Observations of EDA and PTL anisotropy in shear wave VSPs

Bush, I.; Crampin, Stuart

doi:10.1190/1.1892023

Cited by 22 publications

(35 citation statements)

References 2 publications

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“…These will lead to anomalies in the shear-wave amplitudes, polarizations, and time delays (Crampin 1991a). Such anomalies have been identified in multi-offset VSPs in the field (Bush 1990;Bush & Crampin 1987Crampin 1988). The inner circle marks the edge of the shear-wave window at the free surface (Evans 1984;Booth & Crampin 1985).…”

Section: Equal-area Projectionsmentioning

confidence: 95%

“…In a detailed analysis of shear-wave splitting in a multi-offset vertical seismic profile (VSP) study of the Paris Basin (Bush & Crampin 1987Bush 1990), it was found that the azimuthal variations of synthetic seismograms and synthetic polarization diagrams could not match observed seismograms and polarization diagrams unless both PTL and EDA anisotropy were included. The prevalence of EDA and PTL anisotropy, identified separately, suggests that combinations of EDA and PTL anisotropy are common in many sedimentary basins.…”

Section: Introductionmentioning

confidence: 96%

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The range of effects of azimuthal isotropy and EDA anisotropy in sedimentary basins

Wild

Crampin

1991

Geophysical Journal International

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S U M M A R Y Combinations of bedding-or lithology-induced azimuthal isotropy, with an axis of symmetry perpendicular to the bedding plane, and crack-induced extensivedilatancy anisotropy (EDA), with a horizontal axis of symmetry, are believed to be common in sedimentary basins, and cause the widely observed phenomenon of shear-wave splitting. Combinations of two such transversely isotropic forms of anisotropy with orthogonal axes of cylindrical symmetry lead to orthorhombic symmetry. This has two major effects: (1) the polarizations of the faster split shear waves may no longer be parallel to the strike of the cracks, or fractures, even for near-vertical propagation; and (2) such orthorhombic symmetry systems necessarily have a number of directions, called shear-wave point singularities, where shear waves display disturbed or anomalous behaviour, again possibly in near-vertical directions. Unless these effects are correctly identified, they could be interpreted mistakenly for the effects of structural irregularities or discontinuities. In contrast, recognition of the 3-D geometry of this behaviour places comparatively tight constraints on possible combinations of anisotropy in the rockmass. In order to give some understanding of the geometry of these phenomena, this paper presents 3-D patterns of the behaviour of shear-wave splitting that have been computed for a range of combinations of crack-and bedding-induced anisotropy .

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Section: Equal-area Projectionsmentioning

confidence: 95%

Section: Introductionmentioning

confidence: 96%

The range of effects of azimuthal isotropy and EDA anisotropy in sedimentary basins

Wild

Crampin

1991

Geophysical Journal International

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“…Bush & Crampin (1987) and Bush (1990) have shown that many features of shear-wave propagation in the Paris Basin can be simulated by propagation through uniform rock with combinations of (P)eriodic (T)hin (L)ayer anisotropy ( P T L anisotropy ) and parallel vertical inclusions (EDA anbotropy). PTL anisotropy has hexagonal symmetry (transverse isotropy) with a vertical axis of cylindrical symmetry, whereas EDA anisotropy has hexagonal symmetry with a horizontal axis of symmetry, parallel to the minimum horizontal compressional stress.…”

Section: A N I S O T R O P Y O F S E D I M E N T a R Y Basinsmentioning

confidence: 98%

Effects of point singularities on shear-wave propagation in sedimentary basins

Crampin

1991

Geophysical Journal International

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S U M M A R YIn most directions of propagation in anisotropic solids, seismic shear waves split in regular and predictable ways that, in principle, can be directly related to the degree of anisotropy and the anisotropic symmetry of the rockmass. In all anisotropic solids, however, there are directions of propagation, known as shear-wave singularities, where the split shear-waves have the same phase-velocities. For directions of propagation near the commonest type of singularity, the point singularity, the relationship between the phase and group-velocities may undergo rapid variations for small changes in direction. This results in shear-waves along rays (propagating at the group-velocity) behaving anomalously, with irregular polarizations and amplitude changes as if they were propagating near cusps, although the degree of anisotropy may be too small to cause conventional cusps on the group-velocity wave surfaces.The effects of propagation near such point singularities have been identified in sedimentary basins where they are features of the well-established phenomenon of azimuthal isotropy (transverse isotropy with a vertical axis of symmetry) caused by horizontal lithology, or by fine layering (PTL anisotropy), combining with the more recently recognized azimuthal anisotropy, caused by distributions of near-parallel near-vertical fluid-filled inclusions (EDA anisotropy). This paper demonstrates these irregular effects by calculating synthetic shear waves in directions near a point singularity in a material simulating a possible sedimentary basin. Such anomalies may be important in exploration seismology as point singularities can occur along nearly vertical ray paths in sedimentary basins. If not identified correctly, the effects of such point singularities could be mistakenly attributed to structural irregularities, and if correctly identified, the directions of such singularities can place tight constraints on possible combinations of PTL and EDA anisotropy in sedimentary basins.

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“…Rayleigh wave dispersion splitting at 45°to the major joints coincides with S-wave splitting; the latter is regarded as direct evidence of medium anisotropy (Crampin, 1981;Bush and Crampin, 1987). The arrival time difference between the fast and low S-waves reaches the maximum at 45°, leading to a clear view of the S-wave splitting phenomenon.…”

Section: Simulation Results and Discussionmentioning

confidence: 94%

“…Layering anisotropy and azimuthal anisotropy are two common types of anisotropy in geologic strata (Bush and Crampin, 1987). A stack of non-horizontal isotropic layers produces layering anisotropy for wavelengths that are appreciably greater than the layer thickness, with symmetry axis perpendicular to the layering.…”

Section: Introductionmentioning

confidence: 99%