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  • 1
    Electronic Resource
    Electronic Resource
    The scattering of shear-waves in the crust (1990)
    Springer
    Pure and applied geophysics 132 (1990), S. 67-91 
    Details
    ISSN: 1420-9136
    Keywords: Shear-wave splitting ; extensive-dilatancy anisotropy ; EDA ; stress-aligned cracks ; surface interactions ; localSP-wave
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract The two major sources of scattering for shear-waves in the crust, interactions with the topography at the surface and the effective anisotropy of aligned cracks throughout the rockmass, introduce first-order changes to the shear-wave particle-motion. At the surface, shear-waves are scattered by the topography within a wavelength or two of the recording site so that, unless the effective incidence angle is less than the critical angle sin−1 V S/V P, the recorded waveforms may bear little relationship to the waveforms of the incident wave. Within the rockmass, shear-waves are scattered by extensive-dilatancy anisotropy (EDA), the distribution of stress-aligned fluid-filled cracks, microcracks, and preferentially oriented pore-space pervading most rocks in the crust. Analysis of this shear-wave splitting yields new information about the internal structure of thein situ rockmass which is not otherwise available.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00874358
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  • 2
    Electronic Resource
    Electronic Resource
    Inversion of arrival-times in a region of dilatancy anisotropy (1985)
    Springer
    Pure and applied geophysics 123 (1985), S. 375-387 
    Details
    ISSN: 1420-9136
    Keywords: Inversion ; Dilatancy ; Anisotropy ; Local Earthquakes
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Shear-wave splitting has been identified in many three-component seismograms from two separate field experiments on a section of the North Anatolian Fault in North-West Turkey. These observations are consistent with shear-wave propagation through a zone of extensive-dilitancy anisotropy. A preliminary attempt has been made to confirm this interpretation by simultaneously inverting suites of arrival-times for hypocentral locations and for parameters describing an anisotropic halfspace. Although the inversion procedure is not globally convergent, it is possible to recognize the true solution by systematically varying the initial conditions. Applied to selected data sets, the inversion defines several anisotropic models that fit the data significantly better than a simple isotropic model, and display the anisotropy required by the shear-wave splitting. However, most of these anisotropic models are not superior when they are used to individually locate events in a much larger data set. However, for each experiment, there is a single model that produces clearly superior locations for the larger data sets than those of other anisotropic or simple isotropic models. Both models display similar velocity variations which are characteristic of propagation through distributions of biplanar cracks displaying orthorhombic symmetry. The principal axes of the two models are oriented in similar directions and are within 20° of the principal axis of regional stress derived from fault-plane solutions. The solutions indicate low velocities close to the tensional axis, as would be expected in extensive-dilatancy anisotropy.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00880737
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  • 3
    Electronic Resource
    Electronic Resource
    Dashti Biaz, Iran, Earthquake of August 1968 (1968)
    [s.l.] : Nature Publishing Group
    Nature 220 (1968), S. 904-905 
    Details
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] During the first 24 h of operation, 2 weeks after the main shock, 580 local events were recorded, two-thirds of which were clear enough to give direction and distance. The number of events decreased rapidly and levelled out so that 6 weeks after the main shock, when operations discontinued, there ...
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1038/220904a0
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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
    Wave propagation through fluid-filled inclusions of various shapes: interpretation of extensive-dilatancy anisotropy (1991)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 104 (1991), S. 0 
    Details
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Previous examinations of the effects of extensive-dilatancy anisotropy (EDA) on seismic waves have been largely restricted to discussions of parallel fluid-filled cracks of small aspect ratio. It is recognized, however, that EDA may be caused by stress-aligned fluid-filled cracks, microcracks, and preferentially oriented porespace, in a variety of shapes, dimensions, and distributions, which may not be adequately modelled by the effects of uniform distributions of thin parallel cracks. The effects on seismic waves are examined for distributions of inclusions ranging from spherical pores and oblate spheroids (bubbles), to penny-shaped cracks. Unlike thin cracks, distributions of aligned oblate spheroids induce significant P-wave velocity anisotropy. In contrast, the parallel polarizations of the leading split shear waves within the shear wave window, which is one of the most distinctive features of shear waves in the crust, are preserved for all aspect ratios except spherical bubbles. The 3-D effects show minor variations that are most distinctive at small aspect ratios. Shear waves are very sensitivie to changes in the geometry of such thin inclusions, and there is some observational evidence for temporal variations in splitting as the stress acting on the rockmass changes. If this sensitivity is confirmed it would suggest that the detailed geometry of a reservoir during production or enhanced oil recovery (EOR) might be monitored by repeated shearwave VSPs.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1991.tb05705.x
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  • 7
    Electronic Resource
    Electronic Resource
    Observations of shear-wave splitting from marine VSPs in the southern North Sea (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: Four offset marine Vertical Seismic Profiles (VSPs) are analysed for shear-wave splitting in a southern North Sea gas reservoir using mode-converted shear waves. Automatic estimates of fast shear-wave polarizations and time delays indicate the presence of stress-induced cracks striking at between N35°W and N60°W, corresponding to independent measurements of stress from borehole breakout data and earthquake studies. Time delays suggest that shear-wave splitting occurs only in the overlying sealing rock with no measurable anisotropy in the reservoir sands. All measurements are contaminated by scattered energy, probably resulting from non-planar interfaces and/or lateral variations in lithology. More reliable results may have been obtained if the VSP offsets were not almost parallel and perpendicular to the maximum horizontal stress direction.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1991.tb01407.x
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  • 8
    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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  • 9
    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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  • 10
    Electronic Resource
    Electronic Resource
    Preface (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
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1991.tb01400.x
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