155 6. Hybrid seismic imaging field data within a predetermined fitting level; it shows smoothly varying velocities ranging between 100 and 2,000 m/s, with a low velocity layer, approximately 5 m thick, at the surface. Figure 6.6 shows the layout of the seismic acquisition setup, with 240 geophones and 25 shots, the shot-gather for a source located at 120 m and the final P-wave velocity model obtained from P-wave travel time tomography. Figure 6.6 (a) Layout of the seismic acquisition setup, with 240 geophones (gray triangles) spaced every 1 m and 25 shots (gray stars) spaced every 10 m. (b) Example of a shot gather for a source located at 120 m (red star in a). (c) Final P-wave velocity model obtained from P-wave travel time tomography. The topography extracted from airborne LiDAR data is represented with a solid black line. From Pasquet and Bodet (2017). The second step of the velocity-estimation procedure is related to the S-wave velocity model extracted from the surface waves. The processing of the surface waves data was carried out using SWIP and readers can find supplementary information about this practical processing sequence in Pasquet and Bodet (2017). Therefore, very few details are provided here and we restrict our comments to the main results only. After field data windowing for validation of the 1D model hypothesis, the seismic record from its original time–distance domain was transformed into the frequency– phase-velocity domain. This step results in a set of frequency–phase-velocity pairs specifying dispersion curves. The experimental dispersion curves were identified in the f-k domain and the location of maxima energy were picked. The dispersion
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