The last special section on migration (December 2002) contained numerous examples that demonstrated the flexibility and robustness of the Kirchhoff method, including some enhancements to better preserve amplitudes and handle very steep dips. At that time, wave-equation migration was in its infancy and data usually had to be decimated to keep processing costs down. However, during the last two years, the clustering of high-speed, Linux-based computers and the development of clever algorithms have made wave-equation migration a more feasible option. For decades, geophysicists have known that wave-equation migration is a better solution than Kirchhoff since it can address the propagation of the wavefield recursively in depth over an incremental step and, thus, generally yields better subsurface images. However, a major drawback of early wave-equation-migration algorithms was their inability to generate angle gathers for residual moveout information and velocity model updates. Not anymore. As this special section shows, there are now at least two techniques in which angle gathers can be approximated and generated via the common-azimuth and shot-based methods.

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