Richard Wombell

Multiple Attenuation Using an Apex Shift Radon Transform

Multiples from seafloor scatterers and peg-leg multiples in complex geology are often resistant to conventional multiple removal techniques such as Radon demultiple. They have a complicated moveout behaviour in prestack gathers which can only be approximately represented by a conventional parabolic or hyperbolic Radon decomposition. Such multiples split into pairs of events, one for each of the shot or receiver side of the multiple. They are approximately parabolic after NMO correction with primary velocities but have their minimum travel times shifted to either side of zero-offset.

Long Offset Acquisition and Processing for Sub-Basalt Imaging

Summary Imaging though basalt can be difficult using standard seismic techniques. Modeling suggests that the large velocity contrast at the top basalt should be an efficient generator of mode-converted energy and that, if recorded, this data might be suitable for more successful sub-basalt imaging. In this paper we describe a long-offset acquisition experiment designed to record such data and initial processing results.

Fracture orientation estimation from azimuthal AVO, offshore Abu Dhabi

A 3D OBC survey was acquired over an offshore Field in Abu Dhabi, Middle East during 2000. The acquisition geometry of the survey provides data that is well sampled with respect to azimuth and offset range. Sensitivity analysis indicated that the data is suitable for azimuthal Pwave AVO studies. Data processing was designed to preserve offset-azimuth amplitude variations. A subset of the 3D data volume was extracted for azimuthal P-wave AVO analysis. The results suggest that there is a dominant trend of open fractures within the study volume.

Q-guided wavelet-domain amplitude correction

This paper presents a novel way of treating seismic reflection amplitudes for transmission effects due to the overburden. The process can have benefits for structural interpretation due to the resulting improved visibility of events, and further processing of the data can also benefit from a reduction in processing artefacts such as migration smiles.

Multiple diffractions and coherent noise in marine seismic data

Summary 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 geologyrelated 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.