REFINING PRESTACK DEPTH MIGRATION IMAGES WITHOUT REMIGRATlON

Prestack depth migration utilizes two imaging conditions to create a fully-imaged depth section: time equal zero (t = 0) and offset equal zero (h = 0). When a migration-velocity field is in error, however, reflected energy focuses best at depths that are inconsistent with the t = 0 imaging condition. The nonzero time at which focusing actually occurs can be measured from a display of the wavefield near t = 0 at each step of downward continuation. This process is called depth-focusing analysis. Interpreted depth errors can be used to create a new velocity field for another iteration of prestack depth migration. Unfortunately, the extreme computational requirements of the repeated migrations needed to converge on a final velocity field may restrict the number of iterations used in practice.