The dispersive characteristics of multimode guided waves in stratified media are studied experimentally. Ultrasonic surface detections are performed for two layered specimens: lucite/steel half-space and aluminum/lucite/steel half-space. Characteristics of the multimode dispersion curves are analyzed using the frequency-wave number method. The dispersion curves of two modes are obtained for the first specimen of a low-velocity layer on a fast substrate. The dispersion curves for the second specimen containing a low-velocity midlayer are discontinuous and correspond to different modes branches due to mode jumping. Further analysis demonstrates that the mode jumping is caused by the different surface displacement distribution with frequency for each mode. This indicates that the surface displacement distributions of the modes should be also accounted for in the inversion problems, especially in stratified media with a low-velocity midlayer.
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Lu et al. (2005) studied this question.
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