Vibroseis land survey
A land survey layout is shown in Figure 12.7a. A single swath used 112 geophone groups spaced 110 ft apart on each of six east-west lines (solid lines) with vibrators traversing north-south lines (dashed) with vibrator points spaced 220 ft apart. For the next swath to the north, the three southernmost lines of geophones were moved to new lines north of swath #1. The entire area of 8.2 square miles was thus mapped in four swaths, but the layout pattern had to be modified in the north because of permit restrictions.
In the southern two-thirds of the area where spacing was regular, what is the smallest bin size that should be used? What is the best multiplicity achieved? How wide is the multiplicity taper area? What is the smallest bin size if square bins are desired, and, for the best multiplicity bins, what are the offset and azimuth ranges? How will this change if four of the smallest square bins are combined to give larger square bins?
Midpoints in the cable direction will be spaced at half the group interval or 55 ft; in the source direction at half the vibrator-point spacing, or 110 ft. Thus the minimum bin size is as any smaller size will leave many empty bins. If square bins are desired, bins would have a multiplicity of two in the corners of the survey area and build up to 28 along the southern edge (see Figure 12.7b). This figure shows the multiplicity of bins in each rectangle (each rectangle contains bins, each bins). The buildup of multiplicity along the east and west edges of the survey area will increase by two up to a maximum of eight. Azimuths for the center bins cover all directions. If bins are used, the multiplicity values will simply be the sums of the four bins that are combined. The second swath will overlap the two northernmost sets of rectangles, resulting in zones of coverage along the east and west edges of 2, 4, 6, 8, 6, 6, 6, 6, 8, and then repeats in descending order, and in the north-south direction through the center of the area, 28, 56, 84, 112, 112, 84, 84, 84, 84, 112, 112, etc.
Access restrictions in the northwest portion of the area limited the vibrator lines, which results in irregularities in the multiplicity but not to a major degree. The multiplicity degradation will not be very severe since no change in geophone layout is indicated and only a few vibrator lines are missing completely.
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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