Search

A theory is presented for the upper bound on the mean time τ to the detection of an undersea acoustic communications network by an energy detector whose initial position and heading are uniformly distributed random variables. The network is an infinite square grid of omnidirectional transmit-receive nodes on a flat bottom. Each node transmits to one nearest neighbor at a bit-rate R equal to Shannon’s capacity, maximizing τ. The network sets a signaling bandwidth W and node-spacing D. The detector sets a false-alarm rate, integration time, height above the bottom, and speed. For W=5 kHz and D=1 km, τ is computed as a function of R in 200 -m water with propagation varying from spherical to cylindrical spreading, volume absorption of 2 dB per km (corresponding approximately to 15 kHz), and illustrative values for other parameters. [ABSTRACT FROM AUTHOR]

Acoustic signals from three long-range (500-700 km) transmission paths in the Northeast Pacific were examined for multipath structures. Sound propagation along each path encountered both different sound-speed provinces and unique bathymetry which, together with the range differences, caused characteristic pulse arrival patterns at each hydrophone site. The receivers were all on a sloping bottom at depths between 1200 and 1400 m and the source depth was at 450 m in deep water. Ray-path arrivals were modeled using impulse, a new ray-theoretical impulse response code written by one of us (RCS). This code uses piecewise continuous cubic polynomials to fit the sound-speed profile and bathymetric profile, and Runge-Kutta methods to solve the ray equation of motion. It allows arbitrary ray density in launch angle, and identifies eigenrays by searching for rays whose depths bracket the receiver and which were adjacent at the source. Using this procedure, we were able to identify most of the major pulse arrivals observed at each of the recording sites. [ABSTRACT FROM AUTHOR]

We present a ray-trace numerical study of the effects of mesoscale structure on the group velocity (defined as the average horizontal ray speed) of deep-refracted acoustic signals using data from a large-scale, near synoptic survey of the thermal structure in the upper 800 m of the Northwest Pacific ocean reported by Emery et al. [W. J. Emery, T. J. Reid, J. A. de Santo, R. N. Baer, and J. P. Dugan, J. Acoust. Soc. Am. 66, 831-841 (1979)]. A representative sample of twelve sound speed profiles (SSPs) from the survey show maximum variation of 10 m/s in the group velocity of a ray with a given J value, J being the phase integral, and illustrate how particular features in the SSPs cause local maxima in the group velocity as a function of J. [ABSTRACT FROM AUTHOR]

Signal speeds (average horizontal ray velocities) are calculated on the basis of bathymetry and nine sound speed profiles (SSPs) for three great circle paths from shot detonation sites off Perth, Australia to receivers near Fernando de Noronha (14 552 km) and Bermuda (19 766 and 19 810 km). We use two signal speed algorithms, one a long-range ray-tracing code and the other an adiabatic invariant approximation (AIA) method, which assumes J = ∮ c-1 sin [variant_greek_theta] dz is constant for a given ray (where ∮ dz is an integral over one-ray cycle, c the local depth-dependent sound speed, [variant_greek_theta] the ray angle, and z the depth coordinate). Comparison with field measurements made 21 years ago shows that the ray-tracing and AIA methods, when applied to the same eigenrays, yield values averaging, respectively, 5.9±2.3 and 4.5±2.4 m/s below the observed values. The AIA method, however, yields values only 2.0±1.6 m/s below measurements when applied to rays vertexing just above the chief bathymetric obstructions near the Crozet and Kerguelen Islands. The specific reason for the offset is unclear, but this study shows AIA methods can be accurate for very general range-dependent SSPs. [ABSTRACT FROM AUTHOR]

The adiabatic invariance of the phase integral c-1 sin[variant_greek_theta] dz≡J leads to new analytic results for the average horizontal ray speed in constricting parabolic, Hirsch, and bilinear sound velocity profiles (SVPs). Theoretical predictions agree well with exact numerical raytracing solutions even for nonslowly varying SVPs, and show that the average ray speed may either increase or decrease in a contricting channel, depending on the SVP type. The adiabatic invariant approximation (AIA) appears to provide physical insight, an accuracy check for present range-dependent codes, and an alternative in some cases to long-range raytracing, for conditons typical of SOFAR propagation. We illustrate the latter point with an application of the AIA to a set of SVPs representative of the eastern North Pacific. [ABSTRACT FROM AUTHOR]

Acoustic signals from three long-range (500-700 km) transmission paths in the Northeast Pacific were examined for multipath structure. Sound propagation along each path encountered both different sound-speed provinces and unique bathymetry which, together with the range differences, caused characteristic pulse arrival patterns at each hydrophone site. The receivers were all on a sloping bottom at depths between 1200 and 1400 m and the source depth was at 450 m in deep water. Raypath arrivals were modeled using IMPULSE, a new ray-theoretical impulse response code written by one of us (RCS). This code uses piecewise continuous cubic polynomials to fit the sound-speed profile and bathymetric profile, and Runge-Kutta methods to solve the ray equation of motion. It allows arbitrary ray density in launch angle, and identifies eigenrays by searching for rays whose depths bracket the receiver and which were adjacent at the source. Using this procedure, we were able to identify most of the major pulse arrivals observed at each of the recording sites. [ABSTRACT FROM AUTHOR]

Beamforming for an array typically requires array-element localization (AEL) to within one-tenth of a wavelength. Often it is impractical to install dedicated, large-bandwidth AEL transponders with an array. This paper presents an approach to AEL for fixed, bottom-mounted arrays without transponders. High-SNR signals from multiple near-field imploding lightbulbs and interpolated correlations allow a statistical reduction in the inherent uncertainty in arrival-time measurements imposed by the bandwidth of the sensors. The method is illustrated with an application to the SWSS (shallow-water sensor string) array offshore of San Diego, at depths from 180 to 200 m. [ABSTRACT FROM AUTHOR]