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Next: Conclusions and future work Up: Curry and Shan: Near-offset Previous: Interpolation of field data

Pseudoprimaries in 3D

The pseudoprimaries generated so far have been either for a two-dimensional synthetic model or for two-dimensional field data example with strong multiple reflections. We now attempt a 3D pseudoprimary generation on 3D prestack synthetic data where the geology is fully known and the acquisition is along ideal overlapping straight lines.

3dpseudo
Figure 16.
Crossline source distributions for a pseudoprimares generated from a single receiver cable at zero offset. $ [{\bf NR}]$
3dpseudo
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Figure 16 is an illustration of the source distribution for a single receiver line from a 3D synthetic dataset, courtesy of ExxonMobil.The data are composed of a horizontal water-bottom reflection under which there is a prism filled with point diffractors. At slightly below the arrival of the diffracted multiples, three reflectors are present. The acquisition geometry, shown in Figure 16, is ideal, so that at each subsequent sail line the cables shift by a distance exactly equal to that between four receiver cables. As such, any one receiver cable location occurs for three sail lines given the recording aperture of $ 550$ m, which when multiplied by the two crossline source positions per sail line, gives six crossline source contributions for any one crossline receiver location under these ideal circumstances.

3dsynpseudo
3dsynpseudo
Figure 17.
pseudoprimaries generated for a single receiver cable from six different crossline source positions. $ [{\bf CR}]$
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Figure 17 is the result of creating pseudoprimaries for zero crossline offset for a single receiver cable, where each active receiver location is cross-correlated with every other receiver location along the same cable for all sources in the three sail lines. These $ 640$ sources for each of the three sail lines are added together to produce the pseudoprimary image in Figure 17, generated from roughly $ 4$ TB of individual crosscorrelations. While the water-bottom reflection initially appears to be in the correct place, the diffracted multiples are a blurred mess at near offsets and are absent at the far offsets where the limbs of the recorded diffractions are in the recorded data. There are also apparent reflectors below the water bottom that are created from the correlation of the water-bottom primary reflection with the much deeper series of three primary reflections at below the water-bottom multiple arrival time, illustrating the potential pitfalls of unwanted correlations.

In order to determine the variability of the predicted pseudoprimares with crossline source position, we have produced a crossline gather similar to the pseudoprimary contribution gathers we produced earlier in this chapter. While the full 3D uncollapsed cube of crosscorrelations would have $ 7$ dimensions: time, inline and crossline source, inline and crossline first receiver, and inline and crossline second receiver, we only show the predicted multiples for a single first receiver (virtual source), second receiver, and inline source position, showing how the predicted multiples vary as the crossline location of the source varies. Figure 18 contains two of these gathers, where 18a is a zoom in on an area of the data with little diffracted multiples at the water bottom. The arrival time of the water bottom changes as a function of distance from the receiver cable to the source, with the stationary phase point at zero. Looking at a larger section of the time axis for a different location located in the cloud of diffractions below the water-bottom in Figure 18b, there is some similarity between the traces in adjacent flip-flop shots, but very little similarity between the three different sail lines, which in part explains the incoherence of the diffracted pseudoprimaries in Figure 17.

3dsynpseudogather
Figure 18.
A crossline pseudoprimary contribution gather. The location of the water-bottom reflection changes as a function of the crossline distance from the inline location of the pseudoprimaries to the six sources. $ [{\bf CR}]$
3dsynpseudogather
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next up previous [pdf]

Next: Conclusions and future work Up: Curry and Shan: Near-offset Previous: Interpolation of field data

2009-04-13