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  • 1
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Annals of the New York Academy of Sciences 600 (1990), S. 0 
    ISSN: 1749-6632
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Natural Sciences in General
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 112 (1993), S. 0 
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: The scattering of acoustic waves in random media is investigated numerically by a finite difference method and is compared with the predictions of single scattering theory. the random media are characterized by autocorrelation functions which allow the construction of spatially anisotropic random structures with different correlation lengths a and b perpendicular and parallel to the propagation direction. If a equals b, the attenuation of the transmitted wave can be successfully explained by single scattering theory. the attenuation maximum occurs at kb≈ 1-2, where k is the wavenumber. For media with a 〉 b we observe a stronger attenuation than expected from single scattering theory for kb greater than 6. the attenuation peak is shifted to smaller kb values when the spatial anisotropy of the random fluctuations is increased. the investigation of the seismic coda shows that the single scattering theory cannot explain the time dependence of the coda. Coda Q, as determined from the coda decay rate under the single scattering assumption, does not describe the scattering attenuation. In 1-D random media the decay rate of the coda observed in transmission decreases with increasing standard deviation of the impedance fluctuations. In the 2-D case the decay rate increases slightly with the standard deviation.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 102 (1990), S. 0 
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Energy flux models which have been proposed lately offer a unified concept of scattering attenuation and coda wave excitation. Here, an energy flux model for a plane wave propagating through a scattering layer above a homogeneous half-space is developed. It takes into account the intrinsic attenuation of the layer as well as diffusion of coda energy out of the layer into the half-space. All three parameters of the model—scattering attenuation, intrinsic attenuation and diffusion losses—can be determined from the coda strength relative to the direct arrival and the coda decay rate, if one makes some assumptions on the frequency dependence of intrinsic attenuation.The energy flux model has been tested with synthetic finite difference seismograms and has been found to give reliable results for both weak and strong scattering environments. It was then used for the analysis of short-period data from deep events recorded at the Warramunga seismic array in central Australia. an average scattering Q of about 640 at 1 Hz was found for the lithosphere beneath the array with a strong increase of Q with frequency above 1 Hz. Intrinsic Q is much higher than scattering Q and most of the coda decay is caused by diffusion of coda energy into the half-space below the scattering layer.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 110 (1992), S. 0 
    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
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