An automated, passive algorithm for detecting and localizing small boats using two hydrophones mounted on the seabed is outlined. This extends previous work by Gebbie et al. [(2013). J. Acoust. Soc. Am. 134, EL77 - EL83] in which a similar two-hydrophone approach is used to produce an ambiguity surface of likely target locations leveraging multipath analysis and knowledge of the local bathymetry. The work presented here improves upon the prior approach using particle filtering to automate detection and localization processing. A detailed analysis has also been conducted to determine the conditions and limits under which the improved approach can be expected to yield accurate range and unambiguous bearing information. Experimental results in 12 m of water allow for a comparison of different separation distances between hydrophones, and the Bayesian Cramér-Rao lower bound is used to extrapolate the performance expected in 120 m water. This work demonstrates the conditions under which a low cost, passive, sparse array of hydrophones can provide a meaningful small boat detection and localization capability.

Harbor security and protection of maritime assets are issues of increasing concern. Outstanding research questions exist in terms of the optimal protection methodology needed for the wide variety of surface and submerged threats and diverse geographical locations. Economic costs and environmental concerns are also significant overriding issues. Acoustic methods have the advantage of being amenable to tracking and detecting targets both above and below the ocean surface. Moreover, passive acoustic methods are nonintrusive and capable of covering extensive ranges. Acoustic arrays offer significant advantages in terms of gain and signal processing capabilities over discrete, single hydrophones. We investigate the use of horizontal and vertical arrays for the detection and tracking of a remote environment monitoring system (REMUS) autonomous underwater vehicle as well as open-circuit divers in a noisy, shallow water environment. Using conventional beamforming techniques, we obtain positive preliminary results for detection and tracking, which highlight the overall merits of an acoustic array implementation.

This paper presents an analysis of the acoustic emissions emitted by an underway REMUS-100 autonomous underwater vehicle (AUV) that were obtained near Honolulu Harbor, HI using a fixed, bottom-mounted horizontal line array (HLA). Spectral analysis, beamforming, and cross-correlation facilitate identification of independent sources of noise originating from the AUV. Fusion of navigational records from the AUV with acoustic data from the HLA allows for an aspect-dependent presentation of calculated source levels of the strongest propulsion tone.

Mapping and profiling the ocean floor has, until recently, relied on the use of active sonar. These systems use transducers to transmit a known waveform toward the seabed, and hydrophones to listen for its reflections. In recent years, it has been shown that passive sonar systems, which use only hydrophones, can also be used for such purposes. Instead of artificial sounds, these techniques utilize naturally occurring noise on the ocean surface as a sound source. One of these techniques, called the passive fathometer, is able to produce a vertical profile of the seabed from breaking wave and wind noise. The time series it produces is intimately related to the acoustic properties of the seabed, as well as a quantity called the Rayleigh reflection coefficient (RRC). This paper presents a technique for estimating the normal incidence RRC using the passive fathometer. This is accomplished by time-gating the output and scaling the result using an incoherent estimate of the total power loss. Determination of the time-gate range is based off of a closed form solution that approximates the output of the passive fathometer. Using an idealized model of a vertical line array beamformer, it is shown that when surface noise undergoes several bottom-surface bounces, the correlation of end-fire beams produces a time domain representation of the RRC along with reverberations. Moving to a more realistic beamformer, it is shown how leakage at the opposite end-fire direction of the array produces additional artifacts in the time series output. In addition, leakage at oblique angles is used to explain artifacts that appear at the start of the time series that span twice the length of the array. By being able to predict the output of the passive fathometer for a given RRC, it is shown how an estimate of that coefficient can be extracted from ambient noise data. A simulation using the Ocean Acoustics and Seismic Exploration Synthesis (OASES) tool is presented which illustrates this technique. [Work supported by ONR]

Previous studies [Tiemann et al., J. Acoust. Soc. Am. 120, 2355-2365] have reported the localization of marine mammals in 3-D from their clicks using multipath arrivals. Bathymetric variations were advantageously used to predict multipath arrival times with a raytracer. These arrivals are directly discernible from the time series for impulsive sources, such as whale clicks, but extension of the method to continuous broadband sources presents additional complications. By pulse compressing noise emitted from a small boat using two hydrophones, the hyperbolic direct-arrival ambiguity can be refined in both range and bearing. Acoustic-derived results are validated with target GPS measurements.