Reverse-time migration (RTM) provides superior images in areas where there are steep salt flanks or other complex geologic structures. However, the high cost of running RTM often makes it difficult to use RTM for routine large volume production. By dividing the subsurface into two or three regions in depth according to the complexity of the velocity model and applying RTM only in the region where we can benefit from it, we are able to make RTM very cost effective for routine production usage. Since the top region is often structurally simple, we use one-way WEM to image the region and save the downward continued wavefields at the base. We then use the saved wavefields as an input to RTM to image the middle region where there are complex structures and save the wavefield at the base if there is a third region with a simpler structure. We switch back to one-way WEM to image the third region using the saved data. By combining the strength of different migration algorithms, this hybrid approach not only significantly reduces the computation time and memory requirement, but also generates images of higher qualities.


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