Striation-Based Beamforming for Estimating the Waveguide Invariant with Passive Sonar

The waveguide invariant summarizes the pattern of constructive and destructive interference between acoustic modes propagating in the ocean waveguide. For many sonar signal-processing schemes, it is essential to know the correct numerical value for the waveguide invariant. While conventional beamforming can estimate the ratio between the waveguide invariant and the range to the source, it cannot unambiguously separate the two terms. In the present work, striationbased beamforming is developed. It is shown that the striation-based beamformer can be used to produce an estimate for the waveguide invariant that is independent of the range. Simulation results are presented

Exploitation of Frequency Information in Continuous Active Sonar

In pulsed active sonar, short duration coded waveforms insonify the area of interest. The low duty cycle limits detection opportunities and decreases average energy. A recent concept is continuous active sonar (CAS), which has continuous source transmission over a broad frequency band. The low duty cycle limits detection opportunities and decreases average energy. A recent concept is continuous active sonar (CAS), which has continuous source transmission over a broad frequency band. Previous work by the authors has investigated the utility of extracting the propagation-induced frequency structure in pulsed sonar. The broadband, continuous CAS waveforms particularly lend themselves to this approach. The presence of active striations in CAS data has been recently identified in the shallow water Target and Reverberation Experiment (TREX13). In this paper we provide additional examples of frequency structure in both the TREX13 and simulated data, and discuss methods for exploiting the striations to improve tracking performance

Mid-frequency sound propagation through internal waves at short range with synoptic oceanographic observations

Preliminary results are presented from an analysis of mid-frequency acoustic transmission data collected at range 550m during the Shallow Water 2006 Experiment. The acoustic data were collected on a vertical array immediately before, during, and after the passage of a nonlinear internal wave on 18 August, 2006. Using oceanographic data collected at a nearby location, a plane-wave model for the nonlinear internal wave’s position as a function of time is developed. Experimental results show a new acoustic path is generated as the internal wave passes above the acoustic source

Counterintuitive Results in Underwater Acoustic Communications

Book chapter. Underwater wireless communication using acoustic signals is a difficult problem and progress in finding robust solutions has been disappointing. Methods employed successfully in terrestrial wireless communications have not always transitioned successfully to underwater scenarios. An engineer’s intuition developed in solving the terrestrial problem may actually become a hindrance to solving the underwater problem. In the present work, several seemingly counterintuitive experimental results are examined: communications performance can be better when the range is longer rather than shorter, when the sea surface is rough rather than calm, when the bathymetry is undulating rather than flat. Physics-based explanations for the observed results are developed. A physicist’s intuition, however, also may fail when trying to develop useful models. A seemingly counterintuitive fact is that acoustic paths that undergo incoherent reflection from a rough sea surface can be shown experimentally to be useful for coherent communications. The requirements for a proper physics-based model are sketched

Developing a Model for Horizontal Array Coherence in the Presence of Random Linear Internal Waves

The resolution of a horizontal array in shallow water is limited by the environmental variability introduced by several oceanographic processes. The present work exercises a numerical model for how one oceanographic process, nearly linear internal waves, affects array coherence. As the linear internal waves can be represented as a random process, the model is statistical and calculates the mean coherence length. Three results from William Carey\u27s pioneering work [J. Acoust. Soc. Am., vol. 104, pp. 831-837, 1998] are examined: an analytical model for the array correlation function, the weak dependence of coherence length on source-receiver range, and scenarios where linear internal waves can produce a coherence length of approximately 30 wavelengths

Sixty Years Studying Wave Propagation in Random Media at the Applied Physics Laboratory

Ocean acoustics has been a useful avenue for testing evolving theories for Wave Propagation in Random Media (WPRM). These theories generally assume that the index of refraction statistics are stable in space and time, an assumption proven reasonably true in the deep ocean for acoustic paths away from boundaries. In the present work, results from 60 years of theoretical and experimental WPRM research at the University of Washington\u27s Applied Physics Laboratory (APL) are reviewed. The first experiment was performed in 1959 to test theories for amplitude fluctuations based on the Born approximation. The Rytov approximation (from Russian literature) for calculating the log-amplitude fluctuations was also evaluated. Conclusion: neither applied. Experiments in 1971 and 1977 measured acoustic fluctuation statistics for an 18 km acoustic path at sonar-relevant frequencies, 2–13 kHz. A 1985 experiment under Arctic ice used 2–16 kHz signals over a 6 km path. These experiments are discussed together with theoretical issues based on the Moment Equation method to provide one viewpoint on the history of ocean acoustic WPRM. The following translation of Voltaire is appropriate: The ancients when reasoning about physics without the enlightenment of experiments are like blind men explaining the nature of colors to other blind men

On the Sign of the Waveguide Invariant

Acoustic propagation in the ocean waveguide is characterized by mutual interference between the multiple ray paths connecting a source-receiver pair. In the Russian literature, these interference effects have been distilled mathematically into a single parameter, the so-called waveguide invariant defined as beta. The conventional wisdom is that the numerical value of beta is negative in deep water and positive in shallow water. In the present work, it is shown how the waveguide invariant can bifurcate and simultaneously have both positive and negative components. When bifurcation occurs, range-frequency mappings of acoustic intensity become fragmented. A method to separate the positive-beta components from the negative is sketched and applied to simulated data. Possible applications are discussed