Subsurface velocity determination by grid tomography of depth migrated gathers

Determination of subsurface interval velocity is important for imaging by prestack migration, AVO studies and full waveform inversion. We propose a method which uses depth migrated gathers directly, and does not necessarily need a depth model. The approach is complementary to horizon based tomography. When a subsurface model can be identified, model-based tomography seems appropriate since it produces geologically constrained updates. However, if no depth model can be determined initially, a grid based approach is advisable, where one can update an interval velocity section without building a depth model.