Proposed Methodology

In 2-D acquisition the geometry of the layout is almost always chosen to be either split-spread (land) or end-on with the shot pulling the cable (marine), so there isn't much flexibility in that respect. Illumination requirements essentially control the maximum offset of interest, which will change along the line profile as the target depths, dips and velocities change. The fold requirements may also change along the line profile but this situation is less common. The other parameter that may change is the receiver group interval but we have a strong limitation in the choice of this parameter because the seismic cables usually have take outs only at predetermined distances.

The proposed strategy for a more flexible acquisition is the following:

1.

Construct a subsurface model of the survey area as accurately as possible in terms of the geometries of the target reflections and interval velocities.

2.

Do non-zero-offset exploding reflector modeling using this model. This means to consider ``all'' points along each of the target reflections as a Huygen's source and to track rays up to the surface at given uniform reflection aperture-angle increments. This would guarantee perfect illumination if in fact the given rays can be generated during the acquisition. For each pair of rays (with the same aperture angle on opposite sides of the normal to the reflector) record their emergence positions at the surface.

3.

Clearly, the optimum source and receiver positions for each target will be different from those of the other targets. Reconciling these optimum illumination source and receiver positions can be posed as a non-linear inversion procedure. Inputs will be the source and receiver positions required to illuminate each target and output will be the source and receiver positions that minimize the sum of the deviations from the requirements for each target. For this process to be useful, several constraints must be imposed. For land acquisition the most important ones will be:

(a)

The receivers must be equally spaced at a distance consistent with the cable take-outs of the seismic cables.

(b)

The number of shots should be kept to a minimum.

(c)

in 2-D, source and receivers should be kept along the profile line.

More subtle, and perhaps more difficult constraints to honor, will be those related to spatial sampling considerations for prestack migration. Since the emphasis of the acquisition design will be placed on maintaining regularity of subsurface illumination, surface parameters such as offset and fold will not be uniform. Adequate sampling of offsets and azimuths, however, is a stringent requirement that must be honored by the inversion procedure. It is also important to note that the optimum geometry may not provide uniform illumination, but will likely provide better illumination than the standard approach.

4.

Finally, the optimum source and receiver positions will be output in a suitable format such as Shell's SPS geometry format. These geometry files can be readily input to the acquisition instrument.