Effects of Rayleigh Waves Scattered at Shallow Cavities on Apparent Phase Velocity

In a homogenous half space acted by surface sources,the shallow wave-fields are dominated by Rayleigh waves.When the forward Rayleigh waves encounter a shallow cavity,the waves are scattered at the cavity.Taking the cavity as the reference position,the wave-field can be divided into the front,above and rear regions in sequence along the forward direction.The scattered waves in the far field of the front region are dominated by the reflected Rayleigh waves,which travel at a velocity the same as that of the forward Rayleigh waves.The guided waves with apropagation behavior similar to Lamb waves may be formed at certain geometrical sizes of the cavity.The far field of the rear region is controlled by the transmitted Rayleigh waves.Influenced by the scattered waves,the apparent phase velocity curves of surface waves are disturbed.The characteristics of the disturbances differ at different positions.If the shallow cavity is present under the measuring line,the cavity can be localized from the disturbance differenc