Shallow water beamforming with small aperture, horizontal, towed arrays

Abstract

This paper shows that the performance of plane-wave beamformers is significantly degraded in shallow water for horizontal arrays, even for short aperture arrays. Horizontal towed arrays must have short apertures for practical reasons, and the plane-wave beamformer needs to be augmented to optimize performance in shallow water. Matched-field processing (MFP) is a beamforming technique which allows for the detection and localization of an acoustic source in range and depth using passive sonar. The performance of MFP to augment plane-wave beamforming in shallow water environments using a short aperture horizontal line array is investigated. It is realized that MFP performance is enhanced when vertical arrays are used in deep water, or large aperture horizontal arrays are available in shallow water. However, this paper addresses the realistic case of operational, tactical horizontal arrays which are practical to deploy in shallow water. This paper attempts to integrate inverse beamforming (IBF) [A. H. Nuttall and J. H. Wilson, J. Acoust. Soc. Am. 90, 2004–2019 (1991); J. H. Wilson, J. Acoust. Soc. Am. 98, 3250–3261 (1995)], a plane-wave beamformer, with MFP so that the strengths of each are utilized. Results of test cases using synthetic data are presented to evaluate the effects of frequency, bottom type, relative target bearing, and sound-speed profile on MFP and IBF performance. IBF consists of three algorithms: the Fourier integral method (FIM) beamformer; the eight nearest neighbor peak picker (ENNPP); and a sophisticated M of N tracker. It is shown that MFP performance is significantly better than plane-wave beamforming for higher frequencies, for more reflective bottom types in shallow water, and for relative target bearings away from the broadside beams. IBF, or plane-wave beamforming, performs well at very low frequencies (VLF), in mud/silt-clay bottoms, and at beams near broadside. It is also shown that the performance of any plane-wave beamformer, including IBF, is severely degraded in shallow water for relative bearings away from the broadside beams, particularly at higher frequencies. Finally, the MFP algorithm in conjunction with the ENNPP and M of N tracker are shown to perform well in the real, deep water ocean environment. No appropriate shallow water measured data is available to evaluate the integrated IBF/MFP algorithm in shallow water.