112 Seismic Imaging 4.4 A Hydrogeology example This example concerns the imaging of a near-surface karstic reservoir at the Hydrogeological Experimental Site of Poitiers. The field case has already been discussed in the “Refraction surveying” chapter. Preliminary studies led to the selection of the following spreads for 3D imaging. The detonating impulse source was selected to record high frequency data and to reduce the air wave effect. To preserve the high frequency content of the data and to have an accurate picking of the refracted wave, a single geophone per trace was used. a b Figure 4.19 Seismic spreads and field implementation of seismic lines. (a) Seismic spreads - A direct shot and reverse shot are fired in line to obtain 2D seismic images. Several cross-line shots are fired to obtain 3D seismic images. (b) Seismic line implementation: the distance between 2 adjacent lines is 15 m. Red points indicate well locations. To avoid spatial aliasing, a 5 m distance between two adjacent geophones was selected. Due to the dimensions of the area, it was not possible to extend the length of the seismic line over 250 m in the in-line direction. Consequently, a 48-channel recorder was used. In the cross-line direction, the extension of the area did not exceed 300 m. As a result, 21 receiver lines were implemented, with a 15 m distance between adjacent lines. For the refraction survey, a direct shot and a reverse shot were recorded per receiver line. For the reflection survey, 3 shot points in the cross-line direction were fired per receiver line. The range of offsets was selected to optimize the quality of the seismic image in the reservoir zone, between 40 and 130 m. The minimum offset distance was chosen as 40 m to reduce the
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