Abstract：Common poststack seismic velocity analysis is on the basis of three hypotheses:uniform lateral velocity, small offset and horizontal reflector，which is inconsistent with the feature of geologic sedimentary structure in Ghina. Therefore，the paper presented the method of analytic velocity analysis in prestack time migration. By studying the rule of variation of event depth versus offset on common-imaging-point gathers,the method can resuit in following conclusions:1.the depth-ditance curves of events are hyperbolic;2.the errors of event depth are directly related to relative errors of velocity;3. the errors of event depth are inversely related to true depth;4.imaging velocities are true velocities when events are flat. The main implementing steps are:using moveout of events on third-order prestack time-migrated gathers to analytically compute the velocities when the moveout(or called as delay)is zero,which are needed imaging velocities;then using these velocities for prestack time migration. The method is used for practical data processing,it's seen from the results that the method is simple and practical and effects are evident.
