In most areas, estimating the presence and distribution of cryptic marine mammal species, such as beaked whales, is extremely difficult using traditional observational techniques such as ship-based visual line transect surveys. Because acoustic methods permit detection of animals underwater, at night, and in poor weather conditions, passive acoustic observation has been used increasingly often over the last decade to study marine mammal distribution, abundance, and movements, as well as for mitigation of potentially harmful anthropogenic effects. However, there is demand for new, cost-effective tools that allow scientists to monitor areas of interest autonomously with high temporal and spatial resolution in near-real time. Here we describe an autonomous underwater vehicle – a glider – equipped with an acoustic sensor and onboard data processing capabilities to passively scan an area for marine mammals in near-real time. The glider was tested extensively off the west coast of the Island of Hawai'i, USA. The instrument covered approximately 390 km during three weeks at sea and collected a total of 194 h of acoustic data. Detections of beaked whales were successfully reported to shore in near-real time. Manual analysis of the recorded data revealed a high number of vocalizations of delphinids and sperm whales. Furthermore, the glider collected vocalizations of unknown origin very similar to those made by known species of beaked whales. The instrument developed here can be used to cost-effectively screen areas of interest for marine mammals for several months at a time. The near-real-time detection and reporting capabilities of the glider can help to protect marine mammals during potentially harmful anthropogenic activities such as seismic exploration for sub-sea fossil fuels or naval sonar exercises. Furthermore, the glider is capable of under-ice operation, allowing investigation of otherwise inaccessible polar environments that are critical habitats for many endangered marine mammal species.

This paper examines array gain and detection performance of single vector sensors and vector sensor line arrays, with focus on the impact of nonacoustic self noise and finite spatial coherence of the noise between the vector sensor components. Analytical results based on maximizing the directivity index show that the particle motion channels should always be included in the processing for optimal detection, regardless of self noise level, as long as the self noise levels are taken into account. The vector properties of acoustic intensity can be used to estimate the levels of nonacoustic noise in ocean measurements. Application of conventional, minimum variance distortionless response, and white-noise-constrained adaptive beamforming methods with ocean acoustic data collected by a single vector sensor illustrate an increase in spatial resolution but a corresponding decrease in beamformer output with increasing beamformer adaptivity. Expressions for the spatial coherence of all pairs of vector sensor components in homogeneous, isotropic noise show that significant coherence exists at half-wavelength spacing between particle motion components. For angular intervals about broadside, an equal spacing of about one wavelength for all components provides maximum directivity index, whereas each of the component spacings should be different to optimize the directivity index for angular intervals about endfire.

An underwater glider based on a flying wing design (Jenkins et al., 2003) presently is under development by the Marine Physical Laboratory, Scripps Institution of Oceanography and the Applied Physics Laboratory, University of Washington. This design maximizes the horizontal distance between changes in buoyancy to minimize mechanical power consumed in horizontal transport. The prototype wing has a 6.1 m wing span and is 20 times larger by volume than existing gliders. Initial at-sea tests indicate that the lift-to-drag ratio is 17/1 at a horizontal speed of about 1.8 m/s for a 38-liter buoyancy engine. Beamforming results using recordings of the radiated noise from the deployment ship by two hydrophones mounted on the wing verify aspects of the prototype wing flight characteristics. The payload on the new glider will include a low-power, 32-element hydrophone array placed along the leading edge of the wing for large physical aperture at midfrequencies (above 1 kHz) and a 4-component vector sensor. Data previously collected by these types of arrays illustrate the performance of narrow-band detection and localization algorithms. Flight behaviors are being developed to maximize the arrays’ detection and localization capabilities. [Work sponsored by the Office of Naval Research.]

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Liberdade, a new class of underwater glider based on a flying wing design, has been under development for the past 3 years in a joint project between the Marine Physical Laboratory, Scripps Institution of Oceanography and the Applied Physics Laboratory, University of Washington. This hydrodynamically efficient design maximizes the horizontal distance traveled between changes in buoyancy, thereby minimizing average power consumed in horizontal transport to achieve ‘‘persistence.’’ The first fully autonomous glider of this class, ‘‘XRay,’’ was deployed and operated successfully in the Monterey Bay 2006 experiment. Communications, including real-time glider status reports, were accomplished using an underwater acoustic modem as well as with an Iridium satellite system while on the surface. The payload included hydrophone array, with 10 kHz per channel bandwidth, located in a sonar dome along the leading edge of the 6.1-m-span wing. Narrowband tones from 3.0 to 8.5 kHz were transmitted from a ship-deployed controlled underwater source. During the glider’s flight, lift-to-drag ratios (equal to the inverse of the glide slope) exceeded 10/1. However, specific flight behaviors that deviated from this efficient horizontal transport mode allowed for improved detection and localization by the hydrophone array. [Work sponsored by the Office of Naval Research.]