Astronomy and Astrophysics – Astronomy
Scientific paper
Mar 2007
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007geoji.168.1235l&link_type=abstract
Geophysical Journal International, Volume 168, Issue 3, pp. 1235-1246.
Astronomy and Astrophysics
Astronomy
3
Inversion, Laboratory Measurement, Low-Velocity Layer, Rayleigh Waves
Scientific paper
The dispersion and the surface displacement as a function of frequency of multiple modes guided waves in stratified media including a low-velocity layer are studied by numerical simulation and experiment. A method is developed to determine the thickness and the shear wave velocity of individual layers. First, the modal analysis of Rayleigh wave is investigated numerically for three layered media. Then, ultrasonic surface measurements are performed for three specimens: Steel half-space, Lucite/Steel half-space and Aluminum/Lucite/Steel half-space. The Characteristics of the dispersion curves are analyzed using the frequency-wavenumber method. The non-dispersive Rayleigh wave is obtained for the first simple specimen. The dispersion curves for two modes are obtained for the second specimen with a low-velocity layer on a fast substrate. The dispersion curves for the third specimen containing a low-velocity layer are apparently discontinuous and correspond to different mode branches. Further analysis demonstrates that the apparent discontinuity is caused by a rapid change of mode excitation with frequency at the surface. While one mode vanishes from the recorded wavefield, the other appears. This indicates that the surface displacements of the modes should be also accounted for in the inverse problem, especially in stratified media with a low-velocity layer. Finally, shear wave velocity profiles are inverted based on the experimental (maybe discontinuous) dispersion curves of fundamental or/and higher modes using a Genetic Algorithm(GA). Besides the dispersion characteristics of each mode, the surface displacement distribution is also taken into account for the case of a low-velocity layer, and as a result, the mode-misidentification is avoided.
Lu Laiyu
Wang Chenghao
Zhang Bixing
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