Diffraction and scattering at the seafloor

Abstract

In bottom-interacting ocean acoustics discrete, deterministic arrivals with significant energy are returned from the seafloor at angles that are not predicted by Snell’s law. This observation has led to some confusion between the terms “diffraction” and “scattering” as used in ocean acoustics. The common dictionary definitions of diffraction, as the process by which a beam is spread out or bent after passing the edge of an obstacle, and of scattering, as deflecting waves in a more or less random fashion, do not describe the observed seafloor interaction where a pulse of energy is returned deterministically, not randomly, from the seafloor in a non-Snell’s law direction. In ocean acoustics and marine seismology, it is rare to consider “bending diffraction” but it is quite common to see “scattering diffraction,” for example, as diffraction hyperbolas on bottom profiling and seismic records. For impulsive sources, in addition to specular reflections and random scattering from the seafloor, a significant aspect of bottom-interacting acoustics in deep water is diffraction of energy from discrete seafloor locations along repeatable paths in three-dimensions. [Work supported by ONR.]