HR: 0800h
AN: S31B-1045    [Abstracts]
TI: Travel Times of Rays and Waves in Strongly Heterogeneous Media
AU: * Yang, H
EM: r92224204@ntu.edu.tw
AF: Department of Geosciences, National Taiwan University, No.1, Sec. 4, Roosevelt Road, Taipei, 106 Taiwan
AB: Numerical validation experiments demonstrate that the finite-frequency travel times predicted by Born-Fr\'{e}chet kernel theory agree well with those measured by cross-correlation of synthetic seismograms ({\it Hung et al.}, 2001; {\it Baig et al.}, 2003). However, the agreement wanes with the enhanced strength of velocity heterogeneity. When the non-linear effect of high-order perturbations becomes important, the genuine path trajectory of a ray or wave that gives the minimum travel time differs from the stationary unperturbed one assumed in both linearized kernel and ray theories. In addition to the predictions from linearized ray theory and Fr\'{e}chet kernel theory, we extend the study of {\it Baig et al.} (2003) by exploring the ground-truth travel times in 3-D acoustic media of large velocity perturbations ($\varepsilon\ge$4%) predicted by generic ray theory based on a ray-bending method. The ground-truth time shifts are determined by cross-correlation of pairs of synthetic waveforms in one homogeneous medium and the other with Gaussian random wavespeed perturbations. The results reveal that (1) the kernel-predicted travel times are consistently better than those from ray theory in all circumstances for small perturbations ($\varepsilon\le$2%). Nevertheless, the predictions from both linearized theories are significantly deviated from the ground-truth ones for $\varepsilon\ge$4% and the heterogeneity scale $a<\sqrt{{\lambda}L}$, where $\lambda$ and $L$ are the characteristic wavelength and propagation distance, respectively. (2) Generic ray theory is superior to finite-frequency kernel theory for $\varepsilon\ge$3%, as long as $a$ is greater than $\sim{0.4}\sqrt{{\lambda}L}$. (3) While $a\le\sim{0.4}\sqrt{{\lambda}L}$, the effect of wavefront healing becomes pronounced and thus high-frequency approximation of generic ray theory begin to overestimate the travel times for all the heterogeneity strengths. In short, Fr\'{e}chet kernel theory taking finite-frequency diffractive effects into account can interpret seismic travel times very well and is tenable to resolve small-scale, weakly heterogeneous structures. Whereas generic ray theory including detoured path trajectories provides better approximations to high-frequency travel times in strongly but large-scale heterogeneous media.
DE: 7260 Theory and modeling
DE: 7203 Body wave propagation
DE: 3230 Numerical solutions
SC: Seismology [S]
MN: 2004 AGU Fall Meeting