Inline image gathers

Figure  shows the same inline section as in Figure  but this time taken from the migration of only one sail line. Notice that the salt boundaries are poorly imaged because of the complexity of the salt geometry in the cross-line direction. Much more data needs to be migrated from the cross-line direction in order to get these reflections as can be seen by comparing with Figure . The right hand panel is now a subsurface offset gather taken at the center of the window in Figure  (CMP position 13200 m). Although not all the multiples are imaged in this migration because of the lack of cross-line data as indicated for the salt reflections, those imaged can be identified by their moveout away form zero toward the negative subsurface offsets in the offset gather. Notice in particular the multiple at about 4 km depth and -400 m subsurface offset.

 

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Figure 10 Migration of one sail line. The left panel is an inline subsurface offset common image gather.

Figure  shows three subsurface offset gathers taken at different lateral positions. The first SODCIG (panel (a)), taken at about the center of the window in Figure (a) (CMP position 13200 m), shows evidence of multiples at about 4000 m depth at -400 m half-subsurface offset. This multiple is difficult to identify in Figure  because it essentially stacks out. It is, however visible in the constant subsurface offset section of panel (b) in Figure  (left oval). The second SODCIG (panel (b)), taken below the big salt body in Figure (a) (CMP position 20800 m) shows no clear evidence of multiples below the salt. The strong event below 20000 m and about 5000 m in Figure  is actually the base of the salt and the events below it are subsalt primaries (see also the oval in panel (a) of Figure ). The last SODCIG, (panel c), taken where the sediments meet the salt flank (CMP position 24900 m), show evidence of multiples below 4000 m depth. To further illustrate this point, Figure  shows a comparison between the image taken at zero subsurface offset, which should be mostly (but not exclusively) primaries, and that obtained at -400 m half-subsurface offset, which should be mostly multiples. Notice in particular the multiple above a depth of 3000 m to the right of the inline distance of 24000 m as indicated by the right oval. Also notice the flattish event at about 4000 m depth and 13000 m inline distance (left oval). This is likely to be a multiple obscuring the weak primaries.

 

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Figure 11 Subsurface offset common image gathers taken at lateral positions (a): 13200 m, (b): 20800 m and (c): 24930 m.

 

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Figure 12 Common subsurface offset sections taken at (a): 0 and (b): -400 m half-subsurface offset.

Since I expect to attenuate the multiples in ADCIGs rather than SODCIGs, it is interesting to see how the multiples in this 2D migration are mapping to ADCIGs. This is illustrated in Figure  that shows ADCIGs for the same SODCIGs shown in Figure . Again, notice in panel (a) the down curvature of the multiple at about 6000 m at zero aperture angle. There seems to be another multiple at above 7000 m depth. In contrast, no clear evidence of multiple can be seen in the ADCIG in panel (b). In panel (c) there is some evidence of multiples at a depth of about 6300 m depth but at the same depth there seem to be primaries as well. The situation will probably be more clear once the ADCIG is computed after migrating all the data.

 

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Figure 13 Angle domain common image gathers corresponding to the subsurface offset domain common image gathers in Figure