Example common-image gathers

Figure 3 illustrates the effect velocity plays on offset-domain common-image gathers. The three panels show CIGs produced by migrating the Marmousi synthetic dataset with three different velocity models: the correct velocity [panel (a)], a velocity that is too low [panel (b)], and a velocity model that is too high [panel (c)]. Interpreting patterns in the offset-domain CIGs is difficult; however, after transformation to the angle domain (Figure 4, standard residual-moveout patterns indicates the sign of the velocity error: events curving up meaning too low, and events curving down meaning too high.

To illustrate the problems associated with sampling for shot-profile migrations, Figure 5 shows the same image gathers as Figure 3, but after migrating only every twentieth shot. Even if the velocity is correct, energy does not cancel at non-zero offsets, and so events are not flat in the angle-domain (Figure 6). 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 1991 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.

 

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Figure 3 Offset-domain common-image gathers. Panels (a), (b), and (c) were migrated with velocity models that were correct, 6% too low, and 6% too high, respectively.

 

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Figure 4 Angle-domain common-image gathers. Panels (a), (b) and (c) were migrated with velocity models that were correct, too low, and too high, respectively.