Figure 11

To verify the results of our geometric analysis of the kinematic properties of CIGs, we modeled and migrated a synthetic data set with a wide range of dips. The reflector has spherical shape with radius of 500 m. The center is at 1,000 meters depth and 3,560 meters horizontal coordinate. The velocity is constant and equal to 2,000 m/s. The data were recorded in 630 shot records. The first shot was located at a surface coordinate of -2,000 meters, and the shots were spaced 10 meters apart. The receiver array was configured with an asymmetric split-spread geometry. The minimum negative offset was constant and equal to -620 meters. The maximum offset was 4,400 meters for all the shots, with the exception of the first 100 shots (from -2,000 meters to -1,000 meters), where the maximum offset was 5,680 meters to record all the useful reflections. To avoid boundary artifacts at the top of the model, both sources and receivers were buried 250 meters deep. Some of the reflections from the top of the sphere were muted out before migration to avoid migration artifacts caused by spurious correlations with the first arrival of the source wavefield. The whole data set was migrated twice: first using the correct velocity (2,000 m/s), and second after scaling the slowness function by a constant factor (corresponding to a velocity of 1,923 m/s). The ADCIGs shown in this section and the following section were computed by merging the ADCIGs computed from both the HOCIGs and VOCIGs according to the robust algorithm presented in the previous section.

Figure a shows the zero-offset section (stack) of the migrated cubes with the correct velocity and Figure b shows the zero-offset section obtained with the low velocity. Notice that, despite the large distance between the first shot and the left edge of the sphere (about 5,000 meters), normal incidence reflections illuminate the target only up to about 70 degrees. As we will see in the angle-domain CIGs, the aperture angle coverage shrinks dramatically with increasing reflector dip. On the other hand, real data cases are likely to have a vertical velocity gradient that improves the angle coverage of steeply dipping reflectors.

Figure 12

- Transformation of HOCIGs and VOCIGs to GOCIGs
- Image mispositioning in ADCIGs migrated with wrong velocity

7/8/2003