Ocean-bottom cable surveys

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Problem 12.6a

A marine survey (Figure 12.6a) used four parallel ocean-bottom cables (OBC), each having 48 geophone/hydrophone groups spaced at 50-m intervals, the receiver lines being 400 m apart. A source boat towing an air-gun source traversed 19 lines spaced 250 m apart perpendicular to the receiver lines, each of the source lines being 2000 m long with air-gun pops every 50 m, thus covering nearly double the area occupied by the receiver lines. What minimum bin size can be used, and what multiplicity will be achieved?

Figure 12.6a.  Configuration used in ocean-bottom survey (only the left half of the area is shown, i.e., 10 of 19 source lines).

Background

Ocean-bottom recording permits recording among obstacles such as platforms where ships towing long streamers cannot operate. The use of geophone/hydrophone combinations permits excellent attenuation of surface multiples (see Sheriff and Geldart, 1995, section 7.5.5), usually the strongest multiples encountered. Ocean-bottom cables are also used to record three-component data so that S-waves (converted waves) can be studied, a technique called 4-C recording (three orthogonal geophones plus a hydrophone). Laying the cables is time consuming and not very precise, so that the receiver groups are not positioned as regularly as on land. The actual locations of the receiver groups can be determined from the arrivals of waves traveling directly from the various sources.

A reflection recorded by a detector is the sum of the waves reflected from all points of the reflecting surface, the major portion coming from a small circular area called the Fresnel zone (see problem 6.2). To achieve the correct amplitude for any point on the reflector, in migration we must sum all of the traces to which the point contributes, that is, over an area of the surface equal to the Fresnel-zone area.

Solution

For orthogonal source and receiver lines, the minimum bin size in the direction of source movement is half the source spacing and in the receiver direction, half the geophone group interval. With 50-m spacing for both receiver groups and source pops, the minimum bin size is .

Since data from all source locations are recorded at all receiver locations, the multiplicity for the first row of midpoints increases from five ones starting at the corners of the covered area, then five twos, five threes, etc., until it reaches nineteens. This multiplicity will be repeated for the first eight inlines of bins, then the multiplicity will double as receivers on the second cable begin to contribute, and for an area in the center the multiplicity will triple. Then it will decrease symmetrically toward the other edge of the survey. A portion of a corner of the covered area is shown in Figure 12.6b. However, if the cables are then moved forward and the pattern repeated, the coverage to the left of the cables will compensate for the taper and produce a more uniform coverage. Irregularities in the cable layout will produce minor variation in the uniformity of coverage. The long dashes outline the area over which CMP subsurface coverage is obtained.

Problem 12.6b

Assume that a deep objective horizon is a nearly horizontal erosional surface and that the trapping is stratigraphic, so that amplitudes must be mapped accurately. How large an area can be mapped with confidence?

Figure 12.6b.  Corner of Figure 12.6a showing bins and multiplicity in some of them.

Solution

Three factors affect the size of the areas that can be mapped with confidence.

  1. A region around the periphery of a survey area involves a taper zone where multiplicity decreases, and often this zone is about half the length of the spreads employed in acquiring the data; thus the useful area of full multiplicity is smaller than the acquisition area by the amount of this taper zone.
  2. Data usually have to be migrated to position features correctly and this introduces another peripheral zone whose dimensions depend on the depth and the angles that need to be incorporated in the migration, typically up to or . Migration is needed even for horizontal reflectors to sharpen fault evidences.
  3. A given point in the subsurface affects all detectors within an area equivalent to the Fresnel zone area so that, if amplitudes near the survey edge are to be compared to those in the central area, data must be available to be stacked. This area increases with the depth of the reflector. It involves a distance of the order of , where is the average velocity, the traveltime, and the frequency.

The result of these factors is that the acquisition area has to be larger than the area to be mapped with confidence. The effective peripheral fringe zone is at least as large as the largest of the individual fringe zones, but smaller than the sum of the three individual fringe zones. Since they will overlap. It should be considered in planning a survey.

Problem 12.6c

Assume that the objective formations dip away from one edge of the area, how does this affect the area that can be mapped confidently?

Solution

Dip offsets the area of confident coverage in the updip direction, but, if the dip is uniform, it does not significantly shrink the area of coverage. Dip of foreshortens coverage by only or 6%. The distance that dip moves the subsurface coverage depends on the raypath curvature, which, in turn, depends on the velocity gradient; it is apt to be about , where is the depth and is the dip angle.

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