Angle-domain common-image gathers extracted from pre-stack RTM images

Common-image gathers (CIGs) in the angle domain contain important information for velocity analysis and subsurface attribute analysis; and they may also be used to improve stacked migration images. In this paper we propose a new approach to compute the angle-domain CIGs for RTM based on an inverse-scattering imaging condition, in which the reflection angle presents itself implicitly as weighting of the wave fields. Two images are generated with and without the implicit reflection angle factor. The reflection angle is computed by dividing one pre-stack image by the other. Then pre-stack images from all the shots are mapped and binned into angle domain to form CIGs. Since the angle gathers are extracted in a postmigration step, it adds little memory requirement to RTM; and the computation is very efficient, especially for isotropic media. We demonstrate the method with several synthetic examples.