54 Seismic Imaging a b c d Figure 2.14 Seismic acquisition. a) 2D in line acquisition geometry, b) 3D cross-line acquisition geometry, c) Example of in-line shot gather, d) Example of crossline shot gather with 60 m of lateral offset. A direct and a reverse shot were recorded per receiver line (Figure 2.14-a). Three shot points in the cross-line direction were fired at distances of 40, 50 and 60 m from the receiver line under consideration (Figure 2.14-b). Figure 2.14-c shows an example of an in-line shot gather and Figure 2.14-d a cross-line shot gather with a lateral offset of 60 m. The range of offsets was selected to optimize the quality of the seismic image over the reservoir depth interval, between 40 and 130 m. The minimum offset distance was chosen as 40 m to reduce the influence of surface waves. The time sampling interval was 0.25 ms and the recording length was 0.5 s. To obtain the velocity of the refractor (top of the reservoir) and its depth, the Plus– Minus method was used. To apply this method the recordings must be carried out where geophones are aligned with shot points. The arrival times of the direct and refracted waves were picked on all the in-line shots. The picked times from the in-line shots (direct and reverse) were used to compute the t − and the t + curves to obtain the velocity V2 of the refractor and the delay time curve. Figure 2.4 shows the results obtained on line 9. It shows the direct and reverse shot points, the raw t − curve
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