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Changes in seismic velocity and apparent attenuation due to isotropic and anisotropic scattering : results from physical modeling

Much work is presently being done concerning small scale heterogeneities in the
earth's crust. These heterogeneities range from pores in sedimentary rocks up to
fluctuations in the density and seismic constants of the earth's crust with scale lengths
of kilometers. The ability to study and quantify these heterogeneities using seismic
methods would be a major advance in the earth sciences.
Physical modeling has been shown to be a useful technique for investigating
various aspects of wave propagation. In this thesis, two physical modeling
experiments (one three-dimensional and one two-dimensional) are used to investigate
the scattering of seismic waves from small scale heterogeneities and the changes in
seismic velocity and apparent attenuation resulting from this scattering. The effects of
both isotropic and anisotropic scattering on velocity and apparent attenuation are
calculated. These experimental results are compared to theoretical results.
The theory used for isotropic scattering for the three-dimensional experiment is a
modified version of Wu's single scattering theory, where instead of calculating the
scattering for a single scatterer using the Born approximation, the exact results for
scattering from a cylindrical shape are used. While the results for compressional
waves and both components of shear waves compare reasonably well for small
scatterer volume fractions, at larger scatterer volume fractions, where the need for
multiple scattering is more likely, the results for all waves do not compare as well.
Many theories used to test anisotropic scattering predict changes in velocity rather
than changes in apparent attenuation. The velocity changes are used primarily in this
work due to geometrical focusing by a seismic lens that biases the amplitudes (and
hence the estimates of apparent attenuation) at low frequencies where most theories
predict apparent attenuation. Velocities are calculated from the data using travel times
and low frequency phase shifts for the compressional waves and for one component of
the shear waves measured in this two-dimensional experiment. Theories that are used
to predict compressional and shear wave velocities for both isotropic and anisotropic
scatterers are based on a fractional volume method (isotropic), two crack methods
(isotropic and anisotropic), and a finely layered method (anisotropic). The isotropic
experimental results have much larger, non-linear changes in the velocities than do the
isotropic theoretical results. The anisotropic experimental results have similar shapes
to both theoretical anisotropic methods for compressional waves and to the theoretical
anisotropic crack method for shear waves. Attenuation is computed using log spectral
ratios and compares as well with the theoretical results as can be expected within the
limits set.
A method using anisotropic apparent attenuation to help quantify the scatterers is
developed for use with field data. / Graduation date: 1987
Date29 April 1987
CreatorsDubendorff, Bruce H.
ContributorsMenke, William
Source SetsOregon State University
Detected LanguageEnglish

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