Elimination of seismic multiples by anisotropic, pre stack depth-migration and filtering

Robert James Ferguson

Seismic multiples associated with the sea floor can distort the interpretation of seismic reflection data where they are mistakenly interpreted as primaries, or where they overly reflections of interest. Numerous techniques exist by which multiples are removed. For example, some predict the amplitude and phase of the multiples so that they may be subtracted. Other approaches to multiples elimination cast the entire seismic recording into a domain where the multiples separate from the primaries, and then a filter is used to delete the multiples.

Here, I explore a method by which multiples are focused along with the rest of the recorded wavefield in the space domain by prestack migration. There, the multiples are erased, and the space domain data are unmigrated. Central to this method is the derivation of a migration velocity that focuses the multiples. This velocity is computed based on the Huygens model of seismic reflection (and multiple reflection) as the superposition of point scatterers. This model leads to a relationship between a primary scatterer and a multiple scatterer that is exploited to deduce the required focusing velocity for the multiples. Interestingly, the focusing velocity is anisotropic.

A closed form solution is presented for the group velocity for use in ray-based migration, and an implicit solution is presented for the phase velocity for use in Stolt-like migration methods.

A data example is used to demonstrate this multiples elimination approach -multiples in the data are identified, the corresponding anisotropic multiples velocity is computed, and the multiples are erased from the data following migration. The data are then unmigrated.