Complex Imaging Challenges: Offshore South East India

Summary Imaging in deep water environments poses a specific set of challenges, both in the data pre-conditioning and the imaging. These challenges include scattered complex 3D multiples, aliased noise; and low velocity shallow anomalies associated with channel fills and gas hydrates. In this paper, we describe our approach to tackling these problems, concentrating our attention on multiple suppression, scattered noise attenuation, iterative velocity model building and depth imaging. Deep Water Issues Off the east coast of India, the transition from the shallower coastal waters to the deep shelf often encounters significant topographical variation in the sea bed, which gives rise to numerous effects which must be dealt with by the processing geophysicist. In addition to deep channels and steep slopes, we also encounter buried channels with low velocity fills and gas hydrates. Diffracted and "out-of-plane" multiples are the norm in these environments To address multiples, differential velocity based methods such as Parabolic Radon have often been used in deep water. To some extent, the problem of aliasing of the multiples on far offsets can be addressed either by interpolation and/or use of a de-aliased ('beam') Radon transform. However, Radon-based techniques fail for complex multiples, as the apex of the events in the CMP domain does not fall on zero offset for ray paths not in the plane of the shot-receiver axis. In these cases, an alternative method must be employed. In recent years, the SRME technique ha s become popular in deep water. Near offset multiples in particular are better attenuated than with Parabolic Radon technique. Cascading 2D SRME and Radon has become an industry standard approach. However, the complexity of the multiple generator and "out-of-plane" effects can severely limit even this combination. With the advent of 3D SRME, a theoretically more correct approach has become available, and here we demonstrate its effectiveness as compared to the 'conventional' approach. In I