Abstract:The surface particle velocity responses of a solid medium with a cavity under dynamic loads are numerically obtained by the finite element method. Mechanism of disturbance of apparent phase velocity is analyzed from distribution of energies of Rayleigh waves and the reflected body waves in the responses. Influences of frequency and relative distances among the source,receivers and cavity on apparent phase velocity of surface waves are discussed. Disturbance degree of the apparent phase velocity increases with decrease of frequency until a maximum one occurs. The frequency corresponding to the maximum disturbance is related to fundamental frequency of the reflected body waves. The fundamental frequency is varied with relative distances among the source,the receivers and cavity. The relationship between the depth of cavity below the surface and the wavelength corresponding to the maximum disturbance is discussed. The empirical method is suggested for the estimation of the depth of cavity.