Case Study Highlights of Advances in Multiple Attenuation

Abstract The "data-driven" feedback method for modeling and attenuation of free-surface multiples was applied in two case studies for sub-basalt and for sub-salt imaging, where multiple attenuation is an important and challenging processing step. Despite limitations due to cable feathering in one case and to 3-D wide-tow acquisition geometry in the other case, the feedback method performed better than conventional multiple attenuation methods. Two features of the new method appeared as particularly attractive during the case studies: velocity picking or interpretation of horizons were not required prior to the application of the method, and, primary events, including locally converted PSP events with slow apparent velocities, were well preserved. Further improvements in the performance of the feedback modeling and attenuation method could be expected as a result of optimized acquisition geometries and of recently introduced acquisition hardware for streamer steering and control. Introduction Several recent papers1, 2, 3, 4 have documented benefits from "data-driven" algorithms which model and attenuate surface related multiples without requiring knowledge of sub-surface velocities, interpretation of main multiple generators or periodicity of the multiples. It is also well established that the performance of such methods is degraded when conditions of "dataset integrity" are not met3. In this paper we discuss two case study applications of the feedback approach5 to modeling and subtraction of freesurface multiples. In each case study, strong multiples are a key processing issue not well solved by conventional velocity filtering or deconvolution methods. On the other hand, the feedback and related multiple attenuation methods have to overcome problems due to the sub-optimal wavefield sampling that can be currently achieved in practice. The first case study discusses multiple attenuation in the context of sub-basalt imaging using long offset (11.4 km) streamer data. The feedback multiple attenuation method improves upon the results of conventional multiple attenuation methods and in addition demonstrates preservation of locally converted PSP primaries. The performance of the multiple attenuation algorithms is related to the feathering observed in the data, which is variable and occasionally very strong. The second case study compares multiple attenuation results obtained for a typical wide-tow 3-D survey acquired in the Gulf of Mexico for deep-water sub-salt exploration. A pragmatic extension6 of the feedback method to 3-D geometries performs better than the conventional parabolic Radon transform, even though the performance of the method degrades significantly as the cross-line distance increases. Sub-basalt imaging using long offset 2-D streamer data The challenges of seismic imaging below basalt7 have led to innovative acquisition and processing approaches. Improvements in data quality have been reported as a result of acquisition and processing using long or ultra-long offsets8 and imaging either with locally converted PSP waves9, 10 or with P-waves11, 12, 13. The rock properties of basalts and their seismic responses, in particular the relative strengths of PSP and P waves reflected beneath basalt, vary considerably14, 15, 16.