Prestack time migration for a converted wave based on the independent gamma field

Prestack time migration based on a convented wave time scale isn't suitable for P-wave and S-wave velocity analysis and can't ensure the imaging accuracy.Prestack time migration for converted wave can be realized according to the converted wave time scale or the reflection time scale.Both methods were compared and analyzed.The method using reflection time scale directly quotes the P-wave velocity obtained from reflection data, and the double square root time formula can be simplified to a separate problem for calculating the S-wave velocity, so as to effectively reduce the uncertainty.We introduce the independent gamma to the course of prestack time migration for a converted wave based on the reflection time scale; the P-wave and S-wave average velocity ratio can be acquired by time matching, accordingly.It can eliminate the effects of the offset, stratigraphic dip and other factors.On this basis, we calculated the S-wave velocity iteratively and updated the P-wave velocity continuously, and the P-wave velocity, S-wave velocity, and P-and S-wave average velocity ratio can be obtained with clear physical meaning.The practical application shows that the method has high computational efficiency and good imaging effect.