The bandwidth of conventional marine streamer seismic data (i.e., that collected with a streamer comprising hydrophones only) is generally limited at both high and low frequencies by the presence of both source and receiver ghost notches. Recently, a method was introduced that uses non-linear optimization to derive a stable operator which, when applied to conventional marine streamer seismic data, recovers much of the signal present in the recorded data that is weakened by the presence of the ghosts at the notch frequencies. In this paper, we present a case study of the use of such a method to derive a broadband image from conventionally acquired data from offshore West Africa.
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 IssuesOff 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-ofplane" multiples are the norm in these environments (Stewart, 2004), and must be dealt with in order to subsequently derive a reliable velocity model in order to deliver an acceptable structural image (Stewart et al, 2006).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 has 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 figure 1, we show data sorted to CMP gathers after application of the SRME technique (which is applied to shot gathers). We compare results from 2D SRME with those from 3D SRME. Complex ray-paths for the first seabed multiple and associated sedimentary layers, give-rise to a shifted-apex aspect to the moveout behaviour as seen in the CMP domain. Following either 2D or 3D SRME, additional de-noise techniques can be applied to deal with the aliased noise and other classes of noise. CDPs : 2D SRME 3 4 5 6 s CDPs : 3D SRME 3 4 5 6 s Figure 1: example deep water CMP gathers with 2 nd order NMO. Top:-after application of 2D SRME. Bottom -after application of 3D SRME. Maximum offset is 6000m. 433 SEG Las Vegas 2008 Annual Meeting Main Menu 433 SEG Technical Program Expanded Abstracts 2008 Downloaded from library.seg.org by University of California, San Diego -UCSD on 06/21/16. For...
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