Acquisition direction for marine 3D surveys

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Problem 12.10

Does it make any difference whether a 3D marine survey is recorded in the dip or strike direction?

Solution

The common assumption is that reciprocity applies, i.e., that one gets the identical result when source and receiver are interchanged. Both source and receiver groups have linear dimensions that are orthogonal to each other, and, hence, the smearing effects of group lengths will be in different directions. However, the dimensions are so small that this aspect is probably not important. Differences in midpoint locations within a bin will make minor differences when stacking, but this also is probably not important. Hence, it is probably unimportant that the entire system is moving during the recording. While the offsets will be slightly different for different streamers where multiple streamers are used, such effects can be accommodated in processing. We generally assume that the earth is isotropic, and this assumption probably does not create significant differences. Thus, we generally do not expect the shooting direction to introduce differences in the survey results.


Figure 12.9c.  A time slice.


Figure 12.9d.  Interpretation of Figure 12.9c showing possible faults.


However, where there are appreciable lateral changes in velocity, the raypaths for common reflecting points in different directions may differ appreciably, and this can produce quite different effects, especially if they are not allowed for before stacking. Poststack migration is especially apt to produce different results.

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