Advanced Topics In 3-D Survey Design
Relatively simple geometries for design are effective on flat layers and gently dipping surfaces. In the presence of folds, faults, domes, reefs, and overthrusts, however, wavefield behavior becomes difficult to analyze.
An extreme case is imaging the overhang area of a salt dome, as illustrated in Figure 141.
Only upcoming waves can strike the overhang zone directly. Turning waves have been used successfully to image this part of the structure. Turning waves require long offsets and certain velocity gradients. Modeling would seem to be a way to improve the survey design to be sure to include such waves.
Consider a simpler problem with an obstacle and a slope on the target horizon as shown in Figure 142.
The raypath from the source is seen to surface under the obstacle. Conventional design would be along the path from the source to the marked receiver. It would be assumed that the obstacle area was covered by undershooting. Actually, the surface emergent response is under the obstacle where no receivers are possible. No amount of DMO, migration, or other exotic data processing can recover the sloping portion of the target horizon. This might be puzzling to the data processors and chalked up as just another one of those unexplained problems in imaging structure. This structure is not complex, but illustrates that 2-D modeling, when the nature of the target is known, may avoid disappointment. Of course, the slope may not be known prior to the survey. In many cases, shelf-edge margins, large faults,