Introduction

The Gulf of Mexico dataset, provided by VeritasDGC, was acquired over a complex salt body with structure in both the inline and the cross-line directions. The water-bottom dips in some places as much as 11 degrees in the cross-line direction making the mapping of the multiples in the image-space cube difficult to predict. The presence of the salt distorts the multiples so much that in many cases it is difficult to discern with any certainty which events are multiples and which events are primaries in the migrated cube. An important tool for that purpose are the SODCIGs where the multiples can be identified by their tendency to map away from zero inline and crossline subsurface offsets, and the ADCIGs where primaries should be flat as a function of aperture angle while multiples exhibit residual moveout as I showed in Chapter .

I first migrate a sail line, with shot profile migration, to assess the possibility of discriminating between primaries and multiples on inline subsurface offsets, where, according to the results of Chapter , there is better chance of imaging the multiples. The results are encouraging and show in several places that enough differential curvature exists between primaries and multiples in inline SODCIGS and ADCIGs even below salt. The image cube, however, is poor because of the large crossline dips that require that much more than one sail line be migrated in order to capture the flanks of the salt bodies and in one case even its top and bottom. The multiples, of course, are also improperly migrated in just one sail line. I then migrate the entire dataset with a large crossline migration aperture. Due to computer limitations, only a relatively small inline migration aperture was used and no prestack image gathers were created. The image cube shows great improvement in the image of the salt flanks as well as the top and bottom salt reflections that were missing from the migration of only one sail line. The steepest salt flanks are still poorly imaged because of the limitations in the amount of available data. This image cube was used to select a smaller dataset below a small salt body, to perform a full-fledged shot profile migration with the computation of prestack images in both the inline and crossline directions.

In this chapter I will compare the different prestack image domains and show the behavior of the primaries and multiples, in particular subsalt multiples. I will defer until the next chapter the actual attenuation of the multiples.