The paper describes a collaborative project between Total and WesternGeco on quantifying structural uncertainty in application to the development of the Hild field. The key motivation for the project was to improve the seismic uncertainty inputs into the workflows for assessing uncertainty for the gas reserves and risks associated with the development well placement. The main focus of this work was related to the anisotropy ambiguity analysis for more accurate quantifying the structural uncertainty of the BCU top. The workflow used a novel technology of uncertainty analysis in anisotropic model building based on eigen decomposition of the tomography operator and a null-space projection. The main lesson learned was that prior information about anisotropy should be defined thoroughly from the geologic and rock physics perspective and also to be calibrated with the available well data. Overall, the project turned out to be successful and the results enabled more confident decision making in the Hild field development.

Advances in acquisition and processing technology help overcome imaging challenges in complex structural settings. The widespread adoption of wide-azimuth (WAZ) and the move towards full-azimuth (FAZ) acquisition geometries, both combined with increasing offsets, result in significantly improved illumination. Reduced compute cost and improved performance enabled reverse time migration (RTM) to emerge as the imaging algorithm of choice in such settings. Of course, an accurate velocity model is a key component in realizing the full potential of these acquisition geometries and algorithms. The trend is towards increasingly more complex anisotropic models, with a move from vertical transverse isotropy (VTI) to tilted transverse isotropy (TTI) and even orthorhombic. In the Gulf of Mexico (GoM), though the importance of defining an accurate anisotropic model in the supra-salt section cannot be understated, the largest contributing factor to a good image subsalt is often the correct delineation of the "salt body" itself. Without an accurate definition of the salt geometry, the subsalt image invariably remains distorted and poorly resolved. In this paper, we will focus on this portion of the depth imaging workflow and illustrate how the techniques of RTM scenario testing and image partitioning can be used in combination to both help define the salt geometry and improve the final post-migration image. We will describe a practical workflow and the key components that we feel are necessary for its success. In addition, we will illustrate a number of lessons learned during the course of recent projects executed in the GoM.

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Following the introduction of wide-azimuth towedstreamer data as an exploration imaging prerequisite in the deep-water Gulf of Mexico in 2006, anisotropic imaging with vertical or tilted transversely isotropic (VTI or TTI) models has become the dominant practice in the industry. Today, it is widely recognized that incorporating anisotropy in imaging increases our ability both to focus the seismic data and to accurately position the reflectors for drilling decisions. We show three successful anisotropic model building strategies applied on wide-azimuth (WAZ) data from the Walker Ridge and Green Canyon areas of the deep-water Gulf of Mexico. We demonstrate the benefits of incorporating geology information and data from boreholes into all stages of velocity model building.

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