The high frequency ambient noise in warm shallow waters is dominated by snapping shrimp. The loud snapping noises they produce are impulsive and broadband. As the noise propagates through the water, it interacts with the seabed, sea surface, and submerged objects. An array of acoustic pressure sensors can produce images of the submerged objects using this noise as the source of acoustic "illumination." This concept is called ambient noise imaging (ANI) and was demonstrated using ADONIS, an ANI camera developed at the Scripps Institution of Oceanography. To overcome some of the limitations of ADONIS, a second generation ANI camera (ROMANIS) was developed at the National University of Singapore. The acoustic time series recordings made by ROMANIS during field experiments in Singapore show that the ambient noise is well modeled by a symmetric α-stable (SαS) distribution. As high-order moments of SαS distributions generally do not converge, ANI algorithms based on low-order moments and fractiles are developed and demonstrated. By localizing nearby snaps and identifying the echoes from an object, the range to the object can be passively estimated. This technique is also demonstrated using the data collected with ROMANIS.

Flow noise levels for a digital thin line towed array for normal operating speeds of an AUV were estimated using empirical models and towing experiments. The wavenumber filtering associated with the finite size and distribution of hydrophones in acoustic element of DTLA was applied to the empirical models of turbulent wall pressure spectra to estimate the expected flow noise levels. The empirical results were then compared with the noise spectra measured during towing experiments conducted in a quiet lake. The results showed rapid loss of flow noise energy with increase in frequency arising due to the very nature of turbulent noise spectra and flow noise averaging introduced by the finite size of the hydrophone.

Abstract-Underwater Ambient Noise Imaging (ANI) systems rely on the acoustic illumination produced by natural noise sources to image an object of interest. Snapping shrimp are a dominant natural source of illumination in tropical waters and their snaps occur randomly. Hence incoherent energy detection methods, which require no knowledge of the source locations, are usually employed to form images of the objects. This approach, although simple, only produces images when the anisotropy in ambient noise is conducive. Even in anisotropic noise, some sources 'illuminate' the target while others obscure it. In this paper we describe a different approach. We use a snap detection algorithm to estimate the locations of the noise sources on the sea-bottom and then use the sound from these sources to passively range and form images of the objects. By using only the noise from sources that provide us good illumination and rejecting undesirable sources, we improve the image quality. The feasibility of this approach has been experimentally demonstrated with the data collected during a recent deployment of ROMANIS, an ANI camera developed in Singapore.

A nominally circular 2-D broadband acoustic array of 1.3-m diameter, comprising 508 sensors and associated electronics, was designed, built, and tested for ambient noise imaging (ANI) potential in Singapore waters. The system, named Remotely Operated Mobile Ambient Noise Imaging System (ROMANIS), operates over 25-85 kHz, streaming real-time data at 1.6 Gb/s over a fiber optic link. By using sensors that are much larger than halfwavelength at the highest frequency of interest, so with some directionality, good beamforming performance is obtained with a small number of sensors compared to a conventional half-wavelength-spaced array. A data acquisition system consisting of eight single-board computers enables synchronous data collection from all 508 sensors. A dry-coupled neoprene cover is used to encapsulate the ceramic elements as an alternative to potting or oil filling, for easier maintenance. Beamforming is performed in real-time using parallel computing on a graphics processing unit (GPU). Experiments conducted in Singapore waters yielded images of underwater objects at much larger ranges and with better resolution than any previous ANI system. Although ROMANIS was designed for ANI, the array may be valuable in many other applications requiring a broadband underwater acoustic receiving array.Index Terms-Ambient noise imaging (ANI), broadband array design, underwater acoustics, data acquisition.

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