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Experiment and inversion studies on Rayleigh wave considering higher modes

LU Lai-Yu~(1,2),ZHANG Bi-Xing~2,WANG Cheng-Hao~21 Institute of Geophysics,China Earthquake Administration,Beijing 100081,China2 Institute of Acoustics,Chinese Academy of Sciences,Beijing 100080,China  
The dispersive characteristics of the Rayleigh waves propagated in the layered media are investigated experimentally by ultrasonic detection test,especially in the presence of a low-velocity layer.Ultrasonic 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-wavenumber analysis method.The dispersion curves of two modes are obtained for the first specimen of a slow layer on a fast substrate.The dispersion curves for the second specimen containing a low-velocity mid-layer are discontinuous and correspond to the different modes branches due to the mode jumping.Further analysis based on the experimental data demonstrates that the mode jumping is caused by the different surface displacement distribution with frequency for each mode.This indicates that surface displacement distributions of the modes should be also considered for the inversion problems,especially in the stratified media containing a low-velocity mid-layer.Parameters of two specimens are inversed by Genetic Algorithm(GA) using the experimental dispersion curves of surface waves.The displacement distributions of each mode are considered in inversion in the presence of a low-velocity layer.Mode-misidentification is avoided using the inversion method presented in this paper and satisfied results are obtained.
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