f Model-based stack: A method for constructing the true zero-offset section

Usually it is assumed that a stacked time section represents a zero-offset section. But in many areas with complex structures or streng lateral variations it is not valid. In this case a poststack depth migration gives a wrong image of the model. We propose an 'optimal' way to produce the 'true' zero offset section. We call this section the Model-Based Stack. This method consists in calculation the actual traveltime trajectories by ray tracing for each CMP and for each time sample according to the velocity-depth model obtained from the coherency inversion, and stacking the traces within CMP gather along these trajectories. Now a poststack depth migration applied to Model-Based Stack (MBS) produces a better image of the subsurface because MBS represents a zero-offset section even in cases of complex geology. We believe that the processing procedure including Model-Based Stack followed by poststack migration is a fast and reliable alternative to prestack migration.