Comparison of common-azimuth migration with offset plane wave migration

The velocity above the salt body does not vary rapidly in the lateral directions. Therefore the images obtained by the hybrid offset plane wave migration (B) and the ``real'' offset plane wave migration (C) are very similar. On the contrary, below the salt body, the approximation introduced by downward-continuing the offset plane wave separately causes significant problems in the imaging of the sub-salt reflectors. The next four figures compare the deeper portion (starting from a minimum depth of 1,500 m) of two in-line sections and two cross-line sections obtained by migrations (A) and (C). The sub-salt images are considerably noisier than the above-salt images. To facilitate the analysis, I show also the corresponding sections of the velocity model.

Figure 8 compares the in-line sections obtained by common-azimuth migration (A) and the offset plane wave migration (C); Figure 9 shows the corresponding section of the velocity model. The bottom of the salt body, and in particular the dipping segment at the left edge of the salt, is better imaged by migration (A) than by migration (C). In the shadow of the salt body, the image of the basement is also more continuous in panel a) than in panel b).

Figure 10 compares the in-line sections obtained by common-azimuth migration (A) and the offset plane wave migration (C); Figure 11 shows the corresponding section of the velocity model. Both sand lenses visible in the right part of the sections are better imaged by migration (A) than by migration (C); in particular, the deeper lens just above the basement reflector is more clearly defined in panel a) than in panel b). The image of the basement is also more continuous in panel a) than in panel b). Possibly the most interesting difference is in the imaging of the anticlinal structure broken by converging normal faults visible in between the two sand lenses. Migration (A) produces an image, though fairly faint, of this structure even in the shadow of the salt body. In contrast, the structure disappears from the image produced by migration (C) as soon as it gets in the shadow of the salt body.

Figure 12 compares the cross-line sections obtained by common-azimuth migration (A) and the offset plane wave migration (C); Figure 13 shows the corresponding section of the velocity model. The dipping segment of the bottom of the salt that belongs to the fault plane cutting through the salt body (cross-line location 8,000 m) is better imaged by migration (A) than by migration (C). The basement reflector is also more continuous in panel a) than in panel b).

Figure 14 compares the cross-line sections obtained by common-azimuth migration (A) and the offset plane wave migration (C); Figure 15 shows the corresponding section of the velocity model. As for the in-line sections shown in Figure 10, both sand lenses visible in the right part of the sections are better imaged by migration (A) than by migration (C). Though discontinuous in places, the basement reflector is also better imaged by migration (A) than by migration (C). The bottom of the salt reflector right below the deep canyons in the salt body (in-line location 8,000-9,200 m) is fairly coherent in panel a), while it is discontinuous in panel b).

 

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Figure 8 In-line sections (cross-line location 6,300 m) from the images obtained with a) common-azimuth migration (A), b) offset plane wave migration (C).

 

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Figure 9 In-line section of the velocity model at cross-line location 6,300 m.

 

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Figure 10 In-line sections (cross-line location 9,980 m) from the images obtained with a) common-azimuth migration (A), b) offset plane wave migration (C).

 

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Figure 11 In-line section of the velocity model at cross-line location 9,980 m.

 

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Figure 12 Cross-line sections (in-line location 5,180 m) from the images obtained with a) common-azimuth migration (A), b) offset plane wave migration (C).

 

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Figure 13 Cross-line section of the velocity model at in-line location 5,180 m.

 

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Figure 14 Cross-line sections (in-line location 7,300 m) from the images obtained with a) common-azimuth migration (A), b) offset plane wave migration (C).

 

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Figure 15 Cross-line section of the velocity model at in-line location 7,300 m.