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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 ͑RTM͒ tool progressively becoming a routine, the extension of this subsalt-velocity-perturbationscan 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 DIT͒ 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.

To attain the benefits of simultaneous shooting in marine data acquisition, either the simultaneous sources have to be placed far apart to minimize the interferences from one another or sufficient attenuation of the interferences is necessary. Attenuating the interferences in a domain where they are randomly distributed has a potential of attenuating the signals particularly when they have conflicting dips. Our experiments indicate that it is easier to preserve the signals if the interference is removed by modeling them in a domain where they are coherent and adaptively subtracting them from the data. In addition, with effective attenuation of the interference, the distance between the simultaneous sources can be substantially reduced.

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