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  • 1
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    Search for non-Newtonian gravitation-a gravimetric experiment in a hydroelectric lake (1990)
    Oxford University Press
    Details
    Publication Date: 1990-05-01
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Royal Astronomical Society.
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  • 2
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    Seismic-wave attenuation operators for arbitrary Q (1991)
    Oxford University Press
    Details
    Publication Date: 1991-09-01
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Royal Astronomical Society.
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  • 3
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    Seismic-wave traveltimes in random media (1992)
    Oxford University Press
    Details
    Publication Date: 1992-07-01
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Royal Astronomical Society.
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  • 4
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    Migration of Synthetic Seismograms For Crustal Structures With Random Heterogeneities (1993)
    Oxford University Press
    Details
    Publication Date: 1993-04-01
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Royal Astronomical Society.
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  • 5
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    Velocity shift in random media (1993)
    Oxford University Press
    Details
    Publication Date: 1993-11-01
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Royal Astronomical Society.
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  • 6
    Electronic Resource
    Electronic Resource
    Velocity shift in random media (1993)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 115 (1993), S. 0 
    Details
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Seismic waves in a random medium (with standard deviation ε and correlation distance a of the relative slowness fluctuations) prefer fast paths, and therefore the apparent velocity of wave propagation is larger than the velocity which corresponds to the volume average of slowness. This velocity shift can be determined by ray perturbation theory (Snieder & Sambridge 1992), by the Huygens method (Podvin & Lecomte 1991) and by wave theory (Müller, Roth & Korn 1992). We apply all three methods to plane-wave propagation through a 2-D acoustic medium with Gaussian or exponential autocorrelation function of the slowness fluctuations. Ray perturbation theory gives numerical and analytical results, but has path-length (L) limitations. The Huygens method, which also gives the ray-theoretical velocity shift, can be used for L/a ratios of seismological interest. Wave theory shows that the velocity shift also depends on the wavelength λ and that for λ/a less than about 0.1 the velocity shift agrees with the result of the Huygens method. For λ/a= 1 the wave-theoretical (i.e. true) shift is lower than the Huygens-method shift by a factor of 0.25 to 0.5. Simple formulae for the ε dependence of the Huygens-method shift at long path lengths (L/a≤ 80) are given, and a correction factor is derived which approximately transforms plane-wave 2-D into spherical-wave 3-D velocity shifts; the latter correspond to 3-D two-point ray tracing.For short-period seismic waves, propagating to teleseismic distances, mantle heterogeneity with ε= 1 per cent and a= 100 km produces a velocity shift of about 0.2 per cent. Shifts of this order can explain the difference in earth models, derived from free oscillations on the one hand and from short-period body waves on the other. A velocity shift (or velocity dispersion) due to anelasticity would be additional.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1993.tb01206.x
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  • 7
    Electronic Resource
    Electronic Resource
    Seismic-wave traveltimes in random media (1992)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 110 (1992), S. 0 
    Details
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: A ray-theoretical relation is established between the autocorrelation function of the slowness fluctuations of a random medium and the autocorrelation function of the traveltime fluctuations on a profile perpendicular to the general propagation direction of an originally plane wave. Although this relation can be inverted exactly, it is preferable for applications to use the results of a forward calculation for a modified exponential autocorrelation function which represents slowness fluctuations with zero mean. The essential parameters of this autocorrelation function, standard deviation ε and correlation distance a, follow by simple relations from the maximum and the zero crossing of the corresponding autocorrelation function of the traveltime fluctuations. The traveltime analysis of 2-D finite-difference seismograms shows that ε and a can be reconstructed successfully, if the wavelength-to-correlation-distance ratio is 0.5 or less. Otherwise, ε is underestimated and a overestimated; however, both effects can be compensated for.The average traveltime, as determined from the finite-difference seismograms, is slightly, but systematically shorter than the traveltime according to the average slowness, i.e. the wave prefers fast paths through the medium. This is in agreement with results of Wielandt (1987) for a spherical low-velocity inclusion in a full-space and with results of Soviet authors, summarized by Petersen (1990). The velocity shift is proportional to ε2, it has dispersion similar to the dispersion related to anelasticity, and it increases with the pathlength of the wave.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1992.tb00710.x
