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  • 1
    Electronic Resource
    Electronic Resource
    A New Field Experiment in the Greenland Ice Cap to Test Newton's Inverse Square Law (1989)
    Oxford, UK : Blackwell Publishing Ltd
    Annals of the New York Academy of Sciences 571 (1989), S. 0 
    Details
    ISSN: 1749-6632
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Natural Sciences in General
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1749-6632.1989.tb50553.x
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  • 2
    Electronic Resource
    Electronic Resource
    Elastic full-waveform inversion for earthquake source parameters (1996)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 127 (1996), S. 0 
    Details
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: 2-D full-waveform inversion of double-couple earthquake sources is implemented. Temporally and spatially extended sources are represented by superposition of double-couples. The source parameters solved for are the spatial location, origin time, amplitude and orientation of each double-couple. The velocity and density distribution and source time function are assumed to be known a priori but may be arbitrarily complicated. The non-linear inverse problem is solved by iterative linear approximation. The Jacobian matrix elements for source depth and rupture angle are computed by wavefield extrapolation forward in time, while those for origin time and amplitude are computed analytically. A smoothing technique that results in faster convergence and avoids local minima associated with cycle skipping is applied at each iteration. A spatial sampling interval, between discrete sources, of one-quarter wavelength of the dominant shear wave is optimal for inversion if high uniqueness of the result is desired. The presence of a fault is inferred from the spatial continuity of the rupture solution, rather than being imposed a priori. The method is illustrated by successful application to three synthetic source models: a single double-couple, a single extended rupture and a double extended rupture. The resolutions of the source depth and origin time are higher, and their posterior covariances are lower than those of the amplitude and rupture angle at each source point. Source depth, origin time and amplitude are primarily determined by the data; the rupture angle is more strongly influenced by the a priori information.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1996.tb01535.x
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  • 3
    Electronic Resource
    Electronic Resource
    Anisotropy and non-linear polarization of body waves in exponentially heterogeneous media (1995)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 123 (1995), S. 0 
    Details
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: In all media in which the elastic stiffness tensor and density vary in space, an implicit, frequency-dependent anisotropy is present, even if the medium is locally isotropic. This predicts a non-linear, frequency-dependent polarization of body waves, and explains shear-wave splitting in heterogeneous media. Examples are presented for compressional and shear waves in a medium in which the elastic properties vary exponentially; this specific form of heterogeneity allows solutions in terms of plane waves.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1995.tb06902.x
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  • 4
    Electronic Resource
    Electronic Resource
    A new approach to pre-stack seismic processing (1995)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 121 (1995), S. 0 
    Details
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Pre-stack processing of seismic reflection data is significantly simplified if the data organization is the same as that in which the data were acquired in the field; that is, in time slices through common-source gathers. For an impulsive source, the entire processing stream reduces to two elements: velocity analysis and reverse-time pre-stack migration. Many of the steps that are applied as independent operations in standard processing (including demultiplexing, sorting into common-midpoint gathers, elevation corrections, near-surface velocity static corrections, first break muting, ground roll removal, and both normal and dip move-out corrections) either are eliminated, or are applied implicitly during migration. This approach is ideally suited to parallel processing, and can be implemented in machines with small processor memories.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1995.tb03525.x
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  • 5
    Electronic Resource
    Electronic Resource
    Simulation of long-period 3-D elastic responses for whole earth models (1995)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 120 (1995), S. 0 
    Details
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Improvements in GDSN seismic data quality and advances in numerical algorithms for solution of the wave equation now make it useful and feasible to simulate long-period elastic responses for 3-D whole earth models. The algorithm used is eighth-order finite-differencing on a staggered grid; this allows stable computation at boundaries with large contrasts in Poisson's ratio, such as the core-mantle boundary. As the solution automatically generates all wave types, it may lead to prediction of previously unobserved (or misinterpreted) phases and allow more detailed analysis of all phases. Simulations of large (e.g. nuclear) explosions and double-couple earthquake sources reveal the effects on relative amplitudes, of their respective radiation patterns, in both wavefield snapshots and seismogram profiles. A large number of phases are affected, even by a single smooth bump on the core-mantle boundary. A 3-D whole earth model implicitly contains the travaltimes for all source/receiver positions, and, for any source configuration, the corresponding amplitudes of all phases. Thus, 3-D numerical modelling of whole earth responses is a potentially valuable diagnostic and interpretive tool, and a basis for globalization (in contrast to regionalization) of traveltime tables.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1995.tb01848.x
