Prestack depth migration needs a good velocity model to produce a good image; in fact, finding the velocity model is one of the goals of prestack depth migration. Migration velocity analysis uses information produced by the migration to update the current velocity model for use in the next migration iteration. Several techniques are currently used to estimate migration velocities, ranging from trial and error to automatic methods like reflection tomography. Here, we present a method that combines aspects of some of the more accurate methods into an interactive procedure for viewing the effects of residual normal moveout corrections on migrated common reflection point (CRP) gathers. The residual corrections are performed by computing traveltimes along raypaths through both the current velocity model and the velocity model plus suggested model perturbations. The differences between those sets of traveltimes are related to differences in depth, allowing the user to preview the approximate effects of a velocity change on the CRP gathers without remigrating the data. As with automatic tomography, the computed depth differences are essentially backprojected along raypaths through the model, yielding a velocity update that flattens the gathers. Unlike automatic tomography, in which an algebraic inverse problem is solved by the computer for all geologic layers simultaneously, our method estimates shallow velocities before proceeding deeper and requires substantial user intervention, both in flattening individual CRP gathers and in deciding the appropriateness of the suggested velocity updates in individual geologic units.

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