Velocity smoothing before depth migration: does it help or hurt?

Abstract

Previous studies of the sensitivity of depth migration to smoothing of the migration velocity model have treated smoothing of an initially correct velocity model. In practice, however, the initial velocity model contains errors, especially in complex areas such as in subsalt and over‐thrust areas. While smoothing the correct velocity model can only degrade imaging quality in wave‐equation migration, we show that smoothing of an erroneous velocity model can improve the depth‐migrated image. Using smoothed versions of erroneous velocity models, we migrated synthetic data for a simple 2D velocity model of reflectors beneath a salt layer with a chirp‐shape top or bottom boundary. Use of this generic model allows us to assess the relative influence of different types and scales of velocity error on image quality in the depth‐migrated sections. The results indicate that, depending on the complexity of the model and on the type and magnitude of the error in the initial velocity model, smoothing can improve imaging quality in depth‐migrated sections, sometimes significantly. This observation is based on numerous tests performed on modeled exploding‐reflector sections for the generic velocity mode supported by tests with prestack‐migration examples.