Abstract: A method for quantitatively determining an accurate subsurface velocity prior to data migration includes steps whereby the accuracy of the velocity can be defined by measuring the deviation in depth as a function of offset in the common reflection point (CRP) gather. A point on reflector is selected and the CRP gather is formed. If the image is not flat, the velocity is adjusted until it is flat. The velocity is decreased and the far offset end of the image will be imaged to shallower depth than the near offset end. The velocity is increased and the image will tilt down at the far offset end. An error is defined which is the theoretical accuracy limit for the determination of velocity using the CRP method. A factor is defined that indicates the reliability of the image for a reflector.

Abstract: A method for performing velocity analysis while eliminating the effects on weak signals caused by strong signals includes migrating each event of the pre-stack trace to a single location instead of all possible locations. This correct location is determined by ray-tracing through a velocity model. The input trace is divided into many windows, and each window is migrated to a place determined by ray-tracing the center of the window through the model. If the velocity model is accurate, each event will be migrated to the proper location yielding an accurate depth section with no migration artifacts. As a by-product, if the model is not accurate, the post-migrated parts (PMP's) provide a clean velocity analysis.

Abstract: In applying residual movement correction to common offset depth migrated data, common offset depth migration is applied using the best available velocity/depth model. Post migrated parts, which are depth migrated common midpoint gathers, are saved. The post migrated parts are treated as if they were in time not depth. Normal movement based on a constant velocity is removed. Velocity functions time-velocity pairs, are derived for the post migrated parts with normal movement removed using a standard velocity analysis program. Normal moveout based on these velocity functions is applied. The events on the post migrated parts ae not imaged to the same depth. The corrected post migrated parts are then stacked and displayed.

Abstract: A method for generating a three dimensional velocity model makes use of two dimensional depth images, which are the result of two dimensional pre-stack depth migration, and corrects the out of plane distortion by ray tracing through a three dimensional model. The three dimensional model boundaries are iterated until the three dimensional effects are minimized. The final model can be used for a final three dimensional pre-stack depth migration, or as a three dimensional interpretation of all the two dimensional depth migration results.

Abstract: A method for derivation of interval velocities from post-migration parts first includes the step of determining the apparent depth and slope of an event. The apparatus depth of an event is measured. The travel time of the recorded reflection for a particular offset is determined by ray-tracing through the old model and recorded. A trial velocity is assigned to the layer between events in the new model. The depth of the reflector is varied up or down until the computed travel time agrees with the measured travel time, keeping the source/receiver separation constant. A new velocity for the layer between reflectors is selected for which the depths at each offset are the same.