An interval velocity field for depth migration derived from Dix conversion of an rms velocity field often is judged to be inaccurate and, consequently, can require an excessive number of iterations to attain a final velocity-depth model with acceptable accuracy. The most likely reason for failing to derive an accurate interval velocity field from Dix conversion is that the input rms velocity field usually is based on velocities estimated from unmigrated data. For instance, a common approach to derive an rms velocity field is to take a set of stacking or DMO velocity functions picked at analysis locations, edit for any outliers, and interpolate them with some lateral and vertical smoothing. The key to an accurate interval velocity field from Dix conversion, however, is to estimate the rms velocity field from prestack time-migrated data—not from unmigrated data. By doing a better initial estimate, you minimize the number of iterations required to derive a final velocity-depth model suitable for accurate ima...