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The split‐step Fourier method is used to prestack migrate two synthetic borehole‐to‐surface shot gathers. Model structures in the zone of specular illumination beneath the shot are reconstructed by using the split‐step Fourier method both to back‐propagate the recorded wavefield and to forward propagate the source wavelet. The overburden is vertically and laterally inhomogeneous. Each depth interval is treated as a homogeneous strip with the mean velocity plus an inhomogeneity correction term. The inhomogeneity correction term is split and spatially multiplied with each spectral component of the wavefield on its entry to and upon its exit from each strip. Propagation through each strip is effected by multiplication in the spatial frequency domain. The split‐step Fourier method offers a valuable alternative to finite‐difference migration for machines with limited memory. Three imaging methods are compared for two signal‐to‐noise ratios. They are: image extraction by traveltime, crosscorrelation with source wavelet, and deconvolution with source wavelet. At high signal‐to‐noise level, the image formed by deconvolution offers better spatial resolution than images formed by crosscorrelation with the source wavelet or by extraction using traveltime. If the signal‐to‐noise level is low, traveltime imaging deteriorates rapidly, while deconvolution images degrade towards those created by crosscorrelation imaging.
JACKSON, G.M., MASON, I.M. and LEE, D. 1991. Multicomponent common-receiver gather migration of single-level walk-away seismic profiles. Geophysical Prospecting 39, 1015-1029.Seismic data are usually separated into P-waves and S-waves before being put through a scalar (acoustic) migration. The relationship between polarization and moveout is exploited to design filters that extract the desired wavetype. While these filters can always be applied to shot records, they can only be applied to a triaxial common-receiver gather in special cases since the moveout of scattered energy on the receiver gather relates to path differences between the surface shots and the scatterer while the polarization is determined by the path from scatterer to downhole geophone. Without the ability to separate wavefields before migration, a ' vector scalar ' or an elastic migration becomes a necessity.Here the propagation of the elastic wavefield for a given mode (e.g. P-S) is approximated by two scalar (acoustic) propagation steps in a 'vector scaiar' migration. 'Vector' in that multicomponent data is migrated and 'scalar' in that each propagation step is based on a scalar wave equation for the appropriate mode. It is assumed that interaction between the wavefields occurs only once in the far-field of both the source and receiver. Extraction of the P, SV and SH wavefields can be achieved within the depth migration (if one assumes isotropy in the neighbourhood of the downhole receiver) by a projection onto the polarization for the desired mode. Since the polarization of scattered energy is only a function of scatterer position and receiver position (and not source position), the projection may be taken outside the migration integral in the special case of the depth migration of a common-receiver gather. The extraction of the desired mode is then performed for each depth migration bin after the separate scalar migration of each receiver gather component.This multicomponent migration of triaxiai receiver gathers is conveniently implemented with a hybrid Split-step Fourier-excitation-time imaging condition depth migration. The raytracing to get the excitation-time imaging condition also provides the expected polarization for the post-migration projection. The same downward extrapolated wavefield can be used for both the P-P and P-S migrations, providing a flexible and effcient route to the migration of multicomponent data.' Based on paper read at the 52nd EAEG meeting, The technique is iilustrated on a synthetic example and a single-level Walk-away Seismic Profile (WSP) from the Southern North Sea. The field data produced images showing a P-P reflector below the geophone and localized P-P and P-S scatterers at the level of the geophone. These scatterers, which lie outside the zone of specular illumination, are interpreted as fauits in the base ZechsteinJtop Rotliegendes interface.
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