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  • 8
    Electronic Resource
    Electronic Resource
    Seismic-wave attenuation operators for arbitrary Q (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: Under the minimum-phase condition attenuation operators are derived from arbitrary intrinsic or scattering Q as a function of frequency, and a simple method is described to calculate these operators with the aid of Fourier transformation. In the case of scattering attenuation, the medium may contain both 1-D, 2-D or 3-D heterogeneities. For illustration, the method is applied to a 1-D model of statistically layered media with 12 per cent standard deviation of relative velocity fluctuation. The theoretical scattering Q, deduced from single-scattering theory, is determined for a numerical autocorrelation function and an exponential autocorrelation function, as well as for a modified exponential autocorrelation function, which is introduced here. The corresponding attenuation operators are calculated. The theoretical Q and simulated seismograms, calculated with the attenuation operators, agree very well with the exact results of synthetic-seismogram calculations.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1991.tb06342.x
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  • 9
    Electronic Resource
    Electronic Resource
    Migration of Synthetic Seismograms For Crustal Structures With Random Heterogeneities (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: Fast frequency-wavenumber migration is used to migrate synthetic seismograms, calculated by a hybrid method for crustal models with complex 2-D scattering structures in the lower crust. Scatterers with predominantly horizontal or vertical orientation and scatterers without preferred orientation (isotropic scatterers) are investigated; horizontal scatterers or lamellae have frequently been suggested for the lower crust on the basis of modern reflection seismological experiments. In none of the cases studied here are the scattering structures imaged correctly. the reason is mainly multiple scattering which can produce coherent arrivals in the seismogram sections, and hence coherent signatures in the migrated sections, which have no relation to structure. Imaging is generally acceptable for horizontal scatterers, but for isotropic or vertical scatterers the migrated signatures are also horizontal and thus do not represent reality. It is concluded that the highly detailed line drawings, which are popular in crustal reflection seismology, are less reliable than believed, as far as the internal structure of scattering zones and the scatterer orientations are concerned. Horizontal or subhorizontal structures in the lower crust may be less common than assumed.The paper also briefly addresses a few methodical aspects of fast frequency-wavenumber migration for depth-dependent background structure, in particular the concept of sounding beams, their width and the construction of a migrated section from several sounding beams.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1993.tb02541.x
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  • 10
    Electronic Resource
    Electronic Resource
    Search for non-Newtonian gravitation—a gravimetric experiment in a hydroelectric lake (1990)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 101 (1990), S. 0 
    Details
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: In August 1988 gravity measurements were made during 22 days in the Hornberg pumped-storage reservoir in the southern Black Forest, SW Germany. Water-level changes with daily variations of 5-22 m and the Earth tides produced gravity signals which were recorded digitally with six LaCoste-Romberg field gravimeters above and below the water level in a 60 m high tower. The reservoir geometry is well known, and water level could be measured continuously with high accuracy. The purpose of the experiment was to search in the gravity signals for deviations from Newton's gravitational law due to a newly postulated interaction (‘fifth force’) whose range is somewhere between 1 and 1000 m. The search technique is to construct residual signals, i.e. to subtract from observed gravity known effects such as the Newtonian attraction of the moving water masses and the Earth tides, and to search by regression analysis in the residual signal for a residual component proportional to the Newtonian attraction; this component could be of non-Newtonian origin. Residual water signals with amplitudes of 0. 25 ± 0.4 per cent of the Newtonian water signal were found. This value implies negative evidence for a residual water signal and, hence, for non-Newtonian gravitation. The experiment can also be considered as one for the measurement of the gravitational constant at effective mass separations of 40–70 m. The deviation from the laboratory value is also 0.25 ± 0.4 per cent and thus not significantly different from zero. The main limitation of the Hornberg experiment is due to calibration errors of the gravimeters used of 0.2–0.4 per cent, which were revealed by comparison of the records of neighbouring instruments.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1990.tb06572.x
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