105 4. Tying surface seismic data Figure 4.1 3D seismic survey; left: acquisition device, right: time sections. The acquisition and processing sequence are described in Chapter 2 and the processing results are presented in Figures 2.11 and 2.12. It should be noted that the depth of the receiver varies between 25 and 85 m, the surface source being slightly offset (5 m) relative to the wellhead. There is a 5 m distance between two successive positions of the geophone in the well. Figure 4.2 shows: • Top left: unprocessed VSP recording; • Top right: the vertical time-depth relationship and the interval velocity law at the seismic sampling interval in depth, i.e. 5 m; • Bottom left: the stacking corridor of the VSP section and the VSP stacked trace; • Bottom right: Tying of the seismic section on the VSP stacked trace at the intersection of the in-line 3 and cross-line 6 seismic sections. The VSP stacked trace, duplicated 5 times, is inserted in the in-line section 3 at the CMP position associated with the cross-line section 6. The correlation coefficient between the seismic trace and the VSP corridor trace at the point of intersection is greater than 0.72, thus showing a good time fit of the seismic horizons. However, slight differences between the amplitudes observed on the surface seismic and the VSP stacked trace can be noted. This difference is due to the choice of amplitude compensation laws used for the two data sets. For surface seismic, an AGC-type
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