Disturbances of surface wave field due to wave scattering at shallow cavities

In homogenous half spaces, the wave fields at shallow depth are dominated by Rayleigh waves activated by the surface sources. When the forward Rayleigh waves encounter a shallow cavity, the displacement shapes and the particle orbit are changed. Rayleigh waves are scattered at the cavity. The wave field is disturbed by the scattering waves. The different features of the disturbances can be found in different regions of the surface wave field. In the region in front of the cavity, the reflected and the incident Rayleigh waves interfere constructively or destructively with each other. The interference fringes are formed. In the region over the cavity, the transmitted waves are dispersive. The phase velocities are different from those of Rayleigh waves. Under certain conditions, the behaviour of the transmitted waves is similar to that of Lamb waves in the free plates. The diffraction of the transmitted waves along the back boundary results in the energy reduction in the surface wave field over the boundary. Influenced by the geometrical attenuation, the transmitted Rayleigh waves gradually dominate the surface wave field in the back region. The components at low frequencies have greater energy reduction as compared to those at high frequencies. Based on the wave scattering theory, the effects of the ratio of the buried depth to the wavelength of Rayleigh waves on the disturbances of the surface wave field are analyzed. The numerical simulations are used to verify the analyses. It is shown from the results that the buried depth can be estimated from the critical wavelength corresponding to the energy reduction in the offset-wavelength domain.