Second iteration

In an attempt to improve the focusing of the reflectors under the salt edge, I performed another iteration of tomography. This time I concentrated on accurately picking reflectors as close to the salt edge as possible. In addition I relaxed the constraint on model smoothness to allow greater changes. Figure 16 shows the updated velocity model. The changes introduced to the velocity model are significantly greater than in the first iteration of tomography (Figures 11 and 12).

 

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Figure 16 Velocity after two iterations of tomography.

If we look at the same two cube views (as Figures 3, 4, 5, 16, 13, 14, and 16), we see improved focusing of the reflectors, especially as we approach the salt edge. In Figure 17 note the improvement over Figures 4 and 13 at `A', `C' and `D'. In Figure 18 we see improvements over Figures 5 and 14 at the same general locations (`A', `C' and D). If we look at the gathers from the same crossline as Figure 6 (Figure 19) we see that they considerably flatter and more coherent than in the initial migration.

 

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Figure 17 Slices from the 3-D cube using the velocity of Figure 16

 

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Figure 18 Slices from the 3-D cube using the velocity of Figure 16

 

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Figure 19 Gathers from the same crossline as Figure 6.

If we take a close up view of the area to the left of the salt, Figure 20, the improvement in image quality is very apparent. The salt bottom reflector (`A') is more continuous. The reflectors along the edge of the salt (`B') extend almost to the edge of the salt and are much more consistent in amplitude. In the crossline view we see reflector continuity (`C') that was not apparent in the initial image.

A close up view of the right portion of the image Figure 21 also shows improvement. The strong reflector above the salt (`A') is discontinuous with the initial velocity model, with the improved velocity model it is much more believable. The reflectors in the basin to the right of the salt `D' are more consistent in amplitude. The fault in the crossline `C' is better focused and we have generally more energy in the crossline reflectors below the fault. Finally, the depth section `B' is higher frequency and shows more continuous reflectors.

 

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Figure 20 A comparison along the left edge of the salt dome. The top image is using the starting velocity, Figure 3, the bottom using the velocity after two iterations (Figure 16).

 

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Figure 21 A comparison along the right edge of the salt dome. The top image is using the starting velocity, Figure 3, the bottom using the velocity after two iterations (Figure 16).