Multiple Diffractions. Attenuation by Multi-Azimuth Streamer Acquisition
James Keggin1, Martin Widmaier2, Stian Hegna2, Einar Kjos3
(1) BP Egypt, Cairo, Egypt (2) PGS Geophysical, Oslo, Norway (3) BP Norway, Stavanger, Norway
Noise caused by multiples from shallow diffractions is a common and serious problem in the Nile Delta and many other deep-water areas. Multiple diffraction noise is typically seen as an abrupt change in image quality just below the first water bottom multiple. Since the ray-paths associated with these events are complex and 3D in nature, conventional acquisition and demultiple processing techniques have trouble attenuating this noise. Move-out based demultiple techniques fail since the apexes of the travel time curves do not necessarily occur at zero offset. Other multiple attenuation techniques such as wave equation based methods fail as well because they are based on 1D or 2D assumptions.
The nature of the problem is discussed in detail and an alternative high-fold multi-azimuth streamer technique is proposed. The basic idea is simple; improved sampling of azimuth and offset allows the CMP stacking process to better attenuate the complex noise patterns. The concept, the acquisition configuration and processing results from a 2001 acquisition test in the Norwegian Sea will be shown.
The resulting multi-azimuth stack shows significantly less multiple diffraction energy compared to data acquired by standard marine streamer acquisition. This result is due to better sampling of the multiple diffraction noise. Future work should be executed to address the optimum acquisition of multi-azimuth data with significant spatial coverage, and the subsequent options (and additional benefits) this would give in processing.