Loop layout for a 3D survey

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Problem 12.8

In one 3D technique, source points () and geophones () are laid out around a loop such as the square shown in Figure 12.8a and all of the geophones are recorded for each source point. This example employs 48 geophone stations spaced 50 m apart and 24 source points spaced 100 m apart. Locate the midpoints and determine their multiplicity.

Solution

The minimum bin size has sides half the geophone spacing; the lines in Figure 12.8b indicate the bin centers.

Ignoring the sources outside the square for the moment, the sources and geophones along the top of the square will produce inline multiplicity along this line. The source and geophone in the upper left-hand corner will produce a zero-offset trace at that midpoint. This source and geophone #2 will give a midpoint trace between geophones 1 and 2. This source and geophone #3 will give a midpoint trace at geophone #2 as will the source at geophone 3 into geophone #1, giving 2-fold data here. However, both of these involve the same travelpath and so do not produce independent information and so only one counts as increasing the multiplicity. The midpoint at geophone #3 will have multiplicity of 2, once for source-geophone locations at 1 and 5 and once for coincident source and geophone at location 3. The sources along the west side of the area and geophones along the north side will give single-fold coverage over much of the interior of the square, but will leave many bins empty because there are only half as many source locations as geophone locations. Sources along the west side and geophone locations along the east side will provide multiplicity along the north-south bisector of the square. Most of these will not duplicate the raypaths from sources on the east and geophones on the west, and hence, the multiplicity down this bisector will be larger than that along the edges of the square. The multiplicity achieved is shown in Figure 12.8b.

Figure 12.8a.  Loop of geophones (), sources ().
Figure 12.2b.  Multiplicity achieved in one quadrant ignoring sources outside the square.

The sources outside the square will expand the coverage area and also increase the multiplicity along the edges of the square, as indicated in Figure 12.8c.

If adjacent squares are shot by repeating locations along an edge of the square, the midpoints outside one square will fall inside another square. But, in general, this will not increase the multiplicity because reciprocal raypaths will be involved.

Figure 12.8c.  Multiplicity achieved in one quadrant from all sources.

If the loop had been irregular rather than square, the irregularity would have produced irregularities in the distribution of the midpoints and changed the multiplicity somewhat.

Working out the multiplicity pattern longhand is not only tedious but also very subject to errors. Ordinarily a computer is used to make maps showing the multiplicity. The mixture of offsets involved is often at least as important as the multiplicity, and so, maps are also made showing the offset distribution. Likewise maps are often made showing the distribution of azimuths.

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