Seismic imaging is generally a technique to figure out the subsurface structure or physical properties of subsurface media. It is often considered the same as seismic migration in reflection seismic. Migration has been the means of collapsing diffraction events to points, moving (“migrating”) reflection events to the proper locations to obtain a true image of subsurface structures.
Over the years, the technique of migration has evolved. It started from NMO (normal move out) for the horizontally layered structure, and then DMO (dip move out) for dipping layer has been developed. For more general velocity structure, a variety of migration techniques has been applied in time and depth domain. In case of small variation of the subsurface velocity, time migration is enough to gain true image, whereas depth migration is required for abrupt lateral change of the velocity. As the computer technology is developed, CPU intensive pre-stack migration methods become into practice use. Pre-stack depth migration (PSDM) is one the final steps of the seismic imaging with proper velocity estimation.
In this paper, we introduce the general ideas of migration as well as some of the issues related to it, such as computation efficiency and velocity estimation, without describing the details of the migration algorithms themselves. This short paper is intended to provide the overview of migration methods for non-geophysicists.