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  • 6
    Electronic Resource
    Electronic Resource
    Estimation of resolution and covariance for large matrix inversions (1995)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 121 (1995), S. 0 
    Details
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Key advantages of conjugate gradient (CG) methods are that they require far less computer memory than full singular value decomposition (SVD), and that iteration may be stopped at any time to give an approximate solution; this means that they may be used to obtain solutions of problems that are too large for SVD. The disadvantage is that CG does not conveniently provide auxiliary information on the quality of the solution (resolution and covariance matrices). This may be overcome by extensions of Paige and Saunders’ LSQR algorithm, which is one of the family of CG algorithms. The extensions are produced by analogy with SVD; bidiagonalization in LSQR produces orthonormal basis vectors that can be used to construct solutions and estimates of resolution and covariance. For large problems, for which SVD can not be performed, the new method provides approximate resolution and covariance estimates that asymptotically approach those of the SVD solutions as the number of iterations increases.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1995.tb05722.x
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  • 7
    Electronic Resource
    Electronic Resource
    Composite memory variables for viscoelastic synthetic seismograms (1995)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 121 (1995), S. 0 
    Details
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: By rearranging terms in the viscoelastic wave equations for the standard linear solid, new composite memory variables are defined. Instead of having one memory variable for compressional relaxation and one for each of the six independent components of quasi-shear stress relaxation, as in the standard relaxation mechanism formulation, the new grouping contains only one for each displacement response component. Thus, only two (rather than three) memory variables are required for 2-D computations, and only three (rather than seven) are required for 3-D computations. The amount of computation is not changed as the new formulation is equivalent to the standard one, but there is a significant saving in computer memory.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1995.tb05738.x
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  • 8
    Electronic Resource
    Electronic Resource
    2-D full-wavefield inversion for wide-aperture, elastic, seismic data (1992)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 111 (1992), S. 0 
    Details
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Full-wavefield inversion of two-component (elastic), wide-aperture, seismic data from surface sources and receivers simultaneously provides 2-D estimates of both P and SV velocity distributions. The algorithm operates on common-source gathers; it involves cross-correlation of propagating source and residual wavefields to define the steepest descent direction used to update the velocities at each point in a 2-D finite-difference grid. The solution is stable even in the presence of random noise and when the input traces are unequally spaced. Examples include a stack of flat layers and a folded and faulted structure.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1992.tb00550.x
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  • 9
    Electronic Resource
    Electronic Resource
    Full-wavefield inversion of wide-aperture SH and Love wave data (1991)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 106 (1991), S. 0 
    Details
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Reverse-time linearized inversion is implemented for synthetic wide-aperture SH and Love wave data, including multiple reflections, to estimate the S-wave velocity in a two-dimensionally inhomogeneous medium. Complete wide-aperture wave-fields, in which triplications, pre- and post-critical reflections, and surface waves (Love waves) are present, can be imaged by reverse-time inversion to estimate the SH velocity distribution. The algorithm operates on common-source gathers and involves cross-correlation of the source and recorded wavefields to define the steepest descent direction to update the velocity at each point on a finite-difference grid. Effects of noise can be reduced by stacking information from different sources.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1991.tb04601.x
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  • 10
    Electronic Resource
    Electronic Resource
    Wavefield extrapolation of body waves for 3-D imaging of earthquake sources (1991)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 106 (1991), S. 0 
    Details
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Reverse-time imaging of earthquake source parameters is extended from two to three spatial dimensions and from two- to three-component recordings. Provided that the recording aperture is sufficiently large, and the data are not spatially aliased, source radiation patterns, in both time and space, can be reconstructed by elastic reverse-time propagation of body waves; such reconstructions are necessarily partial as only the energy that was recorded is available for reconstruction. For a point source, the origin time and the (3-D) spatial location of an event can be reconstructed by extracting the time and position, of the best focused energy from the backward propagating wavefield. For a spatially and temporally extended source, biased estimates of fault position and rupture/slip time history can be estimated if the recording aperture is sufficient; for smaller recording apertures, the time history can still be estimated if the fault location and geometry are known a priori. The latter is viable for a reasonable number (〈200) of three-component recordings, and so is potentially applicable to real data. All these processes assume that a smoothed representation of the 3-D velocity distribution in the volume containing the source and receivers is available.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1991.tb04603.x
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