Shallow water horizontal signal coherence measurements with use of mobile acoustic sources to create synthetic aperture arrays

• Published in: The Journal of the Acoustical Society of America 2009-04, Vol.125 (4_Supplement), p.2611-2611
• Main Authors: Abbot, Philip, Dyer, Ira, Emerson, Chris
• Format: Article
• Language: English
• ISSN: 0001-4966; 1520-8524
• DOI: 10.1121/1.4783948

Horizontal signal coherence length was measured in shallow water using a reciprocal synthetic aperture array created from multiple signals radiated by a mobile acoustic source, and a slowly drifting omnidirectional receiver. The tests were conducted in September, 2008, in shallow water (125 m depth) of the South East China Sea. Source and receiver depths were 61 m. Twelve hyperbolic FM slides per minute were transmitted while the source moved horizontally at 2.5 m/s, maintaining a nearly constant range of ~6 km to the receiver. Received signals were matched-filter processed, phase-corrected to form an equivalent linear array, then coherently combined using a synthetic aperture time delay beamformer. The beamformer output signal gain versus the number of elements in the array, N (using 2 ⩽ N ⩽ 512 transmissions), was compared to the theoretical limit (20 log, N). The coherence length, defined to be L/λ at a point 3 dB down from the limit, was determined. At 900 Hz, the correlation length was L/λ ≈ 40 (N=7, over 40 s). At 600 Hz, it was L/λ ≈ 10 (N=3 over 8 s). The degradation at 600 Hz is due to substantially smaller signal-to-noise levels at the element level. Partial coherence is observed at both frequencies, even at N=512.