Backscattering enhancements by partially exposed spheres due to reflected subsonic Rayleigh waves at air–water interfaces

Air-water interfaces can enable distinct target scattering mechanisms different from the mechanism under free field conditions. In this study, backscattering experiments are performed by lowering an acrylic or polymethylmethacrylate sphere through the air-water interface into the water and insonifying the sphere from below at grazing incidence. Pronounced backscattering enhancements associated with the subsonic Rayleigh wave propagation mechanism are observed before the specular reflection point of the sphere reaches the water. The results indicate that, for a partially exposed sphere, subsonic Rayleigh waves can pass through the air-water interface and circumnavigate the sphere multiple times. The phase velocities of Rayleigh waves are different when propagating above and below the air-water interface. Moreover, subsonic Rayleigh waves are partially reflected when passing through the air-water interface, generating wavefronts that propagate in the reverse direction.