The effect of towed array orientation on the 3D acoustic picture for sound sources and the vertical ambient noise profile

Abstract

The three dimensional (3D) acoustic arrival structure of the undersea ambient noise field is important for many reasons, and can give us significant insights into the Arctic environment. For example, the anthropomorphic sound of the ice cracking along the ice edge could be used to track the location of the ice edge as it advances and retracts throughout the seasons. The noise sources could also be used as a noise source to acoustically map the bathymetry of the largely unexplored Arctic seabed. In addition, vertical arrival structure of the ambient noise field could give hints and clues that allow for improvements in both acoustic communications and target tracking. In this research, we will examine the ability of an autonomous underwater vehicle (AUV) equipped with a towed array in a virtual environment to develop an accurate 3D acoustic picture of the undersea environment. While prior towed array experiments are generally limited to the arrays being towed in a horizontal manner, here a "yoyo" maneuver is introduced. In a yoyo maneuver, the vehicle moves up and down in the water column as it traverses in order to break up the ambiguity of the measured vertical arrival structure. This thesis presents a method to measure the "verticalness" introduced into the towed array by this maneuver, and quantifies how this improves the quality of the 3D arrival structure. The results conclude that within the vehicle maneuvering limits of a Bluefin-21 AUV, a fully pitched yoyo pattern vs. a constant depth pattern results in a relative increase in the maximum beam response of a source by approximately 6.5 dB, and also decreases the 3-dB down bandwidth in the vertical direction by approximately 12°. This is done without any significant losses for the bandwidth in the horizontal direction. When using a towed array to characterize a horizontally isotropic noise field, we find that within the AUV's maneuvering limits, the 3D beam response patterns are not sufficient to produce an accurate acoustic picture. To measure these fields, a vertical array is the most appropriate.