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  • 1
    Electronic Resource
    Electronic Resource
    Transmission Properties of A Plane Fault (1996)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 125 (1996), S. 0 
    Details
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Continuity conditions are derived for a fault modelled as a plane with isolated areas of slip. These slip areas are, for simplicity, taken to be such that their overall effect is that of a distribution of circular cracks; discontinuities in both normal and tangential components of displacement are allowed, depending on the internal conditions. Dry (gas-filled), partial or saturated liquid fill, or a fill of a weak visco-elastic solid are possible within the theory. the results are given in terms of the mean wave, which, at wavelengths long compared with the scale-lengths of the fault structure, is an accurate approximation to the displacement field. the continuity conditions that arise under this scheme are identical to those for a thin layer of visco-elastic material. However, unlike earlier, more empirical models of an ‘averaged’ fault, the parameters involved are directly related to the fault structure and include crack-crack interactions. It is clear from earlier work that a fault of this type is capable of supporting Stoneley waves.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1996.tb00018.x
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  • 2
    Electronic Resource
    Electronic Resource
    Effects of point singularities on shear-wave propagation in sedimentary basins (1991)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 107 (1991), S. 0 
    Details
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: In 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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1991.tb01413.x
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  • 3
    Electronic Resource
    Electronic Resource
    Observations of shear-wave polarizations from rockbursts in a South African gold field: an analysis of acceleration and velocity recordings (1991)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 107 (1991), S. 0 
    Details
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Rockbursts in the deep gold mines of South Africa have seismic magnitudes ranging from ML less than zero to ML more than 5. They occur in very confined volumes surrounding the working face of the mining excavations. An examination of three-component acceleration and velocity seismograms shows that the polarizations of shear waves recorded within the shear-wave window above an active mining area have a nearly uniform alignment. The polarization alignment and the measured time delays are consistent with shear waves propagating through the effective anisotropy of parallel vertical microcracks throughout the rockmass. Polarizations measured from velocity transducers were compared with polarizations measured from strong-motion acceleration recordings to show the amount of scatter in the data. We conclude that the dry fractures caused by the high stresses during normal mining processes have negligible effect at the wavelengths at which shear waves are recorded at the surface. The anisotropy observed at the surface appears to be due to microcracks aligned by the regional stress regime rather than local disturbances to the stress regime due to mining operations.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1991.tb01425.x
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  • 4
    Electronic Resource
    Electronic Resource
    The fracture criticality of crustal rocks (1994)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 118 (1994), S. 0 
    Details
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: The shear-wave splitting observed along almost all shear-wave ray paths in the Earth's crust is interpreted as the effects of stress-aligned fluid-filled cracks, microcracks, and preferentially oriented pore space. Once away from the free surface, where open joints and fractures may lead to strong anisotropy of 10 per cent or greater, intact ostensibly unfractured crustal rock exhibits a limited range of shear-wave splitting from about 1.5 to 4.5 per cent differential shear-wave velocity anisotropy. Interpreting this velocity anisotropy as normalized crack densities, a factor of less than two in crack radius covers the range from the minimum 1.5 per cent anisotropy observed in intact rock to the 10 per cent observed in heavily cracked almost disaggregated near-surface rocks.This narrow range of crack dimensions and the pronounced effect on rock cohesion suggests that there is a state of fracture criticality at some level of anisotropy between 4.5 and 10 per cent marking the boundary between essentially intact, and heavily fractured rock. When the level of fracture criticality is exceeded, cracking is so severe that there is a breakdown in shear strength, the likelihood of progressive fracturing and the dispersal of pore fluids through enhanced permeability. The range of normalized crack dimensions below fracture criticality is so small in intact rock, that any modification to the crack geometry by even minor changes of conditions or minor deformation (particularly in the presence of high pore-fluid pressures) may change rock from being essentially intact (below fracture criticality) to heavily fractured (above fracture criticality). This recognition of the essential compliance of most crustal rocks, and its effect on shear-wave splitting, has implications for monitoring changes in any conditions affecting the rock mass. These include monitoring changes in reservoir evolution during hydrocarbon production and enhanced oil recovery, and in monitoring changes before and after earthquakes, amongst others.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1994.tb03974.x
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  • 5
    Electronic Resource
    Electronic Resource
    Behaviour of shear waves in rocks with two sets of parallel cracks (1993)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 113 (1993), S. 0 
    Details
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: The behaviour of shear wave polarizations and shear wave splitting observed at the surface suggesting propagation through parallel vertical cracks has been the stimulus for many recent investigations, both in earthquake and exploration seismology. Cracks in surface outcrops, however, frequently display multiple sets of parallel vertical intersecting cracks. This paper examines seismic shear wave propagation in media with two sets of parallel vertical cracks (biplanar cracks) to determine whether the behaviour of shear waves can distinguish between the effects of multiple crack sets and the effects of single sets of parallel cracks (monoplanar cracks). This study shows that the difference between the overall patterns of polarizations of biplanar and monoplanar systems of vertical cracks within the shear wave window in many circumstances is marginal, and unlikely to be easily recognized in the field. We conclude that it is frequently not possible, from analysis of surface observations of shear wave polarizations alone, to distinguish between the effects of biplanar sets of parallel vertical cracks and those of a single parallel set. The difference can usually be recognized if an accurate estimate of both polarizations and time delays between the split shear waves is available over a wide range of azimuths and angles of incidence within the shear wave window. However, in areas with complex fracture and stress systems, time delays may be much harder to estimate than the polarization angles of the leading split shear waves, and it may not be easy to distinguish, from seismic data alone, the difference between parallel and multiplanar sets of vertical fractures.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1993.tb00903.x
