VSP reverse-time migration with variable grids and variable time-steps

Abstract

Prestack reverse-time depth migration is currently recognized as high precision seismic imaging method, which is easy to implement, not affected by the dramatic change in lateral velocity of medium and capable of steep dip angle imaging, etc. Reverse-time migration use twoway wave equation, use high order finite difference method to realize wave field numerical simulation, so accuracy of simulation has important influence on the migration result. In relief areas, describing propagations of wave field needs fine grids. But if it's adopted by whole underground, lots of calculation will be wasted. Based on stability analysis, fine grids need to use smaller time step that will further reduce the computational efficiency. In this paper, considering both grid size and time step, we test variable grids and variable time-steps in reverse-time migration. For specific areas that adopt fine grid and small time steps, and the rest region will be calculated with coarse grid and larger time steps, this can save calculation and enhance the efficiency of calculation at the same time. In the processing of migration, boundary conditions, storage and precision are also considered, the absorbing boundary that keeps parts of wave field information is applied to achieve higher accuracy of VSP reverse-time migration. Using the advantages of VSP data to image areas that conventional surface cannot clear imaging. Theoretical model test can prove the effect of method.