Time coherence of acoustic signals transmitted over resolved paths in the deep ocean

Abstract

Long-term coherence measurements have been made of the stability of broad-band acoustic signals transmitted on single paths through the deep ocean over distances of 145, 250, 495, and 635 NM. The stability of individual ray paths was measured by separating the arrivals in time and angle. Twenty-nine acoustic phase records with durations from 5 to 32 h were analyzed. Typical coherence intervals of 2 to 8 min were found in which the phase of the cross-correlation of the received signal with a replica varied by no more than ±π/2 rad (±0.6 msec) at 400 Hz. Coherent intervals of 8- to 12-min duration at 400 Hz were common, while occasional long periods on the order of 20 min could be found. The spectra of these phase records possess a dominant low-frequency peak overlapping the 12- and 24-h tidal frequencies, but also show much weaker peaks at periods of 1 1/2, 3, and 5 h. The latter could be caused by internal waves, nonlinear overtides, or atmospheric pressure variations. No other effects of the moving ocean surface or of higher-frequency internal ocean instabilities were found. Subject Classification; [43]30.20; [43]60.30; [43]30.82; [43]60.20.