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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

Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/29422 |

Date | 29 April 1987 |

Creators | Dubendorff, Bruce H. |

Contributors | Menke, William |

Source Sets | Oregon State University |

Language | en_US |

Detected Language | English |

Type | Thesis/Dissertation |

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