Time migration velocity analysis by velocity continuation

Field Data Example

Figure 13 compares the result of a constant-velocity prestack migration with the velocity of 2 km/s, applied to a dataset from the North Sea (courtesy of Elf Aquitaine) and the result of velocity continuation to the same velocity from a migration with a smaller velocity of 1.4 km/s (Figure 13a). The two images (Figures 13b and 13c) look remarkably similar, in full accordance with the theory.

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Figure 13. Constant-offset section of the North Sea dataset after migration with the velocity of 1.4 km/s (a), migration with the velocity of 2 km/s (b), migration with the velocity of 1.4 km/s and velocity continuation to 2 km/s (c).

Figure 14 shows a result of two-dimensional velocity picking after velocity continuation. I used values of $\epsilon=0.1$ and $\lambda=0.1$. The first parameter controls the vertical smoothing of velocities, while the second parameter controls the amount of lateral continuity.

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Figure 14. Automatically picked migration velocity after velocity continuation.

Figure 15 shows the final result of velocity continuation: an image, obtained by slicing through the velocity cube with the picked imaging velocities. The edges of the salt body in the middle of the section have been sharply focused by the velocity continuation process. To transform the already well focused image into the depth domain, one may proceed in a way similar to hybrid migration: demigration to zero-offset, followed by post-stack depth migration (Kim et al., 1997). This step would require constructing an interval velocity model from the picked imaging velocities.

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Figure 15. Final result of velocity continuation: seismic image, obtained by slicing through the velocity cube.

Time migration velocity analysis by velocity continuation