HR: 0830h
AN: S11E-0346    [PDF]
TI: Modeling of SH Wave Envelopes in Media With Many Cavities
AU: * Kawahara, J
EM: junk@mx.ibaraki.ac.jp
AF: Department of Environmental Sciences, Faculty of Science, Ibaraki University, 2-1-1 Bunkyo, Mito, 310-8512 Japan
AU: Yomogida, K
EM: yomo@ep.sci.hokudai.ac.jp
AF: Division of Earth and Planetary Sciences, Graduate School of Science, Hokkaido University, North 10 West 8, Kita-ku, Sapporo, 060-0810 Japan
AB: For revealing the stochastic nature of the heterogeneous earth by means of seismograms, wave envelopes in randomly heterogeneous media have been theoretically studied, and several models to describe them have been proposed. In such studies, heterogeneities were usually assumed as continuous fluctuation of medium parameters; it is not self-evident whether the models are valid even for discrete heterogeneities with sharp material contrasts, such as distributed cracks or inclusions. In this study, we dealt with 2-D circular cavities as a tractable example of discrete heterogeneities. We synthesized the envelopes of SH waves propagating in media with many cavities, and compared them with the predictions by some of the models stated above, thus examining their validity and limitations. We randomly distributed many identical cavities within a rectangular area, let a plane SH Ricker wavelet be incident on its one side, and synthesized the seismograms at points arrayed along the opposite side. Here we used a boundary integral method (Benites, Aki and Yomogida, 1992); free surfaces were not considered. We then calculated the RMS envelope and compared it with the solutions of the following theoretical models; the Foldy approximation (Kawahara and Yamashita, 1992; hereafter FA), the single isotropic scattering model (Sato, 1977; hereafter SISM) and the energy flux model (Korn, 1990; hereafter EFM), with some modifications according to the present experimental geometry. FA stochastically describes the attenuation and dispersion of direct waves, whereas SISM and EFM predict coda wave envelopes phenomenologically. The latter two require the scattering Q, that is given by FA. The total scattering coefficient, required by SISM, can be analytically evaluated with a wide-angle scattering approximation. It was shown that FA highly agrees with synthesized direct wave envelopes for any cavity volume concentrations, up to 20%. SISM explains well the synthesized coda wave envelopes only for very small concentrations. For higher concentrations (say, $>$1%), SISM underestimates the envelopes, but it always explains them if multiple-scattered coda waves are removed. This implies the dominance of multiple scattering over coda waves in these cases. In contrast, EFM seems to work well when multiple scattering is dominant. Generally, the agreement between the synthetic and EFM envelopes is better for longer lapse times or denser cavity distributions. Other geometries (e.g., cracked media with point sources) should be examined in future.
DE: 7203 Body wave propagation
DE: 7260 Theory and modeling
SC: Seismology [S]
MN: 2003 Fall Meeting