116 Seismic Imaging The second residual section shows high apparent velocity seismic events with a poor lateral continuity. This could be reflected or diffracted waves corrupted by residual noise. The velocity model was used to apply the NMO correction to the residual section to obtain a zero-offset section at normal incidence. The same processing sequence was applied to the 60 individual shot points to obtain 60 zero offset sections. The 60 sections were merged to create the 3D block. The width of the block in the in-line direction is 120 m. The abscissa zero indicates the location of the source line. The abscissa of the reflecting points varies between –60 m and +60 m in the in-line direction; the distance between two reflecting points is 2.5 m. Due to the geometry of acquisition, the shot point recorded on geophone line 11 with a 60 m source offset becomes the in-line section 31 (Figure 4.22, top right). The VSP time versus depth law measured at well C1 was used to convert the time sections into depth sections with a 0.5 depth sampling interval. The depth conversion of time section 31 is presented in Figure 4.22 (bottom right). In the 30 to 120 m depth interval, it can be noted that the vertical seismic resolution is insufficient to describe the heterogeneities inside the reservoir. The only way to increase the vertical resolution is to apply a deconvolution of the wave number to the depth sections. The result for depth section 31 is presented in Figure 4.23-a (upper part). A significant improvement of the vertical resolution is thus obtained. After deconvolution, it was assumed that the seismic trace represents the reflectivity function of the geological model. Integration with respect to depth enabled the deconvolved seismic trace to be constrained to obtain an estimate of the interval velocity function versus depth. For this purpose, after deconvolution and integration, a Wiener filter (Mari et al., 2015) was applied to the seismic traces to convert the amplitude sections into velocity. The Wiener filter is designed to obtain an optimum fit between the acoustic velocity log at well C1 and the associated deconvolved and integrated seismic trace (Figure 4.23-b). The Wiener operator thus obtained was applied to all the deconvolved and integrated traces of the 3D block to transform an amplitude block into a 3D pseudo velocity block in depth. The result obtained with the in-line depth section 31 is shown in Figure 4.23-a (bottom). The procedure was validated by measuring correlation coefficients between estimated seismic pseudo velocity logs and acoustic logs at wells MP6, MP5, M8 and M9 (Figure 4.19-b). The 2D direct and reverse shots were processed in the same way to laterally extend the 3D block. All pseudo-velocity depth sections were merged to create the 3D block. The width of the block in the in-line direction is 240 m, and 300 m in the cross-line direction.
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