Correlation Measurements of Surface-Reflected Acoustic Signals for Several Sea States

Abstract

A study was made at various sea-state conditions of the coherence of an acoustic signal that has undergone one surface reflection. The source and receivers were bottom-mounted in deep water near Bermuda. Two types of correlation measurements were made: (1) crosscorrelation of a surface-reflected signal received at two hydrophones 500 ft apart, and (2) crosscorrelation of a surface-reflected signal with a replica of the transmitted signal. The signal consisted of random noise for (1) and pseudorandom noise for (2). In both cases, the signal was limited to a 300-cps band centered at 750 cps. Results indicate that the coherence decreases with increasing sea state in a linear manner. Normalized crosscorrelation coefficients obtained from the two hydrophones decrease from about 0.9 for a waveheight of 5 ft to 0.5 for a waveheight of 20 ft. Values obtained for the single-hydrophone output and replica fall off at a much slower rate. [Work supported by the Bureau of Ships, U. S. Navy.]