition is valuable for obtaining information about the reflectivity for targets located in the near and/or very near surface. Based on a three-step procedure, the processing of refraction and reflection waves provided two sections, which after gathering produced an extended time reflectivity section starting from the surface.
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