Spatial coherence of acoustic signals measured during the 1998–1999 North Pacific Acoustic Laboratory (NPAL) experiment

Abstract

Little experimental data have been reported in the literature about spatial coherence of low-frequency sound waves propagating over megameter ranges in the ocean. In 1998–1999, the North Pacific Acoustic Laboratory (NPAL) carried out a comprehensive low-frequency long range sound propagation experiment with the use of the receiving billboard acoustic array. By signal-processing of the NPAL data, we calculated spatial coherence of the broadband acoustic signals (with 75 Hz carrier) after their propagation of about four thousand km in North Pacific. The results obtained are presented in this paper. First, the horizontal coherence of the NPAL signals is discussed. The results show that the horizontal coherence radius varies in the range 500–1000 m. This value is in a good agreement with predictions of a 3D modal theory of low-frequency long-range sound propagation in a fluctuating ocean. Secondly, the vertical coherence of the NPAL signals is presented. After averaging over several days of the experiment, the vertical coherence reveals non-monotonic dependence on the distance between hydrophones. Finally, the horizontal-vertical coherence is studied. This coherence rapidly degrades with the increase of vertical separation between hydrophones. [Work supported by ONR.]