Abstract
Since the advent of large-aperture array processing, more and more attention has been paid to the sound field correlation, which has fundamental limit to the array gain of spatial coherent signal processing. The two dominant mechanisms that degrade the spatial coherence are normal modes (or multi-paths) interference and the environmental variability caused by several relevant oceanographic processes. In the present study, the transverse horizontal spatial coherence of explosive signals has been studied experimentally by a bottom-mounted array in the Northern South China Sea. And the effects of normal mode interference on the transverse horizontal spatial coherence have been analyzed numerically. Expressed in terms of wavelengths, the coherence length is shown to be larger than 170λ/185λ at acoustic frequency 508-640Hz/80-101Hz in shallow water. It is much greater than Carey’s shallow-water result 30λ estimated from array signal gain after assuming a specific functional form for the coherence (The Journal of the Acoustical Society of America 104, 831 (1998)). It, however, is consistent with Rouseff’s modelling result of a coherence length larger than 100λ (The Journal of the Acoustical Society of America 138, 2256 (2015)). Both Carey and Rouseff argue that the transverse horizontal spatial coherence length depends only weakly on range, in direct. In the present study, however, the coherence length is shown to depend highly on source-receiver range, and it fluctuates synchronously with the sound-field intensity while range varies.
Highlights
The spatial correlation of acoustic field is an important characteristic in underwater acoustics
We present the experimental result about the transvers horizontal spatial correlations of the sound field in shallow water by using the experimental data obtained in the Northen South China Sea
As for the 508 to 640 Hz frequency band, the transverse coherence length is shown to be larger than 170λ at most ranges less than 50km in shallow water
Summary
The spatial correlation of acoustic field is an important characteristic in underwater acoustics. Carey[8] investigated shallow-water experimental data taken from around the world, and estimated transvers horizontal coherence length Lcoh from array signal gain after assuming a specific functional form for the coherence. Rouseff[1] developed a statistical model that quantifies how linear internal waves affects the transverse horizontal Lcoh, neglecting all other forms of environmental variability. As for a range of 20km, internal wave energy of 400 J/m2 and a water column of 80m, the modeling result of Lcoh is between 70λ and 180λ at acoustic. We present the experimental result about the transvers horizontal spatial correlations of the sound field in shallow water by using the experimental data obtained in the Northen South China Sea. And we investigate the transvers coherence in shallow water by combining the normal modes theory with geometry analysis
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