Land acquisition geometry
Land 3-D acquisition commonly is carried out by swath shooting in which receiver cables are laid out in parallel lines (inline direction) and shots are positioned in a perpendicular direction (crossline direction). Figure 7.1-14 shows swath shooting with 6 receiver cables, each with 80 receiver groups, 50-m apart. The receiver cables are spaced apart from left to right at 100, 200, 100, 200, and 100 m distances. Shooting usually is perpendicular to the swath starting from the far left and moving in and away to the right of the swath. As one shot line is completed, the receiver cables are "rolled along" the swath a number of stations, equivalent to the shot-line spacing, and shooting is repeated.
The recording geometry in Figure 7.1-14 provides a 25 × 25-m bin size. Once one swath is completed, another swath parallel to it is recorded; and this procedure is repeated over the entire survey area. Although not exercised in some 3-D surveys, it is important to leave some receiver cables on the ground to ensure proper coupling of statics between swaths. A complete survey plan, including the 12 swaths of receivers and shot locations, is shown in Figure 7.1-15.
The swath shooting method yields a wide range of source-receiver azimuths, which can be a concern during velocity analysis (processing of 3-D seismic data). The source-receiver azimuth is the angle between a reference line, such as a receiver line or a dip line, and the line that passes through the source and receiver stations. So why use a recording geometry that introduces problems in analyzing data? The main advantage of swath shooting is that it is economical.
Figure 7.1-16 shows the spider diagram at each bin center that describes the source-receiver azimuthal variations induced by two different recording geometries — shots placed in the direction orthogonal to the receiver lines, and shots placed in the diagonal direction at 45 degrees to the receiver lines. Note that the first type of recording geometry yields uniform azimuthal distribution from one bin location to another. The more unusual second type of recording geometry yields variations in the azimuthal distribution from once bin location to another. On the other hand, the offset distribution is more uniform in the case of the diagonal shooting as illustrated by the histogram plots in Figure 7.1-17.
Figure 7.1-14 Swath shooting geometry used in a land 3-D survey. Solid lines indicate the actual receiver cables for a particular shot line aligned with the arrow indicated. Receiver cables (80 groups each) and shots are rolled along the swath. When the end of the swath is reached, the next swath starts until all of the survey area is covered.
Figure 7.1-16 Two shooting patterns used in 3-D land surveys: (a) source locations (solid green circles) and receiver locations (solid black squares) perpendicular to one another; (b) source locations diagonal to receiver locations. The spider diagram within each cell represents the source-receiver azimuthal distribution associated with the common-cell gather. (Courtesy Santos Oil Ltd.)
Fold of coverage maps for the recording geometries illustrated in Figure 7.1-16 are shown in Figure 7.1-18. Because of operating conditions, uniform coverage usually is not achievable over the entire survey area. In the next section, we shall follow through the entire processing sequence for a land 3-D survey data set. We shall confront the challenges of an irregular recording geometry right at the outset of the analysis.
- Migration aperture
- Spatial sampling
- Other considerations
- Marine acquisition geometry
- Cable feathering
- 3-D binning
- Crossline smearing
- Strike versus dip shooting