Vertical Coherence of Shallow-Water Reverberation

Abstract

During a field experiment in 200 ft of water off the west coast of Florida, the vertical coherence of the reverberation following a small explosive charge was measured by recording, and later correlating, the outputs of a string of vertically separated hydrophones. The reverberation coherence, in terms of the cross-correlation coefficient between the sound pressures on hydrophone pairs, was found to increase with time after detonation, so as to indicate that the reverberation reaches the hydrophone string within an ever-narrowing vertical angle as time or range increases. The equivalent vertical angle of the reververation—here called the coherence angle—was found to lie in the range 5°–30° at frequencies between 200–8000 Hz. It is so small that only an extremely long vertical array can yield an appreciable array gain for active sonars operating in a background of shallow-water reverberation.