S U M M A R YThe 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.

The elastic wave field and the excitation mechanism of the surface waves in multilayered elastic solid media are studied in this paper. On the basis of Abo-zena [Geophys. J. R. Astron. Soc. 58, 91–105 (1979)] and Menke [Geophys. J. R. Astron. Soc. 59, 315–323 (1979)], the elastic wave field is further investigated in the B, P, C coordinate system. The so-called new type of propagator matrix introduced by Menke to avoid loss of the precision problem is improved. It presented an important result and some new properties. The dispersion characteristics and excitation mechanisms of the surface waves (Rayleigh and Love waves) are also investigated via numerical simulation. The excitation intensities of the surface waves strongly depend on the frequency range of the source. The source frequency should be controlled in a proper range to effectively excite the surface waves. Two quantities, β1 (the ratio of B to P components of displacement) and β2 (the ratio of B to P components of stress), are defined for the Rayleigh wave. It is found that β1 and β2 are sensitive to the material property of the medium and the layered geometry, and they are two important physical quantities for exploring the structures of the interfaces and the velocity distributions of layers under the free surface. The relative error in estimating the thickness of each medium by β1 and β2 is less than 10%. The effects of the thickness of each layer of media and other factors on the dispersion characteristics of Rayleigh and Love waves and the values of β1 and β2 are also analyzed.

The guided waves in multilayered elastic media are studied. The mechanism of zigzag dispersion curves in the Rayleigh Wave Exploration is analyzed. It is proved that the zigzag dispersion curve can not be obtained by a single guided mode. The relation of the excitation intensity of each mode to the formation parameters is studied. We obtained many results about zigzag dispersion curves that were not available in previous literatures. The effects of the position and thickness of the low-velocity layer and other parameters on the zigzag dispersion curves of Rayleigh wave are also analyzed. Finally, comparative analyses between the practical data and theoretical results are carried out. Theoretical results are consistent to the practical data.