A full 3D migration with large inline and crossline migration apertures is necessary to correctly image the multiples. To attenuate the multiples we need to compute prestack image gathers as a function of subsurface offset or aperture and azimuth angles. Below salt both primaries and multiples are illuminated only by a narrow range of aperture angles. This makes the difference in moveout between primaries and multiples relatively small. In fact, if we consider only inline subsurface offset gathers at zero crossline subsurface offset and vice-versa, the difference in moveout may indeed be too small. But we need to remember that in a way that is the worst case scenario because the difference between the moveout of primaries and multiples in inline subsurface offsets is larger at non-zero crossline subsurface offsets and vice-versa. Similarly for the 3D ADCIGs.
It is challenging to fully appreciate all the information in the five-dimensional SODCIGs or ADCIGs and just looking at individual planes gives only glimpses of the true difference between the primaries and the multiples. The main message of this chapter is that we can indeed discriminate between them even for subsalt reflections. In the next chapter I go into the craft of actually attenuating the multiples in the Radon domain.