76 Seismic Imaging Figure 3.11 (a) S-wave VSP-CDP time transformation compared with S-wave sonic log and synthetic seismogram; (b) S-wave VSP-CDP time transformation compared with P-wave VSP data. Adapted from Becquey et al. (1992). 3.2.3 General interpretation An analysis of the S-wave VSP-CDP image in Figure 3.11 shows, that three main events correlate well with the S-wave sonic log from the vertical borehole: • 1,580 to 1,745 m: Lusitanian marl and limestone layers, giving rise to internal impedance contrasts; • 1,745 to 1,851 m: Callovo-Oxfordian marls; • 1,851 m to the bottom: Limestone reservoir. The S-wave VSP-CDP stack was compared with the P-wave VSP acquired at the vertical borehole and the synthetic seismograms produced by the S-wave sonic log convolved with a characteristic signal with the same bandwidth as the VSP-CDP image. The improvement in the vertical resolution of the S-wave VSP-CDP image is significant, compared to the conventional P-wave VSP (Figure 3.11b).
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