Measuring temperature changes of the deep oceans, important for determining the oceanic heat content and its impact on the Earth's climate evolution, is typically done using free‐drifting profiling oceanographic floats with limited global coverage. Acoustic thermometry provides an alternative and complementary remote sensing methodology for monitoring fine temperature variations of the deep ocean over long distances between a few underwater sources and receivers. We demonstrate a simpler, totally passive (i.e., without deploying any active sources) modality for acoustic thermometry of the deep oceans (for depths of ~ 500–1500 m), using only ambient noise recorded by two existing hydroacoustic stations of the International Monitoring System. We suggest that passive acoustic thermometry could improve global monitoring of deep‐ocean temperature variations through implementation using a global network of hydrophone arrays.

The accuracy of the seismo-acoustic parabolic equation is tested for problems involving sloping fluid–solid interfaces. The fluid may correspond to the ocean or a sediment layer that only supports compressional waves. The solid may correspond to ice cover or a sediment layer that supports compressional and shear waves. The approach involves approximating the medium in terms of a series of range-independent regions, using a parabolic wave equation to propagate the field through each region, and applying single-scattering approximations to obtain transmitted fields across the vertical interfaces between regions. The accuracy of the parabolic equation method for range-dependent problems in seismo-acoustics was previously tested in the small slope limit. It is tested here for problems involving larger slopes using a finite-element model to generate reference solutions.

The accuracy of the seismo-acoustic parabolic equation is tested for problems involving sloping solid–solid interfaces and variable topography. The approach involves approximating the medium in terms of a series of range-independent regions, using a parabolic wave equation to propagate the field through each region, and applying a single-scattering approximation to obtain transmitted fields across the vertical interfaces between regions. The accuracy of the parabolic equation method for range-dependent problems in seismo-acoustics was previously tested in the small slope limit. It is tested here for problems involving larger slopes using a finite-element model to generate reference solutions.

Leaky wave antennas (LWAs) have been shown to be an effective tool for frequency-steerable wave radiation in both the electromagnetic and acoustic wave regimes. LWA’s operate by modifying the impedance on a waveguide such that refraction occurs out of the waveguide at an angle corresponding to Snell’s Law. For a LWA with uniform leaking parameter across the waveguide length, that leakage angle is constant. Using analytical techniques, and by careful geometric design of the waveguide impedance, the leaked beampattern can be tailored. The process of the tapering process for an acoustic LWA is discussed here, and notional examples are presented including sidelobe reduction. [Work supported by the Office of Naval Research.]

A long range Underwater Navigation Algorithm (UNA) is described that provides a geolocation underwater while submerged without having to surface for a Global Navigation Satellite System (GNSS) position. The UNA only uses measured acoustic travel times from a constellation of underwater acoustic sources analogous to the constellation of satellites in GNSS. The UNA positions are calculated without any a priori track, position or sound speed information, and thus provide a “Cold Start” capability. The algorithm was tested using data from the 2010–2011 Philippine Sea Experiment in which six sources were deployed in a pentagon ∼400 km on a side. 502 positions of hydrophones in a bottom-moored vertical line array at depths of 485–3037 m drifting in a tidal watch circle up to 600 m in diameter were computed. The sources were 129–450 km from the hydrophone receivers. The mean UNA position error from ground truth was 58 m with a standard deviation of 32 m. The UNA Cold Start Algorithm position can be used as the point in the ocean for calculating acoustic model runs from the source positions with a four-dimensional sound speed field from a general circulation model to improve the accuracy.

The geometry of non-smooth A n>2 caustics in solutions of the Helmholtz equation is analyzed using a Fock-Schwinger proper-time formulation. In this description, A 3 or cusp caustics are intimately related to poles of a quantity called the einbein action in the complex proper-time, or einbein, plane. The residues of the poles vanish on spatial curves known as ghost sources, to which cusps are bound. The positions of cusps along the ghost sources is related to the value of the poles. A similar map is proposed to relate essential singularities of the einbein action to higher order caustics. The singularities are shown to originate from degenerations of a certain Dirichlet problem as the einbein is varied. It follows that the singularities of the einbein action, along with the associated aspects of caustic geometry, are invariant with respect to large classes of perturbations of the index of refraction.

Correlation processing of ocean noise can be used to develop totally passive ocean monitoring methods. Using various hydrophone pair orientations, this study investigates the frequency dependence, seasonal variability, and emergence rate of coherent arrivals from cross-correlations of low frequency ambient noise (f < 40 Hz) recorded on triangular hydrophones arrays. These arrays are located at five existing hydroacoustic stations of the International Monitoring System (IMS), situated in the deep-sound channel, and distributed across the Atlantic, Pacific, and Indian Ocean basins. For the majority of studied sites, persistent and fast-emerging coherent arrivals are reliably obtained if the axis connecting the selected hydrophone pair has a direct line-of-sight with regions of the globe containing stable and diffuse noise sources (e.g., polar-ice or seismic noise). Furthermore, for this favorable orientation, the emergence rate of coherent arrivals extracted between hydrophone pairs separated by long ranges (here ∼130 km) can be approximated based on measurements made between hydrophone pairs separated by short ranges (∼2 km) in the Atlantic Ocean. Hence, results from this study, obtained using existing hydrophone configurations of the IMS hydroacoustic stations, could be used to guide the placement of other hydrophone arrays over the globe for future long-range passive ocean monitoring experiments.

The acoustic prism (i.e., leaky wave antenna) has been experimentally demonstrated in air as a way to steer an emitted beam using only a single broadband acoustic source. The prism relies on a leaky, dispersive waveguide to provide a unique radiation angle for each narrowband frequency projected by the acoustic source. In air, the leakage occurs through a series of periodically spaced shunts in the waveguide. This study examines an acoustic prism design that is capable of operating underwater, where leakage occurs through the waveguide wall itself due to the much lower impedance contrast of the waveguide material in water to that in air. This results in a geometrically simpler design in the underwater case. However, shear wave effects must be considered in the design of the underwater acoustic prism. The waveguide wall is constructed out of a composite material to have a high impedance but a low shear modulus, which are both necessary conditions to decrease sidelobes in the radiated pressure field. Numerical results indicate that the acoustic prism design is capable of scanning a range of frequencies from broadside to forward endfire. Experimental realization of the underwater acoustic prism is also discussed. [Work sponsored by ONR.]