The Importance of Hybrid Ray Paths, Bottom Loss, and Facet Reflection on Ocean Bottom Reverberation

Abstract

Mackenzie’s deep-water model for bottom reverberation [J. Acoust. Soc. Am. 23, 1498–1504 (1961)] has been extended to handle reverberation arriving later than the second fathometer return. The model uses straight-line ray paths, flat bathymetry, and a short pulse length. Energy is scattered from the bottom in all directions; in particular, the energy can return by a different path from the outgoing one (hence, the term hybrid path). A three-parameter backscattering function has been used which incorporates Lambert’s law at low grazing angles augmented by a facet-reflection process at steep angles. Also, three bottom loss curves based on the Rayleigh reflection coefficients for sand, silt and clay have been used for a sensitivity study. The results indicate that: (i) the facet-reflection process broadens the fathometer returns for a short pulse characteristic of an impulsive source; (ii) the hybrid paths are important (adding about 5 dB reverberation) when the bottom loss is low; and (iii) bottom loss is as important as the backscattering in determining the reverberation level. A correction to the isovelocity model to allow for a sound speed gradient indicates that (for omnidirectional sources and receivers) the effect of the sound speed profile is small. Thus, the isovelocity model seems to be a reasonable approximation to longer times than one might at first expect.