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  • 1
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 123 (1995), S. 0 
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Inclusion of amplitude data in reflection seismic tomography may help to resolve the ambiguity caused in the traveltime inversion by the trade-off between reflector position and velocity anomaly. To illustrate the uses of amplitude data we initially exclude all traveltime information from the inversion. In a previous paper (Wang & Houseman 1994) we have shown, using geologically relevant synthetic models, that the information contained in amplitude versus offset data suffices to accurately constrain the geometry of an arbitrary smooth 2-D reflector separating constant velocity layers. In this paper we investigate the implementation of the inversion for 2-D velocity variations using reflection seismic amplitude data.A stable method of ray tracing in a 3-D heterogeneous velocity medium is presented. The ray-geometric spreading which partly determines the ray amplitude is then calculated according to the propagator along the ray path. The ray-perturbation theory is used to trace the perturbed ray due to the model perturbation. We compare amplitude perturbations arising from slowness perturbations along the whole ray path with those arising from the slowness perturbation close to the interface, and see that in an inversion of reflection seismic amplitude data, the data residuals will have most effect on velocity anomalies near the interface. Synthetic models are used to demonstrate the efficacy of amplitude inversion for velocity variation, using the subspace inversion method, with a 2-D Fourier series parametrization of the slowness distribution. The efficiency of the inversion lies in a judicious partitioning of model parameters into subspaces. A stable strategy for the parameter partitioning is to separate parameters on the basis of the magnitude of rms values of the Frechet derivatives of ray amplitudes with respect to the model parameters. Numerical examples show that the amplitudes of reflected signals are sensitive to the location of the velocity anomalies. Inversions provide an approximate image of velocity variation, demonstrating that amplitude data contain information that can constrain unknown velocity variation.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 125 (1996), S. 0 
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: The geological record of deformation is often characterized by a combination of discontinuous deformation, in which strain is concentrated in faults, and continuous deformation, in which strain is distributed through the material. Where slip occurs on a fault that terminates, the surrounding material is deformed. In the lower crust and in cases where large strains occur over long geological time-scales, it is appropriate to model the deformation using a viscous (probably non-linear viscous) rheology. We describe a method for practical finite-element solution of this problem using a dynamically self-consistent formulation for stress and displacement on a fault of arbitrary geometry; the accuracy of the method is tested by comparison with an analytical solution for the linear rheology. We describe here the instantaneous deformation fields around a mode II fault under both plane-strain and plane-stress conditions, and a range of rheological exponents n (where strain rate is proportional to deviatoric stress to the nth power). the distributions of stress and strain rate around the fault tip are controlled primarily by the rheological exponent n. A localized zone of high strain rate projects beyond the end of the fault if n is about 3 or greater, and the degree of localization of deformation increases with the value of n. the zone of high shear-strain rate can be defined in practical terms by considering (1) the region in which the creep velocity differs by more than 20 per cent from the velocity on the nearby external boundary and (2) the region in which the maximum shear-strain rate is greater than about twice the externally imposed shear-strain rate. For n= 1, the volumes so defined differ considerably, but for large values of n, the two definitions both describe the same narrow zone of deformation beyond the end of the fault. Evaluation of the Navier-Coulomb criterion for brittle failure of the medium surrounding the fault tip shows first that brittle failure is much more likely on the extensional side of the fault than the compressional side. It also shows that the volume of material subject to brittle failure decreases rapidly with increasing n because of the relatively weaker stress singularity. We analyse previously published displacement versus distance data for faults terminating in sedimentary rocks at 0.1 to 100 m length-scales under different tectonic conditions, in order to determine the rheological exponent n. These analyses result in n values between approximately 0.85 and 5 for the different faults, with error bounds on n typically pL 1. the variation in n values may result from differences in pressure, temperature and fluid conditions at the time of faulting. More importantly, the analysis demonstrates a new method for the determination of the effective rheological exponent under in situ geological conditions.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 117 (1994), S. 0 
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Reflection seismic tomography using only traveltime data may be unable to resolve the ambiguity caused by trade-off between reflector position and velocity anomaly. the inclusion of amplitude data in the inversion may help to resolve this problem because the amplitudes and traveltimes are sensitive to different features of the model, therefore providing us with more accurate information about underground structures and velocity distribution. the amplitude of a reflected seismic wave is determined partly by the reflection coefficients and partly by the curvature of the reflector. the latter causes the spherical divergence of the seismic rays to be modified at the reflection point (focused or defocused) and can be represented using a simplified analytical expression. We show, using geologically relevant synthetic models, that the information contained in amplitude versus offset data (here excluding traveltime data) suffices to constrain accurately the geometry of an arbitrary 2-D reflector separating constant velocity layers. the most effective inversion method is a subspace gradient algorithm using a model parametrization in which the interface is described as a discrete Fourier series with fixed upper and lower bounds on the wavenumber. Model parameters are allocated to separate subspaces first on the basis of different physical dimensionality. We also found that declaring separate subspaces for those parameters defining short, intermediate and long wavelength components of the interface geometry, based on the magnitude of singular values of the Frechet derivative matrix, is very effective in accelerating convergence and obtaining a more accurate solution. the inversion is robust with respect to data errors and poor initial estimates.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    Boston, MA, USA : Blackwell Science, Inc.
