SEG-EAGE salt data set migration results

The improvement in image quality achieved by applying source-receiver migration on a narrow range of cross-line offsets in conjunction with the coplanarity condition is demonstrated in the following results obtained from the SEG-EAGE salt data set. Figure a shows the in-line section of the velocity model taken at cross-line location of 5,770 meters. Figure b shows the corresponding migrated image obtained by common-azimuth migration. The section is well imaged everywhere, with the exception of the bottom of the salt around in-line location of 4,000 meters. This inaccuracy in the image is likely to be caused by the common-azimuth approximation.

Figure  compares the results of common-azimuth migration and full source-receiver migration with 8 cross-line offsets and the application of the coplanarity condition. It shows three zooms into the area of interest. Figure a shows the velocity model. Figure b shows the image obtained by common-azimuth migration. Figure c shows the result of stacking the images obtained by applying the coplanarity condition on the azimuthal range defined by $\left\vert\beta\right\vert \leq 16$ degrees. Notice the improved definition of the bottom of the salt in Figure c compared to Figure b.

Similar improvements are visible in the corresponding depth slices. Figure  compares the slices taken at a depth of 2,600 meters. Figure a shows the velocity model. Figure b shows the image obtained by common-azimuth migration. Figure c shows the result of stacking the images obtained by applying the coplanarity condition on the azimuthal range defined by $\left\vert\beta\right\vert \leq 16$ degrees. Now the salt bottom boundary located between in-line locations of 4,000 and 4,500 meters is well defined. Notice that the portion of the salt boundary that is not well delineated by the image (between in-line locations of 3,000 and 3,500 meters) is not properly illuminated by the data.

 

both-sec-y5770 Figure 13 In-line sections (ym=5,770 m): (a) the velocity model, (b) common-azimuth migration.

 

all-y5770 Figure 14 Zooms of the in-line sections (ym=5,770 m): (a) the velocity model (b) common-azimuth migration, (c) full source-receiver migration with 8 cross-line offsets and the application of the coplanarity condition.

 

all-z2600 Figure 15 Zooms of the depth slices (z=2,600 m): (a) the velocity model (b) common-azimuth migration, (c) full source-receiver migration with 8 cross-line offsets and the application of the coplanarity condition.

I presented a ``narrow-azimuth'' generalization of common-azimuth migration that overcomes the accuracy limitations and retains the computational efficiency of the original method. The new method is based on: 1) the definition of an ``optimal'' range of cross-line offset dips for the downward continuation, and 2) application of a ``coplanarity'' condition on the prestack image for enhancing the correctly focused events. The migration examples show that the new method has the potential of correcting the inaccuracy introduced by common-azimuth migration even in challenging situations such as the one presented by the SEG-EAGE salt data set.

I would like to thank Bob Clapp and Paul Sava for developing the WEI library that allowed me to run wave-equation prestack migration using MPI. Without WEI I could not have tested narrow-azimuth migration on the SEG-EAGE data set.