Coherence analysis of multimegameter range ocean acoustic signals

Abstract

The quality of acoustic travel time measurements depends on the coherence of the signal as well as the bandwidth. Ocean acoustic signals exhibit coherence properties in four ways: in time, in bandwidth, and in vertical and horizontal extent. (These are conjugate to Doppler broadening, pulse time spread, and rms vertical and horizontal arrival angle.) These properties have recently been measured and can be compared for several different ranges (80–5000 km), center frequencies (28–250 Hz), and geometries ( source on-bottom versus off-bottom). For signals of less than perfect coherence, techniques from communications theory can be used to improve the quality of travel time measurements by relaxing the resolution requirement. For instance, if the coherence bandwidth is less than the signal bandwidth, it is possible to consider subbands of the signal as separate measurements. How to best combine these measurements then depends crucially on the SNR. While acoustic thermometry has relied on electronically controlled sources, by allowing for the coherence structure of stochastic sources (such as T-phases, explosives, or marine mammals) an improved signal processing strategy may result. a)The ATOC Consortium: A. B. Baggeroer (MIT), T. G. Birdsall (U. Michigan), C. Clark (Cornell), J. A. Colosi (WHOI), B. D. Cornuelle (SIO), D. Costa (UCSC), B. D. Dushaw (U. Washington), M. Dzieciuch (SIO), A. M. G. Forbes (CSIRO, Hobart), C. Hill (MIT), B. M. Howe (U. Washington), J. Marshall (MIT), D. Menemenlis (MIT), J. A. Mercer (U. Washington), K. Metzger (U. Michigan), W. Munk (SIO), R. C. Spindel (U. Washington), D. Stammer (MIT), P. F. Worcester (SIO), and C. Wunsch (MIT).