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f Challenges of depth model building for Tilted TI media
- Publisher: European Association of Geoscientists & Engineers
- Source: Conference Proceedings, 72nd EAGE Conference and Exhibition - Workshops and Fieldtrips, Jun 2010, cp-162-00002
- ISBN: 978-90-73781-87-0
Abstract
Over the last years, Shell has accepted anisotropic velocity model building and pre-stack depth migration as the norm for their seismic imaging. This has reduced the number of sidetracks to be drilled as a consequence of sub-optimal imaging or positioning, which is a clear demonstration of the impact of including anisotropy in the model building cycle. While VTI is a commonly accepted working model, with a workflow that is conceptually clear, this is less obvious for Tilted TI. Tilted TI might be the preferred model at the flanks of salt bodies and minibasins, where the sediments are strongly dipping. We shall demonstrate such a real data case in the Gulf of Mexico, where the application of Tilted TI simultaneously explained mis-ties of well markers and sub-optimal focusing of dipping reflectors. Mis-ties and residual moveout were in conflict for a VTI model. Modelling experiments confirm that this can be the case if the subsurface is Tilted TI, indeed. We shall also see that availability of Wide-Azimuth data should be helpful in recognizing Tilted TI in the subsurface. Usually, anellipticity causes only subtle effects in focusing, considering the trade-off between Eta and moveout-related velocity. For the Gulf of Mexico, one generally finds Eta to be small. Therefore, model building is frequently restricted to vertically elliptic models, at least in the initial phase. Reflection tomography with such a model will flatten residual moveout already to a large extent. If, subsequently, a tilt angle is introduced in such a model, this has effect on the well-ties as well as on the residual moveout in the common image gathers. A method is discussed to fit well-markers in such a situation, aiming at preservation of moveout.