Optimizing field layouts

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Problem

Seismic field work is usually carried out in a uniform manner with group intervals everywhere the same and the spread either symmetrical about, or on one side of, the source point. Layouts tend to be determined by the equipment at hand or by habit rather than by the problem to be solved; for example, the length of geophone strings (several geophones for a single group permanently wired together) may dictate the geophone interval and the available equipment the number of channels, and thus, the effective spread length. Hybrid spread arrangements are sometimes used to make fuller use of equipment.

Assume that you have more channels available than given by the rules stated in problem 8.12; what circumstances might lead you to use the extra channels

  1. to extend the spread to longer offsets than given by rule (1);
  2. to use shorter minimum offsets than given by rule (2);
  3. to interleaf additional groups somewhere in the spread;
  4. to lay out a partial spread on the other side of the source where an end-on arrangement is being used;
  5. to lay out a short cross arm?

    If you have almost but not quite enough channels to use a split arrangement compared to an end-on, what are the advantages and disadvantages of using a split spread with

  6. longer group intervals than given by rule (5);
  7. shortening the maximum offset; or
  8. increasing the minimum offset?

Background

Commonly used spread types are shown in Figure 8.13a. Spreads (ii), (iii), and (v) are used when source noise is a problem (spread (v) is also used to obtain larger offsets). A hybrid spread is one in which the group spacing is different for some groups, usually larger for long-offset groups.

Figure 8.13a.  Typical spreads using 24 groups. Geophone group and source locations are represented by x and o, respectively. (i) Split spread; (ii) split spread with offset source; (iii) gapped split spread; (iv) end-on spread; (v) inline offset spread; (vi) Cross-spread.

Coherence refers to the similarity of an event as seen on successive traces (see also problem 6.1); it is the most important factor in recognizing a reflection (or any event).

The manner in which amplitude varies with offset (or angle of incidence; see problem 3.12), called AVO (or AVA), depends on Poisson’s ratio (or ), which is sensitive to changes in lithology and the fluid contained in pore spaces.

Horizontal resolution is discussed in problem 6.2.

Solution

  1. Extending the spread to offsets larger than the depth of the deepest zone of interest could help in (i) mapping deeper; (ii) increasing the amount of NMO to get better velocity information; (iii) increasing the NMO differences between primaries and multiples to better attenuate multiples; (iv) decreasing source-generated noise; (v) getting better AVO data.
  2. Using minimum-offsets less than the depth of the shallowest zone of interest might yield useful shallow information, for example, it might yield data less confused by source-generated wavetrains, such as ground roll.
  3. Interleafing additional groups in the middle of the spread might (i) improve coherence and thus increase the detectability of weak and steeply dipping events; (ii) decrease the possibility of aliasing.
  4. Laying out a partial spread on the other side of the source where an end-on arrangement is being used would increase the amount of data and, hence, improve noise cancellation, especially shallow noise, and yield better measurement of dip.
  5. Laying out a short cross arm would be valuable to (i) measure the cross-dip or check that the line has in fact been laid out in the direction of dip; (ii) check against the possibility of noise arriving from the cross-spread direction.
  6. If we have almost but not quite enough channels to use a split arrangement compared to an end-on, a split spread with group intervals more than double the desired horizontal resolution may hurt coherence, permit aliasing of steeply dipping data, and degrade horizontal resolution. The split would add redundancy and perhaps attenuate noise.
  7. Without enough channels to use a split arrangement, shortening the maximum offset will give poorer moveout measurements with consequent poorer stacking-velocity values and less multiple attenuation. It will also discriminate against the deeper data. Closer sampling might define events better.
  8. Without enough channels to use a split arrangement, increasing the minimum offset may cause deterioration of the shallow data, but traces near the source may be noisy anyway. The advantages are closer sampling and increased redundancy.

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