A parametric study of effective phase velocity of surface waves in layered media

In layered soils, there are multiple modes in the surface-wave field. The vibrations of these modes interfere constructively or destructively with each other. The phase velocity (i.e. the effective phase velocity) of the superposed particle vibration is different from those of the modes. Because of limitations in the current data acquisition methods in geotechnical engineering tests, the modes are not easily separated and the experimental dispersion data are often corresponding to the effective phase velocity. In the present work, the relationship between the effective phase velocity and the phase velocities of the natural modes (i.e. the modes of planar Rayleigh waves under the natural state) is established by using the thin layer stiffness matrix method. Effects of higher modes and the mode incompatibility on the effective phase velocity are analyzed for three typical layered media. The results show that influenced by both the higher modes and the mode incompatibility, the effective phase velocity depends not only on the frequency but also on the offset. Effects of changes in soil properties such as the density, Poisson’s ratio and the damping ratio, on the effective phase velocity are investigated. The analyses indicate that changes in the density and the material damping ratio have negligible influences on the effective phase velocity and dispersion of modes, whereas change in Poisson’s ratio shows relatively obvious influences in the case that a stiffer layer is trapped into two soft layers. The results are helpful for reducing the non-uniqueness in the back analysis of the experimental dispersion data.