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Summary In a ‘Kirchhoff’ (i.e. weighted diffraction stack) prestack migration, the summation over a complete diffraction surface can be thought of as an average of reflectivity estimates from migrated common-shot, common-receiver or common -offset gathers. The optimal weight for averaged reflectivity should be based on Bleistein et al’s (2001) β common-offset weight. In comparison, the β common-shot and common-receiver weights, although correct for individual gathers, produce average reflectivity estimates with a dip- and depth-dependent bias. Bleistein et al’s (2001) 1 β common-offset weight is more suitable as a basis for practical weights because it downweights by the cosine of the ray half-opening or obliquity angle at the reflector and hence accounts for the corresponding reduced spatial resolution as obliquity angle increases. In this paper, weights for optimal 2.5-D and 3-D diffraction stack migrations are i) tested using simple noise-free constant-wavespeed synthetics, ii) re-expressed for practical implementation as relative-amplitude-preserving prestack time migration weights, and iii) customized for the equivalent offset method (EOM) of prestack time migration. |
