SHALLOW-WATER BEAMFORMING WITH SMALL APERTURE, HORIZONTAL, TOWED ARRAYS

This paper shows that the performance of plane-wave beamformers is sig nificantly degraded in shallow water for horizontal arrays, even for s hort aperture arrays. Horizontal towed arrays must have short aperture s for practical reasons, and the plane-wave beamformer needs to be aug mented to optimize performance in shallow water. Matched-field process ing (MFP) is a beamforming technique which allows for the detection an d localization of an acoustic source in range and depth using passive sonar. The performance of MFP to augment plane-wave beamforming in sha llow water environments using a short aperture horizontal line array i s investigated It is realized that MFP performance is enhanced when ve rtical arrays are used in deep water, or large aperture horizontal arr ays are available in shallow water. However, this paper addresses the realistic case of operational, tactical horizontal arrays which are pr actical to deploy in shallow water. This paper attempts to integrate i nverse beamforming (IBF) [A. H. Nuttall and J. H. Wilson, J. Acoust. S ec. Am. 90, 2004-2019 (1991); J. H. Wilson, J. Acoust. Sec. Am. 98, 32 50-3261 (1995)], a plane-wave beamformer, with MFP so that the strengt hs of each are utilized. Results of test cases using synthetic data ar e presented to evaluate the effects of frequency, bottom type, relativ e target bearing, and sound-speed profile on MFP and IBF performance. IBF consists of three algorithms: the Fourier integral method (FIM) be amformer; the eight nearest neighbor peak picker (ENNPP); and a sophis ticated M of N tracker. It is shown that MFP performance is significan tly better than plane-wave beamforming for higher frequencies, for mor e reflective bottom types in shallow water, and for relative target be arings 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 sha llow water for relative bearings away from the broadside beams, partic ularly at higher frequencies. Finally, the MFP algorithm in conjunctio n with the ENNPP and M of N tracker are shown to perform well in the r eal, deep water ocean environment. No appropriate shallow water measur ed data is available to evaluate the integrated IBF/MFP algorithm in s hallow water. (C) 1997 Acoustical Society of America.