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Reference velocities

The bit precision limiting scheme used in the previous section should either have a relatively uniform, or somewhat unpredictable, amplitude effect. A more interesting test is to apply both difference detecting methodologies on a problem where the locations of the differences are well known.

PSPI migration accurately handles non-overturning waves when the velocity in a given depth layer is constant. When velocity varies laterally the accuracy decreases as a function of propagation angle. The Marmousi example has significant lateral velocity variation and many steeply dipping events. As a result the image quality varies with the number of reference velocities that are used. Figure 9 shows the effect of changing the number of reference velocities used in the migration.The rows represent migrating with a PSPI migration using one, three, and seven reference velocities. The left column is the migrated image, the center panel is the result of using the PEF method for detecting differences. The right panel shows the result of using the histogram matching. In this case, the PEF approach proves to be better at identifying differences in the images. Note how the PEF approach highlights the steeply dipping features, where the PSPI approximation breaks down. With increasing number of reference velocities only steeper dipping features are displayed. The histogram approach seems to simply highlight the major features in the image, showing limited preference towards large dips. Figure 10 shows the total differences as a function of the number of reference velocities. Note how the effect of increasing the number of reference velocities is much more pronounced in the PEF case.

big-v
big-v
Figure 9.
The rows represent migrating with a PSPI migration using one, three, and seven reference velocities. The left column is the migrated image, the center panel is the result of using the PEF method for detecting differences. The right panel shows the result of using the histogram matching.
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v-graph
Figure 10.
A graph of the differences detected by the PEF (solid line) and histogram matching (dashed line) scheme.
v-graph
[pdf] [png]


next up previous [pdf]

Next: Planewave migration Up: R. Clapp: PEF enhancement Previous: Precision

2007-09-18