Bin size in marine work
What spacings are required to achieve a minimum bin size of for a marine operation towing two sources and three streamers? What mix of offsets and azimuths will result? How far apart should parallel ship traverses be to provide continuous coverage?
Traces whose common midpoints fall within small unit areas (called bins) forming a rectangular grid are assumed to lie at the center of the bin and are stacked together for processing, especially for migrating. The signal that the source generates in the earth is the same as if the entire system were at rest, and the effect on the hydrophone groups at any time is almost the same as if they were at rest. Hence, the fact that the entire recording systems is in motion does not change the dimensions of a bin; the fact that the bin is moving slightly with record time makes so little difference that it is generally ignored.
For uniform sampling, lines of common midpoints need to be 25 m apart. We assume a symmetrical array with the sources between the streamers.
One possible arrangement, shown in Figure 12.2a, uses streamers 100 m apart and sources offset 25 m on opposite sides of the ship’s track.
In the inline direction hydrophone group spacing should be no more than 50 m, but this is not a limitation since streamer groups are usually spaced appreciably closer than this. Allowing 15 s between shots implies about 100 m source spacing when using two sources with the ship traveling at about 6 knots (problem 12.4a.) Shots from the two sources will be about 50 m apart in the inline direction so adjacent bins in the crossline direction involve different mixes of offsets. The CMP bins for this arrangement are shown in Figure 12.2a. The numbers within the bins show, respectively, the shortest inline and shortest crossline offsets of gathers (see problem 9.23) within the bins, assuming that the survey is regular and that the shortest inline offset is 100 m.
The azimuths for most of the traces in a gather will be nearly zero, that is, they are mostly inline. The largest azimuth would be for the bin where the crossline offset is 62.5 m and inline offset 100 m and where the angle with the inline direction is . This type of marine surveying is narrow-azimuth. The ship’s track for the next swath to extend coverage in the crossline direction should be moved 150 m assuming regular operations.
Answer the questions for two sources and four streamers
One possible arrangement with four streamers is shown in Figure 12.2b. The streamers are separated by 100 m and the sources are 25 m on opposite sides of the ship’s track.
Some of the CMP bins for this arrangement are shown in Figure 12.2b. The numbers within the bins show, respectively, the shortest inline and crossline offsets of the gathers within the bins, assuming that the survey is regular and that the shortest inline offset is 100 m.
Successive ship tracks for continuous coverage should be separated by 200 m assuming regular operations.
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Also in this chapter
- Spatial sampling restrictions
- Bin size in marine work
- Effect of crosscurrents
- Number of seismic sources
- Circle shooting
- Ocean-bottom cable surveys
- Vibroseis land survey
- Loop layout for a 3D survey
- Fault interpretation using time slices
- Acquisition direction for marine 3D surveys