Adaptive raytracing-based suppression of severe water-bottom multiples in marine seismic data

O'Brien, Simon R. M. (1997) Adaptive raytracing-based suppression of severe water-bottom multiples in marine seismic data. Doctoral (PhD) thesis, Memorial University of Newfoundland.

[English] PDF (Migrated (PDF/A Conversion) from original format: (application/pdf)) - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. Download (21MB) [English] PDF - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. (Original Version)

Abstract

Marine seismic data is generally acquired by towing source and receiver arrays through the water, periodically generating an acoustic pulse and recording the echoes reflected back from the sub-sea layering. Since both the water surface and the sea-floor are good reflectors, much of the source energy reverberates between the two. When the sea-floor is very hard, this 'multiple' energy is recorded at the receivers for some time after the shot instant and can completely mask all of the energy produced by 'primary' reflections from sub-seafloor boundaries. As we are only interested in imaging the primaries, effective multiple attenuation is extremely important to the processing of marine seismic data. While traditional multiple attenuation techniques often produce good results, they all have limited success dealing with extreme sea-floor conditions such as those which are often experienced off the coast of Newfoundland. This thesis develops a new technique which is better able to handle these conditions and makes a comparison with existing techniques. -- The attenuation technique, called Raymult, is based on an adaptive prediction-subtraction approach in which raytracing is used to guide the multiple prediction. The near-trace gather is automatically picked and the picks subsequently migrated to generate a water-bottom model. By raytracing each shot gather, an estimate is generated for the phase, amplitude and arrival time of the multiples on each trace. These estimates are then adjusted until they accurately match the data. Finally, the multiples are subtracted from the gather. The water and sea-floor velocities are the only required input parameters. However, since the routine is able to adapt the raytracing results to fit the data, the accuracy of these parameters is not essential. As a byproduct of the multiple suppression, wavelet estimates are produced for each multiple order. -- Raymult is successfully applied to both synthetic and real data examples, and proves very effective in dealing with substantial sea-floor topography as well as the phase and amplitude problems which are typical of data collected over hard sea-floors. There is only minor distortion of the remaining energy in the gathers, so other multiple attenuation techiuques, e.g. predictive deconvolution or radon filtering, can be applied subsequently, providing even greater attenuation of the multiples.

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