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  • 6
    Electronic Resource
    Electronic Resource
    The range of effects of azimuthal isotropy and EDA anisotropy in sedimentary basins (1991)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 107 (1991), S. 0 
    Details
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Combinations of bedding- or lithology-induced azimuthal isotropy, with an axis of symmetry perpendicular to the bedding plane, and crack-induced extensive-dilatancy 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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1991.tb01412.x
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  • 7
    Electronic Resource
    Electronic Resource
    Dispersion of guided waves in thin anisotropic waveguides (1991)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 107 (1991), S. 0 
    Details
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: This paper shows that: if appropriate source mechanisms and frequency bands can be selected, thin layers in sedimentary reservoirs may act as seismic waveguides, and guided waves may be useful for reservoir characterization in cross-hole surveys; and that the dispersion of such guided waves is sensitive to crack saturations and crack orientations, and could contain important information about the level of saturation in specific thin layers in enhanced oil recovery operations. In the first part of the paper, we extend a technique, previously used for calculating the dispersion of surface waves in a plane-layered anisotropic half-space, to calculating the dispersion of guided waves in the interior of a multilayered anisotropic solid. In the second part, we demonstrate the effects of anisotropy by calculating synthetic guided waves in a thin gas-sand layer displaying crack-induced anisotropy.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1991.tb01414.x
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  • 8
    Electronic Resource
    Electronic Resource
    Viability of shear-wave amplitude versus offset studies in anisotropic media (1991)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 107 (1991), S. 0 
    Details
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: The study of shear-wave splitting can yield information about crack strike, crack density, and other parameters of the cracks and aligned inclusions within the in situ rockmass. Such information is important for reservoir characterization and other hydrocarbon production applications. Crack density is usually inferred from the percentage shear-wave anisotropy which is measured from time delays between the two split shear waves. Reservoirs may be too thin for discernible time delays to build up, and the time delay (and crack density) in a reservoir layer may be unresolvable in conventional arrival-time analysis of reflection surveys or vertical seismic profiles.Shear-wave amplitude versus offset (AVO) techniques are studied to see if they provide a more viable method of determining anisotropic parameters in thin reservoir layers. The behaviour of reflected shear-wave amplitudes with angle of incidence is investigated in simple two-layer models. The variations in shear-wave AVO are calculated for a range of percentage shear-wave anisotropies for water-filled cracks and dry cracks in the second layer (representing a reservoir) to see if characteristic information about reservoir properties can be extracted from the shear-wave AVO signatures. The shear-wave AVO curves, for thin cracks, are sensitive to changes in anisotropy and crack content. Most of the information about crack content is contained in wide offset reflection data which suggests applications in crosshole monitoring of enhanced oil recovery. The variations in shear-wave AVO may be distorted by the acquisition system. Most of the information about the percentage anisotropy is contained in the amplitudes of shear waves reflected at near-vertical angles of incidence. Vertical incidence reflection amplitude methods are reviewed and a simple graphical procedure is suggested for determining their viability in different reservoir environments.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1991.tb01410.x
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  • 9
    Electronic Resource
    Electronic Resource
    Complex component analysis of shear-wave splitting: theory (1991)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 107 (1991), S. 0 
    Details
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: The use of complex arithmetic is a natural way to treat vectorially polarized data, where the real and imaginary components can be taken as two perpendicular axes. This transforms multicomponent data from conventional Cartesian coordinates to polar coordinates, and allows the calculation of instantaneous amplitude and instantaneous polarization. We call this technique complex component analysis. Wave motion can be represented by instantaneous attributes which show distinct features characteristic of the type of wave motion. It is particularly informative to examine shear-wave splitting by instantaneous attributes. The instantaneous amplitude of shear-wave splitting has a number of local maxima, and the instantaneous polarization has a combination of rectangular and semitriangular shapes. Shear-wave splitting can be identified from displays of instantaneous amplitude and polarization, where the polarization of the faster split shear wave and the delay between the two split shear waves can be quantified from colour-coded displays. The instantaneous attributes can be displayed as wiggle-lines of amplitude superimposed on a colour-coded polarization, where the use of colour improves the indentification and quantification of shear-wave splitting.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1991.tb01419.x
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  • 10
    Electronic Resource
    Electronic Resource
    Complex component analysis of shear-wave splitting: case studies (1991)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 107 (1991), S. 0 
    Details
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Complex component analysis has been applied to four shear-wave data sets: a four-component single-offset VSP in the Lost Hills, Kern County; a multi-offset single-source two-component VSP in the Paris Basin; a four-component reflection line in the Lost Hills; and an in-seam crosshole survey at German Creek Mine, Australia. In all cases, the polarization of the faster split shear wave (or channel wave) can be assessed from the colour-coded record sections of the seismic attributes. In particular, the source-independent coherent polarization on the colour sections of the complex components of in-line and cross-line sources allows the shear-wave polarization angle to be determined without need for rotation of the instrument and source axes. We conclude that complex component analysis can aid identification and estimation of shear-wave splitting from seismic sections, help stratigraphic interpretation, and simplify the processing sequence of multicomponent reflection data in the presence of anisotropy.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1991.tb01420.x
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