In this section I introduce the seismic line I use to test the apex-shifted Radon transform. The line is from the Gulf of Mexico and was shot over a large salt body. The presence of the salt creates a host of multiples that obscure any genuine subsalt reflections, as shown in the angle stack of Figure 14. Most multiples are surface-related peg-legs with a leg related to the water bottom, shallow reflectors or the top of salt. Below the edges of the salt we also encounter diffracted multiples (, CMP position 6000 m below 4000 m depth in Figure 14).
Figure 14. Angle stack of migrated ADCIGs of 2D seismic line in the Gulf of Mexico. Notice that multiples below the salt obscure any primary reflections. The ovals highlight diffracted multiples.
Figure 15 shows four ADCIGs obtained with wave-equation migration as described by (). Notice that although the data is marine, the ADCIGs show positive and negative aperture angles. I used reciprocity to simulate negative offsets and interpolation to compute the two shortest-offset traces not present in the original data. The CMP gathers were then migrated and converted to angle gathers. The purpose of having both positive and negative aperture angles is to see more clearly the position of the apexes of the diffracted multiples. The top two ADCIGs correspond to lateral positions directly below the edges of the salt body (CMP positions 6744 m and 22056 m in Figure 14). Notice how the apexes of the diffracted multiples are shifted away from zero aperture angle (, the seagull-looking event at about 4600 m in panel (a)). For comparison, the bottom panels in Figure 15 show two ADCIGs that do not have diffracted multiples. Figure 15(c) corresponds to an ADCIG below the sedimentary section (CMP 3040 m in Figure 14) and Figure 15(d) to an ADCIG below the salt body (CMP position 12000 m in Figure 14). In these ADCIGs all the multiples are specularly-reflected and thus have their apexes at zero aperture angle.
Figure 15. Angle domain common image gathers. (a) under the left edge of the salt, CMP at 6744 m; (b) under the right edge of the salt, CMP at 22056 m; (c) below the sedimentary section, CMP at 3040; (d) below the salt body, CMP at 12000 m.