Subcritical scattering from buried elastic sphere based on subsonic Rayleigh waves

This study investigates the scattering characteristics of a buried elastic sphere under plane wave subcritical incidence at low-frequency. As the penetrating wave in sediment at subcritical angles transitions to an evanescent wave, it decays along the depth. The scattering separation method is utilized to explore the form function of a polymethyl methacrylate (PMMA) sphere and the multiple scattering effects between the sphere and the interface under evanescent wave incidence. Additionally, a Ray approach is utilized to predict the far-field scattering from the buried PMMA sphere, which is verified using experimental results. The findings reveal that the evanescent wave field in the sediment exhibits spatially limited beam characteristics and low-pass filtering features. At low frequencies, echoes from the PMMA sphere are primarily contributed by Rayleigh waves. Furthermore, under evanescent wave incidence, the interference between clockwise and counterclockwise Rayleigh waves in the bistatic scattering of the PMMA sphere weakens, and the Specular-Rayleigh wave interference pattern becomes prominent, replacing the typical “X-shaped” distribution of the impulse response with a “backslash-like shaped” distribution in the time-angular domain. The smoothed Wigner-Ville distribution (SWVD) analysis of the echo signals indicates that the group velocity of lower frequency Rayleigh waves is biased higher, leading to a faster arrival of low-frequency components. This research may provide a valuable reference for detecting artificial targets at low-frequency and long distances.