In this abstract we will present a theoretical framework for demigration of seismic image data under<br>the frame work of Gaussian Beam theory. The underlying methodology has no dip limitation and<br>handles multi-valued arrivals well. The practical use of this technology is to perform prestack<br>demigration of existing image volumes (CIG gathers) followed by prestack depth remigration using a<br>replacement velocity and/or anisotropy model. The result is a fast, accurate, and cost effective way to<br>improved seismic imaging as compared to proprietary reprocessing from field tapes.<br>Ross Hill published seminal works in common offset Gaussian Beam prestack depth migration (Hill,<br>1991 and 2001). Chevron has been largely the only beneficiary of the Gaussian beam technology. To<br>the best of our knowledge there have been no published work on Gaussian beam prestack demigration.<br>For example, in prestack demigration, one can start with an individual common offset image volume<br>(Kirchhoff migration volume, Gaussian beam migration volume, or fast beam migration volume). The<br>single fold image is demigrated to reconstruct the corresponding input unmigrated seismic data. In<br>Gaussian Beam prestack demigration, we actually reconstruct the individual beams used as input to<br>the original Gaussian Beam migration. The individual beams are then synthesized to form an<br>unmigrated seismic volume.<br>There are many uses of Gausian Beam prestack demigration technology. For example, it can be used<br>as a fast forward modeling tool to generate synthetic data for migration and inversion. Another use of<br>this technology is to perform prestack demigration of existing image data, followed by prestack depth<br>(re)migration with a replacement velocity and/or anisotropy model. This is a faster, more accurate and<br>cost effective way of seismic imaging over proprietary reprocessing from field tapes. Both synthetic<br>and field data examples of this will be shown.


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