As described in basic data processing sequence, processing of 2-D seismic data is done in midpoint-offset coordinates. This requires, first, sorting the recorded data into common-midpoint gathers. Similarly, processing of 3-D seismic data requires binning the recorded data into common-cell gathers. Albeit we shall devote the next section to processing of 3-D data, it is imperative to discuss 3-D binning here, for it is fundamentally related to the acquisition geometry.
To perform 3-D binning, first, a grid is superimposed on the survey area as illustrated in Figure 7.1-5. This grid consists of cells with dimensions of half the receiver group spacing in the inline direction, equivalent to the CMP spacing in 2-D processing, and the line spacing in the crossline direction.
By precisely determining the shot and receiver coordinates, we can determine the midpoint locations and gather them into bins (or cells) for stacking and migration. In reality, midpoint distribution within a cell is not necessarily uniform since cable shape varies from shot to shot and line to line (Figure 7.1-5). Midpoints may be clustered at one part of the cell; that is, the centroid of the midpoints is not necessarily at the center of the cell. Midpoint distribution also can vary from cell to cell. Some cells may contain more traces than others, while some cells may have less uniformly distributed midpoints than others.
Figure 7.1-5 shows the actual midpoint locations over a portion of a 3-D survey area generated by an 8-cable configuration. The grid with dimensions 12.5 × 25 m represents the bins, and the traces at the midpoint locations that fall within each bin constitute a common-cell gather. The number of the midpoints within each bin defines the fold for that bin. Note that the fold and midpoint distribution vary significantly from one bin to another.
Figure 7.1-6a shows the fold of coverage map of a marine 3-D survey. While the average fold is 30, note that the fold of coverage is not uniform over the survey area. Fold of coverage varies from one range of offsets to another. Figures 7.1-7a, 7.1-8a and 7.1-9a show the coverage maps for the near-offset, mid-offset, and far-offset ranges. Irregularities in recording geometry — specifically, nonuniform fold of coverage, irregular offset distribution per bin, and irregular midpoint distribution within each bin, can cause problems in processing of the 3-D data. Lack of uniformity in the fold of coverage causes inconsistency in the accuracy of velocity estimation from one analysis location to another and variations in the stacked amplitudes. Processes such as 3-D dip moveout (DMO) correction, 3-D prestack time and depth migration, and amplitude variation with offset analysis are adversely affected by the acquisition footprint.
Figure 7.1-5 Midpoint locations associated with an actual marine recording geometry using eight cables and two source arrays. The inline direction is from left to right. The letters represent the individual cables that contribute to the midpoint locations indicated by black dots. The grid indicated by the red lines represents the bins into which the midpoints are assigned. The red dots indicate the bin centers. (Courtesy PGS.)
Low-fold areas in a 3-D survey must be filled in during acquisition by shooting more lines at appropriate locations. If these deficiencies are discovered in processing, it is costly to send the vessel back for further acquisition. Quality control work therefore must be carried out on board to monitor the fold of coverage.
Consider some modifications to binning of 3-D data. A slight translation and rotation of the grid imposed on the survey area sometimes significantly reduces problems associated with binning. This type of grid optimization may yield a more uniform midpoint distribution within each cell and even improve the uniformity of the fold of coverage over the survey area.
It is common not to restrict the gridding to cells of equal size. More midpoints may be included in the cell from the neighboring cells by expanding the cell size in the crossline direction as needed, typically up to 50%, to achieve a uniform fold of coverage. Although the same midpoint could be used in more than one cell, restrictions often are imposed on the offset range and multiplicity to prevent an excessive use of each midpoint. Adjustment to cell size to achieve a uniform fold of coverage and offset distribution is called flexible binning.
Figure 7.1-6b shows the fold of coverage map as in in Figure 7.1-6a after flexible binning. Except for a few locations, note that the fold of coverage is fairly uniform over the survey area with a nominal value of 30. Following the flexible binning, the fold of coverage maps for the near-offset, mid-offset, and far-offset ranges are shown in Figures 7.1-7b, 7.1-8b, and 7.1-9b. Although, there still exist some variations in the fold over the survey area within the specified ranges of offsets, flexible binning has helped to remove the anomalous fold distribution present in the maps shown in Figures 7.1-7a, 7.1-8a and 7.1-9a.
- Migration aperture
- Spatial sampling
- Other considerations
- Marine acquisition geometry
- Cable feathering
- Crossline smearing
- Strike versus dip shooting
- Land acquisition geometry