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 | Interpolation of near offsets using multiples and prediction-error filters |  |
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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.
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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
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.
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3dsynpseudo
Figure 17. pseudoprimaries generated
for a single receiver cable from six different crossline source
positions.
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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
sources for each of the three sail lines are added
together to produce the pseudoprimary image in Figure
17, generated from roughly
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
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.
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 | Interpolation of near offsets using multiples and prediction-error filters |  |
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Next: Conclusions and future work
Up: Curry and Shan: Near-offset
Previous: Interpolation of field data
2009-04-13