Abstract: Rayleigh waves in layered half spaces are dispersive, and the excitabilities of Rayleigh waves are varied at different frequencies. When a cavity full of air is present in the underlying half space, the incident Raleigh waves are scattered at the cavity. In this paper, the waves in the layered plates are studied using the plates with the layer properties and thicknesses equal to those over the cavity. Effects of the ratio of buried depth (plate thickness) to wavelength on differences between the dispersion curves of the diffracted and Rayleigh waves are analyzed. The dispersion of the diffracted waves is obtained from the spectrum of synthetic responses over the cavity in the frequency-phase velocity domain. The dominant diffracted waves are identified by comparing the dispersion curves of the diffracted waves and the plate waves. It is shown from the results that the behavior of the dominant diffracted waves is similar to that of the fundamental plate waves; for the incidence of Rayleigh waves with the wavelength longer than the buried depth, the phase velocity of the diffracted waves is slower than that of Rayleigh waves; with the wavelength of incident Rayleigh waves decreases, the phase velocity of diffracted waves gradually approaches that of Rayleigh waves. Compared with the displacement structure of Rayleigh waves, the variation of the diffracted wave displacement with the depth is flatter. This variation is related to the layer structure and frequency. Due to the displacement structure of the diffracted waves, the vibration energy at the surface is reduced. The level of reduction is varied at different frequency components. This phenomenon is a major feature of the spectral disturbances caused by the presence of a cavity. The locations of a cavity can be identified from the spectral disturbances in the offset-frequency domain.

Key words: layered half space, dispersion, scattered waves, spectral disturbance, cavity