Effect of shot sampling on common-image gathers

As discussed previously in Chapter , datasets collected with sparse-shot geometries are most suitable for shot-profile migration. Figure  compares the migrated (zero-offset) images of the two prestack datasets. The dataset that produced panel (a) had a fully-sampled shot axis, while the shot axis on the dataset that produced panel (b) contained only every twentieth shot. Even with the very sparsely-sampled shot axis, the geologic structure is clearly interpretable in Figure  (b).

 

zoff
Figure 4 Migrated images produced with data having a fully-sampled shot axis (a), and a very sparsely-sampled shot axis (b).

To illustrate the problems associated with sampling for shot-profile migrations, Figure  shows the same image gathers as Figure , but after migrating only every twentieth shot. Energy is no longer concentrated around zero-offset.

Figure  (a) shows the equivalent picture in the angle domain. Even if though the velocity is correct and the zero-offset image [Figure  (b)] seems reasonable, coverage in the angle-domain is very irregular. When the velocities are incorrect, the angle gathers remain chaotic: shot aliasing has effectively rendered the angle-gathers uninterpretable in terms of velocities.

Although both de Bruin's 1990 original methodology and the approach described here provide means of obtaining common-image gathers from shot-profile migration, the problem of shot aliasing remains important for the geometries that are best suited to shot-profile migration.

 

offvel4000
Figure 5 Offset-domain common-image gathers corresponding to x=4000 m. Panels (a), (b), and (c) were migrated with velocity models that were correct, 6% too low, and 6% too high, respectively.

 

angvel4000
Figure 6 Angle-domain common-image gathers corresponding to x=4000 m. Panels (a), (b) and (c) were migrated with velocity models that were correct, too low, and too high, respectively.