Measurements of transmission fluctuations at three ranges for refracted paths through the deep ocean

Fluctuations of acoustic transmission were measured for resolved refracted–refracted ray paths at three long ranges in the deep ocean (258, 410, 560 km). A moored broadband source with center frequency of 460 Hz transmitted continuously for a period of several days from each range. Received signals were processed to give time histories of complex pulse response. Fluctuation statistics are computed for the carrier line, the combined multipath signal and for time gated arrivals associated with predicted paths. Refracted arrivals are identified by ray models and statistics are computed for resolved paths, unresolved doublets of upgoing and downgoing rays and the combined multipath. A second mooring was used to measure temperature fluctuations associated with internal waves and for estimating the magnitude and vertical coherence of sound-speed fluctuations during the acoustic experiments. We report statistics of fluctuations and compare measurement with predictions of A Theory of Sound Transmission Through a Fluctuating Ocean, edited by Flatté (Cambridge U.P., Cambridge, 1979). Λ and Φ parameters of the theory are computed numerically using environmental measurements of sound-speed fluctuations as input. Estimates of the mean square phase fluctuations using the Φ parameter are in agreement with measurements for all three ranges. Most paths at all ranges exhibit regular fading characteristics of time varying multipath interferance. Some paths at the shorter range exhibit more rapid fading and have decorrelation times, moments, and pulse spreading more characteristic of full saturation than paths of the longer ranges. Neither variations of environment fluctuations, details of the ray geometry nor computed Λ–Φ values explain these short-range results.