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 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 reverberation was found to decrease with time or range and to lie in the range 5°–30° at frequencies between 200 and 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.