73 3. Seismic tomography For this reason, we repeated the tomographic inversion using both surface and VSP data. The result presented in Figure 3.A.2 revealed some significant velocity anomalies, which can be compared with the homogenous velocity presented in Figure 3.8-b. Figure 3.A.2 A horizontal slice at 28 m depth taken at the 3D velocity cube produced by tomography when combining seismic surface and VSP data. Now, the model is characterized by velocity anomalies contrasting with the high homogeneous velocity and very poor resolution produced by the tomographic inversion of the surface data (Figure 3.8-b). Adapted from Galibert et al. (2014). 3.2 Reflection tomography example: cross-hole field data It has been demonstrated that reflection tomography is an appropriate technique for building a good velocity model of subsurface structures based on multichannel seismic data. This section describes the use of a reflection tomography procedure to image a limestone reservoir at a depth of about 1,850 m, utilizing the information present in the travel time of reflected S-waves. These data were recorded during a cross-hole seismic experiment, carried out in the Paris basin. The data processing sequence is detailed in Becquey et al. (1992).
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