Stacking charts

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Problem 8.3a

Draw surface and subsurface stacking charts for a 24-trace split spread where the source is midway between the centers of geophone groups 12 and 13, which are separated by three times the normal geophone-group spacing. Assume a source interval double the geophone-group interval and that source and geophone locations beyond a location at the right-hand end of the line cannot be occupied, but the source can move as far as this location. Thus spreads near the end of the line will be asymmetric; this technique is called “shooting through the spread.”

Background

In seismic field work, several relatively closely spaced geophones are connected together to constitute a geophone group; the group is thought of as equivalent to a single large geophone located at the group center. A spread denotes all the geophone groups that record signals from a single source.

A split spread (also called split dip) has the source located at the midpoint of a linear spread. If the source is moved forward half the spread length and the rear half of the cable and geophones are moved forward to become the front half, each subsurface reflecting point will be sampled once, resulting in single-fold continuous subsurface coverage.

In the common midpoint (CMP) method (see problem 5.12), many source and geophone-group locations are used. Stacking charts are used to keep track of locations; stacking charts have geophone-group locations along one axis and source locations along the other (see Figure 8.3a). A trace observed at group location when the source is at is plotted at in a surface stacking chart, and at at in a subsurface stacking chart (see Figures 8.3a,c). Figure 8.3a assumes that the fifth source location is at rather than at E. On stacking charts, straight lines locate all of the traces involving a common-geophone, common-source, common-offset, or common-midpoint. In the CMP method, traces having the same midpoint are stacked together. The number of traces stacked together (after statics and NMO corrections) is the multiplicity (see problem 5.12).

Solution

Figure 8.3b shows the end of a surface stacking chart where “shooting through the spread” occurs. Where the pattern is regular the multiplicity is 6, as shown by the common-midpoint lines drawn on the figure at angles. The irregularity at the end of the line adds additional longer-offset traces and steepens the end-of-line taper. The resulting multiplicity at the end of the line is 6, 6, 6, 7, 7, 8, 8, 9, 9, 9, 8, 8, 7, 7, 6, 6, 4, 4, 2, 2. The corresponding subsurface stacking chart is shown in Figure 8.3c.

Note that where the source occupies a geophone location, pairs of traces on common-midpoint gathers (see problem 9.23) on opposite sides of the source have the same offset distances, whereas if the source occupies a location between geophone-station locations, then offsets in opposite directions from the source (as in this example) do not pair up. The result is more offset distances on the common-midpoint gathers, which is generally preferable.

Problem 8.3b

Assume that geophones, but not sources, can be located over a distance of geophone-group intervals centered at point in Figure 8.3d. What is the effect on the multiplicity? If the data deteriorate markedly when multiplicity falls, what can be done to alleviate the deterioration?

Solution

Not being able to locate sources for eight geophone group intervals creates tapers on either side of the zone, as indicated by the common-midpoint lines drawn on Figure 8.3d. Additional source locations (for example, midway between the source locations) on each side of zone could add to the multiplicity while creating irregularities in the offset distribution.

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