Neil Hargreaves

Surface multiple attenuation in shallow water and the construction of primaries from multiples

The central insight that comes from the work of Berkhout, Verschuur and others in the development of their method for surface-related multiple elimination (SRME) is that multiples, normally considered as unwanted noise in seismic data, are intimately related to, and can be constructed from, primary reflections. It follows that multiples contain equivalent information about the subsurface to that which is given by the primaries. There are circumstances, therefore, in which the multiples can add to signal, i.e. they can be an aid to seismic processing and interpretation rather than being viewed simply as unwelcome noise.

Similarity and the inverse Q filter; some simple algorithms for inverse Q filtering

A number of authors have addressed the issue of the time variance of the forward and inverse Q filter on the assumption of a frequency‐constant or nearly frequency‐constant Q factor. Early papers by Futterman (1962) and Carpenter (1966) derived time‐domain solutions for the forward Q filter, given an assumed behavior of Q at low and high frequencies, while Kjartansson (1979) used an assumed stress‐strain relationship to derive the forward Q filter for a precisely frequency‐constant Q. Bickel and Natarajan (1985) applied these earlier results to the derivation of various expressions for the inverse Q filter.

Air-gun source instabilities

The signature of an air‐gun array can change over a period of time or even from one shot to the next. If the signature variations are large, then deterministic deconvolution, with an operator designed from a single signature or from an average signature, could produce errors significant enough to affect data interpretation. Possible sources of air‐gun instability include changes in gun positions, firing times, and pressures, gun failures, and scattering from the fluctuating rough ocean surface. If an air‐gun array were perfectly stable, after application of signature deconvolution the residual signatures for a sequence of shots would be identically shaped, broadband, zero‐phase wavelets. In practice, air‐gun instabilities lead to two major defects in band‐ limited residual signatures: the central portion of the wavelet can become asymmetrical, and unsuppressed energy can occur in the residual bubble region. Processing experiments done with synthesized air‐gun array signatures show that of all types of air...

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.

Air-gun signatures and the minimum-phase assumption

The air‐gun array signature is close to minimum‐phase as a function of continuous time, in the sense that for processing purposes its phase spectrum can be derived from the Hilbert transform of the logarithm of its amplitude spectrum. This phase spectrum is different, however, from the minimum‐phase spectrum that is estimated by spiking deconvolution for a sampled and time‐windowed version of the signature. As a consequence, there can be large phase errors when spiking deconvolution is applied to an air‐gun signature or to a recording instrument response. The errors can be shown to consist primarily of a time shift and, at least visually over a limited bandwidth, a phase rotation of the output wavelet. The time shift is introduced by time sampling, while the phase rotation is caused by the spectral smoothing generated by time windowing. If the seismic wavelet as a whole, and not just the air‐gun signature, is minimum‐phase, then the total residual phase error after spiking deconvolution, including also th...

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.

Transition‐zone surveying: Phase‐equalization in the absence of overlap

The transition-zone between land and sea is one of the most challenging environments for the seismic method. Frequent changes in acquisition parameters and in near-surface conditions mean that considerable effort is required to obtain an image of the subsurface that is free from acquisition-related artefacts. In this paper we concentrate on one aspect of this problem removal of the phase changes introduced by a variation of the seismic source. The nature of the problem obliges us to use statistical techniques, including the widely employed, but much maligned, minimum-phase assumption.