In the pursuit of ever improved imaging in the Gulf of Mexico a number of trends in velocity model-building workflows appear to be emerging. A first trend has been the shift towards increasingly more complex anisotropic as part of prestack depth migration (PSDM) workflows. A good example is the move from vertical transverse isotropy (VTI) imaging to tilted transverse isotropy (TTI) imaging in complex areas. Another trend, particularly in production settings, is the growing requirement for delivery of seismic images that tie all available well data together with an understanding of the uncertainty associated with that image. A final trend is the realization that additional drilling regulations are likely to require a much more thorough understanding of the shallow section. A flexible model-building workflow is therefore needed that can be tailored depending on project objectives and the availability of additional data. Such a workflow would allow for a proactive approach in anticipation of additional regulations, while maximizing the value that can be derived from the velocity model. In this paper we present two case studies from the eastern Gulf of Mexico with different degrees of geological complexity and project objectives. We illustrate the use of a flexible and adaptable workflow for anisotropic model building and suggest the trends that such a workflow may capture in the future.
Description A fundamental problem in structural imaging, in addition to the need for fast iterative migration, is that of parametrizing velocity models (Johnson, 1992). Different approaches to parametrizing 3-D velocity models have been previously described in Pereyra (1989), Brat (1992), and Mallet (1989). We have used the Gocad triangular mesh surface representation described in Mallet (1989) to construct 3-D surface models. A simplified version of our approach has been used by Ratcliff(1991) and Ratcliff et al. (1991) to construct salt models in the Gulf Coast. Using parallel 2-D velocity models to construct 3-D velocity models may be adequate in areas of simple structure, however more complex areas require additional control in dip, strike, and depth directions. This is particularly true in areas having complex faulting, overthrusts, salt holes, spires, or complex velocity variations. We will show example models that exhibit these complexities, and illustrate how powerful 3-D surface generation can be coupled with 3-D seismic displays to produce better 3-D velocity models. Application In this talk we will illustrate tools that allow the interpreter to generate a smooth iso-velocity surface while viewing seismic data on a perspective cube. The interpreter can use any combination of dip, strike or depth sections to create iso-veIocity surfaces. In addition, after the surface is generated, the 3-D surface model can be cut by dip strike and depth section planes that can then be posted on the seismic display to verify the fit between the surface model and seismic data. In addition, the user can include the results of velocity analysis or velocity inversion within his model, and visualize velocity variation within layers. We will show examples of how this technique can be used in complex geological regions to produce improved 3-D velocity models, The techniques we will describe do not require hardware graphic accelerators, however when hardware graphic accelerators are available, the software can make use of them. Complex Faults and Overthrust Model This model is from a complex faulted and overthrust geological region. The 3-D iso-velocity surface model was generated from 13 key parallel lines extracted from 250 lines. Since the faults and overthrust were predominantly in the dip direction, 3-D tools in Gocad were used to construct surfaces having nonorthogonal overthrust and pitchouts. The resulting velocity model was compared with the migrated seismic data in both dip and strike and depth sections to verify the accuracy of the model with the seismic data (Figure 1). Volume rendering of the velocity model and migrated seismic was used to visualize the depth migrated seismic along the iso-velocity surfaces across the entire 3-D survey. Complex Salt Body with Hole and Spire This model demonstrates a closed salt body that not only contains a hole or window in the salt, but a salt spire that turns on itself. Construction of this model is very difficult since both the top of salt and bottom of salt are each multivalued functions. This model is further complicated due to the fact a salt window occurs very near a thick vertical salt section.
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