Defining the correct velocity is critical for proper subsalt imaging. A subsalt velocity perturbation scan based on wave equation migration is an effective way to update subsalt velocity and has become a standard industry practice. A factor limiting its use is that the cost of generating migration scans remains comparatively high. With the even more expensive reverse-time-migration (RTM) tool progressively becoming a routine, the extension of this subsalt-velocity-perturbation-scan approach to RTM becomes impractical. We propose a new methodology for updating subsalt velocity using an RTM-based scan composed of multiple migrated images. Rather than perturbing the velocity, each image is produced by a variable time-imaging condition. The zero-time imaging condition generates the normal RTM output, and the positive and negative nonzero-time (delayed imaging time, or DIT) imaging condition generates a scan of images called a DIT scan. The generation of the RTM-based DIT scan requires only a single RTM pass. Synthetic and field data examples confirm the effectiveness of this new approach and suggest the DIT scan approach could serve as an efficient alternative or even a replacement to the standard subsalt migration scan approach for RTM.

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