Angle-domain CIG and velocity

Angle-domain CIGs can be used to update the velocity function after migration similarly to the way that offset-domain CIGs are currently used Brandsberg-Dahl et al. (1999) or for wave-equation Migration Velocity Analysis Biondi and Sava (1999). As for offset-domain CIGs, if the velocity function is correct the reflections are aligned along the angle axis. If the velocity function is too low the reflections will smile upward; if the velocity function is too high the reflections will frown downward. This behavior is demonstrated by the analysis of the gathers in Figure 6. The gathers were extracted from a 3-D prestack wavefield focused using common-azimuth downward continuation. The left gather was obtained using the correct velocity. The right gather was obtained using a low constant velocity. Figure 7 shows the inline migrated section that passes through the gather shown in Figure 6. Notice that the CIG gathers show only the first kilometer of the image.

 

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Figure 6 Left: Angle-domain CIG with correct velocity. Right: Angle-domain CIG with too low constant velocity.

 

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Figure 7 In-line section of 3-D migrated cube