Pre-stack depth migration of seismic multiples

Abstract

Seismic multiples still present a major impediment to exploration on the North West Shelf of Western Australia. Considerable effort is being expended on the removal of multiple events from the seismic record to improve the quality of the image of the subsurface, hence reducing the risk of drilling dry holes. Most of the published multiple attenuation techniques to date are based on the premise that multiples are noise and hence need to be removed. The technique presented in this paper is based on the assumption that multiples are signal, and that image quality will be enhanced by assigning their energies to the appropriate place in the final migrated section. A new procedure has been developed for the pre-stack depth migration (PSDM) of seismic multiples which correctly images both primary and multiple energy to give a depth section without visible multiples. The three key processes in the PSDM process are the determination of a suitable velocity model, a method for the computation of travel-times, and a method of mapping seismic shot record data to the final output depth section. The velocity model in the new procedure can include horizontal, dipping planar, and irregular boundaries. Travel-time mapping is achieved by implementing Huygen's principle (Podvin and Lecomte, 1991) while also incorporating the curved wavefront approach of Schneider et al. (1992). By extending the work of Zhao (1996), reflections are modelled using the generalised exploding reflector model (Lambert, 1996) which was used by Manuel (1998) for modelling multiple reflections. This technique is capable of handling large velocity contrasts in structures with irregular boundaries. Zhao et al. (1998) showed how to map first and later arrivals arising from different travel paths in the subsurface. This new procedure treats primary and multiple events as first and later arrivals respectively, and maps them to the final depth section. Examples based on synthetically modelled data show that significant multiple content can be removed by using this new migration technique. The velocity models used for these examples are reasonably complex which enables the new technique to be tested quite rigorously. In most, if not all, depth migration procedures the resultant image quality is dependent on an accurate velocity model. A slightly inaccurate velocity model was used with the new procedure which still resulted in a reasonably accurate depth migrated section.