To test the effects of fold variations on the quality of the migrated image, I applied three distinct imaging schemes to the data. The first consisted of prestack depth migration (PSDM) of the irregularly sampled input. The second applies NMO-staking followed by inverse NMO and prestack migration. The third uses AMO to organize the data as a zero-azimuth cube with 800 meter effective offset, then applies prestack migration to the regularly sampled partial stack. The results from the different processing flows are compared to the image obtained by migrating an over-sampled subset with the same azimuth and offset ranges. To make the comparison as fair as possible to the conventional methodology of simple NMO-Stacking, the traces after NMO were laterally interpolated before they were stacked for common-offset migration. The traces were also binned at the same resolution used for AMO stacking, that is, half the nominal CMP spacing. A single depth-varying velocity function (migration920) was used for the depth migration of the subset.
Figure migration910 compares the results of migrating the 3D subset using different imaging flows. The Figure represents a depth slice at 910 m. The migration of the oversampled survey shows the presence of a complex morphology of a meandering river system marked by ramification of the major channel. The result of migrating the NMO-stack shows a poor resolution image that makes the interpretation of the channels difficult. Applying AMO to the data preserved the sharp features of the fluvial deposit system. The result of migration after AMO is smoother than the output of migrating the irregularly sampled data. This is due to the fact that the effect of fold variations were effectively reduced by the calibration of AMO.
Given the dominantly flat geology of the survey, it was expected that migration after NMO should provide a good image. Such result can be observed on the 920m depth slice which marks the floor of the river channel migration920. The explanation for this phenomenon is that the morphology of the river system becomes more complex towards the top of the deposition sequence. This results into diffractions from the edges of levees and from possible barrier islands. While these diffractions were preserved by AMO, they were destroyed during stacking by NMO.