94 Seismic Imaging the length of the spread (L/2). If the entire spread (source and receiver spreads) is shifted by L/2 then the reflection zone illuminated by the current shot adjoins that illuminated by the previous shot. Where there is no overlap between the successive reflection zones, the coverage is called single fold coverage. If the distance between shots is m/2 times the receiver interval, where m is a submultiple of the number of receivers N, then there is an overlap between the reflection zones illuminated by successive shots; the reflection coverage is then termed multiple coverage, with the fold coverage being equal to N/m. Figure 4.2-b is an example of multiple coverage. A 12-receiver spread (N=12) is moved up by one receiver interval (m=2) to provide 6-fold coverage. The fold of coverage corresponds to the number of traces having the same common midpoint (CMP). The distribution of offsets is regular in 2D surveys; the azimuth (angle between the theoretical direction of the seismic line and the straight line joining the source and the receiver) is constant (0° for end-on shooting, and 0° and 180° for split spread shooting). Acquisition is more complex for 3D land surveys. Source and receiver lines are laid out to provide the most homogeneous coverage. The most conventional implementation is the cross-spread design with lines of sources perpendicular to lines of receivers (Figure 4.3). Figure 4.3 Cross-spread design: Lines of sources (green triangles) perpendicular to lines of receivers (red dots). In 3D acquisition, the CMP is replaced by a cell or bin, the size of which being the product of half the source interval and half the receiver interval. Traces contributing to the same CMP bin have irregularly distributed azimuths and offsets. Implementation is optimized to ensure the most regular azimuth and offset distribution possible. Figure 4.4-a shows a single fold 3D subset, obtained with an elementary cross spread for which source positions belong to the same source line, and receiver positions belong to the same receiver line. The stacking fold is the number of overlapping elementary cross spreads (Figure 4.4-b).
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