We have developed a simple but practical methodology for updating subsalt velocities using wave-equation, migration-perturbation scans. For the sake of economy and scalability (with respect to full source-receiver migration) and accuracy (with respect to common-azimuth migration), we use shot-profile, wave-equation migration. As input for subsalt-velocity analysis, we provide waveequation migration scans with velocity scanning limited to the subsalt sediments. Throughout the migration-scan sections, we look for the best focusing or structural positioning of characteristic seismic events. The picking on the migration stacks selects the value of the best perturbation attribute (alpha-scaling factor) along with the corresponding position and local dip for the chosen seismic events. The associated, locally coherent events are then demigrated to the base of the salt horizon. Our key observation is that this process is theoretically equivalent to performing a datuming to a base of salt followed by subsalt migration of the redatumed data perturbed-velocity profiles. Thanks to this implicit redatuming of shot profiles, no ray tracing through the salt body is required. Thus, the events picked on the subsalt-velocity scans only need to be demigrated to the base of salt. For the event demigration we use 3D specularray tracing up to the base of the salt horizon within a predefined range of reflection angles. Event demigration produces model-independent data -time and time slope -that are then kinematically migrated using the current tomographic-inversion working model. To find a finalvelocity model that will flatten best the remigrated events on common image point (CIP) angle gathers, we use the same set of demigrated observation data as the input data set for several nonlinear iterations of 3D tomographic inversion.
This paper presents an effort toward interactive migration. A prestack depth-domain fast beam migration algorithm is implemented. To achieve efficiency, only picked events are migrated. Picked events in the time domain form a new dataset which is a small fraction of the original dataset in terms of volume. Furthermore, events are migrated wavelet by wavelet instead of sample by sample. The wavelet is mapped into depth domain in the format of a patch. Coherent noise and migration artifacts in migration image are reduced by multi-path migration and intrinsically used dipping information The idea of multi-path algorithm is borrowed from the Gaussian beam migration technique, but only picked events with certain p values (event slowness) are migrated. The resulting Sigsbee image using fast beam migration is superior to that of conventional Kirchhoff migration with a speed-up of at least one order of magnitude.
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