Backscattering from sub‐bottom microlayers

Abstract

Vertical profiling measurements of the bottom in the frequency band from 1–5 kHz showed the variance of the reflected signal to be proportional to the frequency and the horizontal correlation length to be in the order of 10 m. This indicated that the variance in the backscattered signal is a function of the sonar beam pattern and the pulse shape, frequency, and grazing angle of the incident wave. For a pulse length of 25 ms and a grazing angle of 10°, the predicted backscattering strength was found to be independent of frequency at a level of −40 dB. This behavior agreed with data measured at this angle. As the classical theory of backscattering assumes that the bottom reverberation is due to independent scatterers which add incoherently, this theory will become invalid if the dimensions of the area ensonified on the bottom are less than the correlation length. Thus, for sonars with a short pulse or a narrow beam, the backscattered power will no longer be proportional to the ensonified area. [Work supported by the Office of Naval Research.]