Multipathing in three-parameter common-image gathers from reverse-time migration

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Abstract

Reverse-time migration gives high-quality, complete images by using full-wave extrapolations. It is thus not subject to important limitations of other migrations that are based on high-frequency or one-way approximations. The cross-correlation imaging condition in two-dimensional pre-stack reverse-time migration of common-source data explicitly sums the product of the (forward-propagating) source and (backward-propagating) receiver wavefields over all image times. The primary contribution at any image point travels a minimum-time path that has only one (specular) reflection, and it usually corresponds to a local maximum amplitude. All other contributions at the same image point are various types of multipaths, including prismatic multi-arrivals, free-surface and internal multiples, converted waves, and all crosstalk noise, which are imaged at later times, and potentially create migration artefacts. A solution that facilitates inclusion of correctly imaged, non-primary arrivals and removal of the related artefacts, is to save the depth versus incident angle slice at each image time (rather than automatically summing them). This results in a three-parameter (incident angle, depth, and image time) common-image volume that integrates, into a single unified representation, attributes that were previously computed by separate processes. The volume can be post-processed by selecting any desired combination of primary and/or multipath data before stacking over image time. Separate images (with or without artifacts) and various projections can then be produced without having to remigrate the data, providing an efficient tool for optimization of migration images. A numerical example for a simple model shows how primary and prismatic multipath contributions merge into a single incident angle versus image time trajectory. A second example, using synthetic data from the Sigsbee2 model, shows that the contributions to subsalt images of primary and multipath (in this case, turning wave) reflections are different. The primary reflections contain most of the information in regions away from the salt, but both primary and multipath data contribute in the subsalt region.

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