Technique Enhances Subsalt Imaging

HOUSTON–Technology advances such as wide-azimuth/full-azimuth (WAZ/FAZ) acquisition, reverse-time migration (RTM) and iterative salt imaging in model building have greatly improved subsalt image quality. In the Gulf of Mexico, steeply dipping three-way closures are familiar subsalt targets. However, despite these imaging improvements, some of these targets remain poorly imaged. The fundamental problem is imperfect illumination from limited acquisition. In the poor illumination zones, migration artifacts and coherent noise, such as residual multiples and converted waves, become prevalent and can severely contaminate the images and mislead the interpretation. A specialized reverse-time migration technique addresses this issue by combining a deconvolution imaging condition with vector offset output (VOO). The effectiveness of RTM in tandem with VOO stacking has been demonstrated on WAZ data from the Walker Ridge area in the Gulf, illustrating how this technique can enhance the subsalt signal amplitude and reduce noise, thereby providing a cleaner subsalt image and decreasing subsalt exploration risk. In deepwater Gulf of Mexico subsalt oil exploration, the salt-related structures serve as one of the main structural traps for hydrocarbons. Yet, the salt bodies also present the major obstacles for subsalt imaging. Their irregular 3-D shapes and high seismic velocity compared with surrounding sediments can cause severe wave field dissipation and distortion as seismic waves propagate through the complex salt bodies. During the past few years, advances in both acquisition and imaging technologies have provided unprecedented breakthroughs for subsalt imaging in terms of image quality as well as velocity model accuracy. Most of the subsalt four-way prospects with gentle dips are well imaged with the latest technologies, such as wide-azimuth acquisition and tilted transverse isotropic reverse-time migration. However, other subsalt exploration targets, such as the potential three-way traps next to the salt flank or salt keel circled in Figure 1, remain poorly imaged with existing technology. The target dips, marked by the blue dotted lines, could be as steep as 70 degrees. Because of its oblique angle to the base of salt, most of the reflected energy at the target reaches the base of salt at an over-critical angle, and is reflected back toward the subsalt (yellow lines in Figure 1). Only a limited amount of energy may be reflected back to be recorded at the surface. Therefore, the amplitude of the target section is weaker than the surrounding area. In extreme cases of poor illumination (e.g., where the dipping sediments truncate against the salt flank), the true events can be overwhelmed by noise. To improve the images in illumination shallow zones, illumination compensation must be addressed to enhance the amplitude as well as reduce the noise to enhance the signal-to-noise (S/N) ratio.