Angle-gathers can be constructed by two classes of methods: data-space methods Mosher and Foster (2000); Prucha et al. (1999); de Bruin et al. (1990), with reflectivity described function of offset ray parameter, and image-space methods Rickett and Sava (2001); Sava and Fomel (2003); Weglein and Stolt (1999), with reflectivity described function of scattering angle. The moveout behavior of primaries and multiples are similar, irrespective of the method used to construct them.
Figure shows on the left a CMP gather for a model with flat reflectors and v(z) velocity. Most of the events in the gather are multiples, and just a few of the top-most events are primaries. On the right, Figure shows an angle-domain CIG for the data in the left panel. The primary events are imaged correctly and are flat, but the multiples are not imaged correctly and have strong moveout.
Angle-domain common image gathers are useful for multiple suppression for several reasons. First, events imaged with the wrong velocity show substantial moveout, which allows us to discriminate between primaries imaged with correct velocity and multiples, imaged with incorrect velocity. Second, angle-domain common image gathers describe the reflectivity at the reflection point, independent, in principle, from the actual structure for which they are computed, so they capture all 3-D propagation effects at every individual CIG.