    Restoration ecology 10 (2002), S. 0 
    ISSN: 1526-100X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Notes: Jack pine barrens, once common in northern lower Michigan, mostly have been converted to managed jack pine plantations. Management of the disturbances associated with logging provides the opportunity to maintain the unique plant assemblages of jack pine barrens and nest habitat of the federally endangered Kirtland's warbler. Studies indicate that Carex pensylvanica can develop into dense mats and strongly compete with other barrens species such as Vaccinium angustifolium, which seem to be important species for Kirtland's warbler nest locations. According to forest managers, the most important factors facilitating high cover of V. angustifolium and reducing cover of C. pensylvanica are the amount of shade produced by tree crowns before harvest (pre-harvest shade), the length of time between harvest and planting (planting delay), and fire. We found that high or low levels of pre-harvest shade had no effect on cover of either V. angustifolium or C. pensylvanica. Planting delays of at least three years following prescribed burns generally increased cover of V. angustifolium in forest plots, which are important for warbler nesting. Analysis of community composition in openings indicated that burning enhanced the growth of barrens species. We found only weak evidence for a negative correlation between the cover of V. angustifolium and C. pensylvanica on our study sites. The openings created in the jack pine plantation are important refugia for barrens flora that would likely be lost under forests managed strictly for jack pine. Maintenance of jack pine barrens flora and Kirtland's warbler nest habitat is possible within the context of a heavily managed forest plantation system.
    Type of Medium: Electronic Resource
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  • 5
    Electronic Resource
    Electronic Resource
    [s.l.] : Nature Publishing Group
    Nature 315 (1985), S. 297-301 
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] Unlike rigid oceanic plates, continents show diffuse deformation, for example the Tibetan plateau that has accommodated much of the convergence of India with Asia. We interpret the Tertiary deformation of Asia as the three-dimensional strain of a non-linear, viscous continental lithosphere. Lateral ...
    Type of Medium: Electronic Resource
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  • 6
    Electronic Resource
    Electronic Resource
    [s.l.] : Nature Publishing Group
    Nature 332 (1988), S. 346-349 
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] Many previous two-dimensional convection calculations have been published1'7 in which the planform is necessarily prescribed as a series of alternating (often time-dependent) hot and cold sheet-type structures that extend into the third dimension. In general, such a planform is unstable at large ...
    Type of Medium: Electronic Resource
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  • 7
    Electronic Resource
    Electronic Resource
    [s.l.] : Nature Publishing Group
    Nature 343 (1990), S. 32-32 
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] MANTLE convection has become one of the most exciting fields in the Earth sciences because it is at the heart of the basic physical and chemical configuration of our planet. It is a field that has seen great progress and intense disagreement in the past 20 years. An enormous volume of literature ...
    Type of Medium: Electronic Resource
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  • 8
    Electronic Resource
    Electronic Resource
    [s.l.] : Nature Publishing Group
    Nature 379 (1996), S. 771-772 
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] IT has been speculated that sometimes sub-continental lithosphere is recycled into the convecting mantle below, but the process has never been clearly seen. On page 785 of this issue1, Seber and co-workers describe new geophysical evidence for the process of lithospheric delamination occurring ...
    Type of Medium: Electronic Resource
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  • 9
    Electronic Resource
    Electronic Resource
    [s.l.] : Nature Publishing Group
    Nature 444.2006, 7117, E4-, (1 S.) 
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] Arising from: D. B. Rowley & B. S. Currie Nature 439, 677–681 (2006); Rowley & Currie reply The determination of palaeo-elevation has emerged in the past 15 years as an important tool for constraining physical ...
    Type of Medium: Electronic Resource
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  • 10
    ISSN: 1420-9136
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Type of Medium: Electronic Resource
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