HR: 17:15h
AN: S22G-06    [PDF]
TI: Amplitudes and Traveltimes of Waves in Random Media
AU: * Baig, A M
EM: abaig@princeton.edu
AF: Department of Geosciences, Princeton University, Princeton, NJ 08544 United States
AU: Dahlen, F A
EM: fad@princeton.edu
AF: Department of Geosciences, Princeton University, Princeton, NJ 08544 United States
AB: We measure traveltimes and amplitudes from `ground-truth' seismograms, computed using a numerical wave propagation code to compare with finite-frequency and ray-theoretical values for these quantities. Ray-theoretical traveltimes become invalid whenever the scale-length of 3-D heterogeneity is smaller than half the maximum width of the Fresnel zone; in contrast, ray-theoretical amplitudes have a much more restricted range of validity, that the scale length should be less than one Fresnel zone width. Finite-frequency theory gives better results for amplitudes, suffering no observable degradation for small-scale media for the weakest heterogeneity considered, but suffering appreciable misfit in more heterogeneous media. Using these finite-frequency expressions for traveltime, we derive expressions for the expected variances of traveltimes and amplitudes that act, in most cases, as extensions to the ray-theoretical expressions. Finally, we propose using the amplitude variance as a criterion for delineating the validity of these linear approximations. For traveltimes, provided one rejects waveforms that do not provide a good cross-correlation traveltime, the remaining data are linearly related to the model over the values of theoretical amplitude variance that we probe in this experiment. Amplitudes do not behave as well: when the theoretical amplitude variance rises above $0.1$, significant nonlinearities start to invalidate our linear approximation.
DE: 7200 SEISMOLOGY
DE: 7203 Body wave propagation
DE: 7260 Theory and modeling
SC: Seismology [S]
MN: 2003 Fall Meeting