# Vibroseis land survey

Series Geophysical References Series Problems in Exploration Seismology and their Solutions Lloyd P. Geldart and Robert E. Sheriff 12 469 - 484 http://dx.doi.org/10.1190/1.9781560801733 ISBN 9781560801153 SEG Online Store

## Problem 12.7

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. Figure 12.7a.  Layout for a vibroseis survey shot in four swaths. Layouts for the first and last of the four swaths are shown (after Hardage, 1993). Figure 12.2b.  Zones of multiplicity for $110\times 110\ {\rm {ft}}$ bins for half the swath. Note that each zone is eight bins wide by 12 bins tall. Bins in the top three rows get additional coverage from the swath to the north. The pattern uses six lines of geophones spaced 1320 ft apart and 15 lines of sources spaced 880 ft apart.

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?

### Solution

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 $55\times 110\ {\rm {ft}}$ as any smaller size will leave many empty bins. If square bins are desired, $110\times 110\ {\rm {ft}}$ 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 $8\times 12$ bins, each $110\times 110\ {\rm {ft}}$ 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 $220\times 220\ {\rm {ft}}$ 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.