Anisotropic Imaging in Complex Media: Step Change in Quality and Interpretability of the Results
Authors: Burkepile David , Yang Yi , Hubbard Robert , Zdraveva Olga , Heimach James , Wagner Charles 
Published: May 2013

The soaring cost of drilling in deep-water environments leads to a strong demand for high-quality interpretable images and associated models, both products of seismic imaging, in many basins around the world. Anisotropic models more closely represent the real earth's properties. Using them in migration increases our ability to create accurate images. Overall, this allows for more reliable inversion, interpretation, and drilling decisions. In addition, anisotropic imaging allows us to realize fully the potential of new data types (e.g., wide-azimuth and long-offsets towed-streamer data) and to incorporate systematically our understanding of the geology, the available rock physics and geomechanics information, and borehole data into the model building process. We discuss an anisotropic model building workflow that is easily adaptable to any geologic environment, data type, objectives, or available non-seismic information. We report successful imaging strategies made possible by this workflow and demonstrate the improved results with examples from North America and West Africa.

Introduction

Advances in seismic imaging are, arguably, the most important element in the rising success of deep-water exploration and development because they reduce interpretation and drilling risk to acceptable levels. Historically, anisotropic imaging was applied only on smaller projects in mature fields with abundant well data. In recent years, the demand for high-quality pre-salt and subsalt images made anisotropic depth imaging with vertical transverse isotropic (VTI) and tilted transverse isotropic (TTI) models the dominant industry practice, even for large-scale exploration projects in areas of limited well control, i.e., the majority of the deep-water basins around the world. In parallel, new data types (wide- and full-azimuth towed-streamer) were introduced and more advanced demultiple, migration, and inversion algorithms became available. Figure 1 illustrates the cumulative effect of all these improvements, comparing isotropic imaging with 2006 narrow-azimuth (NAZ) data with a VTI imaging result in the same area using 2011 wide-azimuth (WAZ) data. The much-improved and detailed velocity model follows the geology closely (Figure 1 b) and naturally translates into an image with better focusing, better event continuity, less reflector distortion, and more easily interpretable events at depths more than 30,000 ft below the autochthonous salt.

In complex geologic situations, the improved images resulting from well-designed anisotropic imaging carry potential beyond that of typical structural and stratigraphic interpretation. In cases with thoughtful conditioning of the migrated amplitude data, careful calibration of well and seismic data, validation of amplitude fidelity, and preparation of appropriate background trend models, quantitative interpretation methods can be applied in situations previously thought to be off limits for quantitative work (Wagner et al., 2012). In addition, high-quality velocity models obtained during anisotropic imaging allow quantification of structural uncertainty for volumetric analysis (Osypov et al., 2011). Through such approaches, be it subsalt or pre-salt, reservoir development in deepwater environments can greatly benefit from recent advances in imaging.