Re: Migration in complex geological areas
While Rajesh is totally correct, I think its worth commenting that there are some hybrid pre-stack time/depth migrations that might give a "good enough" solution depending on what you are after.
These are usually Kirchhoff pre-stack time migrations that use more sophisticated approaches to developing an operator which is asymmetric and varies with the (RMS) velocity of the output location - its a better approximation than a standard Kirchhoff preSTM but one that runs many times faster than a standard preSDM.
In general, things like preSDM obey the "80/20" rule - in that 80% of the time/cost is in getting the last 20% of image improvement. Hybrid PreSTM/DM routines can get you to the "80%" solution for 20% of the time/cost.