Migration Results

For each of the three gathers described in the previous section the PP reflections, PS reflections, and PS transmissions were migrated with reduced-time migration. The grid spacing for the migration model was 3 m in both the offset and depth directions. Conventional migration as well as reduced-time migration were applied to these data although no significant difference could be determined between the two. I find that the velocity models accurately describe the true earth velocity. The average time difference between the calculated direct-P traveltime ( $\tau_{sg}^{calc}$) and the picked direct-P traveltime ( $\tau_{sg}^{obs}$) was about 6 ms (6 data time samples). Since there is little difference between the two I only show the reduced-time examples.

I first show the results for the 152 m offset gather followed by the 610 m offset gather. For comparison I generated a synthetic migration section (Figure 4.11 left) based solely on the well log velocity (Figure 4.7).

Using Kirchhoff migration I migrated the 152 m offset Z-component gather with isolated P-wave reflections (Figure 4.10) to obtain the reflection section shown in Figure 4.11. The prominent event at 3171 m is the base of salt reflection and correlates with the same event in the synthetic section. A similar event can be found somewhat higher in the reflected PS image, Figure 4.12. This figure shows another event at 2805 m which correlates with the top of salt. The transmitted PS migration image (Figure 4.13) shows a strong event at 2805 m which correlates well with the top of salt. In this figure I reversed the polarity of the synthetic section to accentuate the top boundary of the salt. Also, in this figure the reader may notice that the events below the top of salt dip steeply to the right. Since the geologic interfaces in this region are assumed to be flat and appear to be so in the other migrated sections, I assume that this dip may be caused by salt anisotropy. A supporting argument could be that the previous image (Figure 4.12) contains the Y-component while this image contains the X-component.

Figures 4.14, 4.15, 4.16, contain, respectively, the reflected P, the reflected PS, and the transmitted PS migrated images for a source offset of 610 m. In Figure 4.16 notice the prominent events at 3171 m, the base of salt, in the reflected images. There is a good correlation between the top of salt, above the uppermost receiver at 3049 m, and the prominent event in the migrated section. Figure 4.17 shows a comparison between the wavepath and Kirchhoff migration images for transmitted PS arrivals from sources at 610 m offset. Although the Kirchhoff images contain most of the same coherent events the wavepath image contains much less noise. Also, the events in the wavepath image have a higher wave number, indicating better focusing, than their counterparts in the Kirchhoff image.

Figure 4.11: Migration of the gather shown at the top of Figure 4.10, the reflected P-waves. A synthetic zero-offset reflection section calculated solely from borehole velocities is shown on the left. There is good correlation with the base of salt and the strong event in the migrated gather.

Figure 4.12: Migration of the reflected PS transmitted wave gather shown at the bottom of Figure 4.10. There are strong events at both the base and top of the salt sheet.

Figure 4.13: Migration of transmitted PS arrivals (right). The traces of the synthetic depth section have been rotated 180 degrees from the previous two figures to emphasize the top of the salt boundary. Notice the excellent correlation between the top of salt in the synthetic and migrated traces. The dip of the lower events may be due to anisotropy because corresponding events in Figure 4.12 (right) are flat.

Figure 4.14: Migration of reflected P-waves from a source position of 610 m. The active receivers range from depths of 3049 to 4482 m.

Figure 4.15: Migration image of reflected PS waves for a source at 610 m offset.

Figure 4.16: Migration of transmitted PS waves. Notice that there are migrated events above to uppermost receiver (3049 m). The dipping events in this gather may be due to shear-wave anisotropy within the salt.

Figure 4.17: Comparison of wavepath and Kirchhoff migration for transmitted PS arrivals. The wavepath image contains much less noise and is richer in high wavenumber energy.