Multiple diffractions and coherent noise in marine seismic data

Abstract

Multiple reflections from irregularities on the sea floor, (multiple diffractions), can share a number of characteristics with the primary scattering that is also generated by such irregularities. The kinematics of the primary scattering were investigated in a classic 1983 Geophysics paper by Ken Larner and his colleagues, as part of a study of coherent noise in marine seismic data. Where the sea floor is generally smooth except for isolated irregularities, many of the conclusions of that paper can also be applied to the multiple scattering. In particular, strategies for removal of scattered noise from sea‐floor irregularities can also be applied to attenuate sea floor and other multiple diffractions. Like the primary scattering, the multiple diffractions are organised in mid‐point gathers in such a way that mid‐point stacking may enhance portions of the multiple. Also like the primary scattering, the multiples can be distinguished from primary reflection events in the data by their dip in pre‐stack shot and receiver gathers. Pre‐stack dip‐filtering in those domains can be used to remove the portions of the multiple diffractions that would otherwise be enhanced by the mid‐point stack, in the same way that dip‐filtering can be used to remove the primary scattering. Shot and receiver dip‐filtering can, however, damage diffractions and lateral amplitude changes from geology‐related primaries in the data. A partial strategy, in less complex structure, is to apply pre‐stack imaging in conjunction with multiple removal, with the aim of simplifying the geology‐related primaries and hence increasing the separation between the primaries and the multiples. The pre‐stack imaging does however increase the complexity of the multiple diffractions, and can also introduce aliasing and dispersion‐related artefacts. The multiples are mis‐migrated, but the degree of mis‐migration varies smoothly with offset. Multiple removal can still be applied, but with a reduced potential for primary damage.