Nonreciprocity of long-range reverberation in a wedged continental shelf.

Abstract

The bottom scattering strength at low frequency and grazing angle in shallow water can only be extracted from long-range reverbertion (with large R/N ratio). Due to multipath effects the conventional model of boundary reverberation, based on geometric ray theory in the deep sea, is no longer suitable for calculating the long-range reverberation in shallow water. Up to date little reliable information on bottom scattering at low frequency and small grazing angle has been given in shallow water. For a frequency range of 50–2000 Hz and at a grazing angle of 0.5°–10°, it is hard to confirm the mechanism of bottom scattering and its dependence on frequency, grazing angle, and bottom type. In such a range, even the angular dependence of bottom scattering (a most basic relationship) is questionable. In this talk, using the WKB approximation to adiabatic normal mode theory, an averaged intensity expression R(r) of long-range reverberation in wedged homogeneous shallow water is derived. For two identical sources directed at each other, the monostatic reverberation intensities, obtained at two terminations with a depth of h1 or h2, would not be reciprocal. It is shown that Rh1(r)/Rh2(r) = (h1/h2)n, where n is an angular index of bottom backscattering. Based on this result, the value of n at small grazing angle can be determined from at-sea long-range reverberation data. A simulation tank experiment is suggested. [Work supported by ONR and the IAAS.]