Application of Radiative Transfer Theory to Acoustic Propagation in the Ocean Bottom

Abstract

Acoustic scattering from the ocean bottom has been extensively studied by applying analytical methods based on the wave equation. This has the advantage of being mathematically rigorous but many of these techniques are not applicable for complex sediment environments with combined volume and rough surface scattering. In this research, the use of the radiative transfer (RT) theory is explored as a way to address layered ocean sediments with embedded scatterers in a computationally tractable manner. The RT formulation has been applied extensively in electromagnetic applications involving layered media and has been considered for acoustic waves with applications to ultrasound but has not previously been applied to acoustic scattering in ocean sediments. The RT theory uses principles of conservation of energy to describe the behavior of a wave propagating in a volume with random scatterers. Using this model, the non homogeneous volume can be characterized by emission and extinction coefficients which can be computed in closed form for scatterers such as elastic spheres, cavities or cylinders or by numerical methods or analytic approximations for more complex geometries. In this paper, an infinite half space of sand with embedded spherical cavities is considered. The Fresnel reflection/transmission coefficients are utilized to provide examples of the backscattered signal level that could be expected due to volume scattering. It is shown that for typical sandy sediments with small shear sound speed, the main contribution to the volume backscattering is due to longitudinal waves that couple from the sediment into the water column, while the coupling of vertical shear waves is weak.