Analysis of internal wave interactions with broadband acoustic signals propagating along the shelf during the SWARM’95 experiment

Abstract

A multi-institution experiment was conducted in 1995 on the New Jersey continental shelf to assess the Shallow Water Acoustic propagation in Random Media (SWARM’95). Two major source/receiver geometries were established. One was parallel to the continental shelf edge and the other was perpendicular. The orthogonal propagation consisted of two broadband sources transmitting signals perpendicular to the direction of internal wave field. The first signal was a transient airgun source (0.1-s duration), while the second was a linear frequency modulated sweep (30-s duration). Placed above and below the thermocline, these sources were transmitting signals every minute for a few hours at one location. Here we present a 2-h segment of these observations during 4 August 1995 when nonlinear internal waves were present. The transmissions were received by two vertical line arrays at different ranges and angles from the source. The dependence on the azimuth between the internal waves and the acoustic transmissions was examined. It is found that sound speed fluctuations induced by internal waves cause a 10–12-min temporal variations in the intensity of received signals. These temporal variations are azimuthally dependent since the environment is anisotropic due to internal solitary waves. [Work supported by ONR.]