The specular and diffuse scattering of the sound in stratified media

Abstract

The atmosphere and the ocean are greatly distinct in two aspects that determine their fundamental qualitative difference of the sound scattering. (1) A sharp distinction between the state equations in both media. This leads to a great difference in mechanisms of the volume scattering which is mainly caused by directional (dipole and quadruple) and omnidirectional contributors, respectively, in the atmosphere and in the ocean. (2) Atmospheric kinetic coefficients (thermoconductivity, diffusion and viscosity) are close to each other and substantially exceed any of those of the seawater; besides, the latter very differ between each other. These features, with account of commonly employed sound wavelengths and intensities of the turbulence in both media, essentially eliminates an existence of sound specular scattering (reflection) in the atmosphere, however, makes it quite possible in the ocean. Reflective properties of the thin layers in the seawater strongly depend on which type of stratification dominates. At initial stage the temperature layer excels the salinity one in reflectivity by factor 4.7; however, the lifetime of the latter is longer in dozens of times. Theoretically derived amplitude and evolution features of the scattering from the salt layer have been confirmed in laboratory experiment. One more experimental series has been carried out to distinguish the scattering from the reflective layer and microstructure, both co-existing in the wake past a cylinder. An employment of bistatic sounding configuration allows separating both the reflection and diffuse scattering effects, the former is stronger by factor 50. The volume scattering cross-sections, obtained both by an acoustic way and derived by processing of the optical image spectra, are in agreement and follow the function k−7 of the wave number k.