Regions that influence acoustic propagation in the sea at moderate frequencies, and the consequent departures from the ray-acoustic description

In the limit where a transient signal is comprised of very large frequencies, spatial regions within an inhomogeneous medium that influence the propagation from a source to a receiver lie along one or more ray paths. At lower frequencies for which the geometrical acoustic approximation is of borderline applicability, the regions that influence such transient signals are extended because of diffraction. Previous research has addressed the numerical determination of those spatial regions that influence propagation at low frequency. The present paper addresses the question of how high the center frequency need be so that the regions of influence are nearly described as ray paths for a model ocean in which the speed of sound increases nearly linearly with depth from a perfectly reflecting surface. Computations indicate that near 2500 Hz and at a range of 50 km, the region of influence resembles a ray. Noticeable departures from the ray picture are found at a range of 500 km. Various physical and mathematical causes for the departures from the ray propagation model for lower frequencies and for greater ranges are identified and discussed

U.S. Navy sources and receivers for studying acoustic propogation and climate change in the ocean (L)

Sounds from a U.S. Navy SSQ-110A source are received at high signal-to-noise ratios at ocean-basin scales at two Sound Surveillance Systems in the Pacific. The sounds have sufficient pulse resolution to study climatic variations of temperature. The acoustic data can be understood using ray and parabolic approximations to the wave equation. Modeled internal waves decrease pulse resolution from 0.01 to 0.1 s, consistent with observations

Acoustic identification of a single transmission at 3115 km from a bottom-mounted source at Kauai

Sounds received in the Gulf of Alaska at 3115 km from the ATOC/NPAL source at Kauai (75 Hz, 0.027-s resolution, bottom-mounted) are compared with acoustic and oceanographic models. Unlike data collected at stationary SOSUS arrays, these data come from a towed horizontal array at 372-m depth of military origin. A plausible identification of the acoustic reception is made despite the fact that only one transmission is collected and sound interacts with the bottom near the source. The similarity between the modeled and measured impulse response here may be useful for understanding the signals between this same source and the NPAL array near southern California. The plausible identification of sound from the horizontal array here appears to point toward the feasibility of using other military platforms of opportunity besides SOSUS to study acoustic propagation and possibly map climatic changes in temperature by means of tomography

Probability distributions for locations of calling animals, receivers, sound speeds, winds, and data from travel time differences

A new nonlinear sequential Monte Carlo technique is used to estimate posterior probability distributions for the location of a calling animal, the locations of acoustic receivers, sound speeds, winds, and the differences in sonic travel time between pairs of receivers from measurements of those differences, while adopting realistic prior distributions of the variables. Other algorithms in the literature appear to be too inefficient to yield distributions for this large number of variables (up to 41) without recourse to a linear approximation. The new technique overcomes the computational inefficiency of other algorithms because it does not sequentially propagate the joint probability distribution of the variables between adjacent data. Instead, the lower and upper bounds of the distributions are propagated. The technique is applied to commonly encountered problems that were previously intractable such as estimating how accurately sound speed and poorly known initial locations of receivers can be estimated from the differences in sonic travel time from calling animals, while explicitly modeling distributions of all the variables in the problem. In both cases, the new technique yields one or two orders of magnitude improvements compared with initial uncertainties. The technique is suitable for accurately estimating receiver locations from animal call

Comparison of Measured and Modeled Temporal Coherence of Sound Near 75 Hz and 3683 km in the Pacific Ocean

The hypothesis to test is internal gravity waves following a Garrett–Munk spectrum are sufficient to explain temporal coherence of sound at 3683 km in the Pacific Ocean for a signal at 75 Hz and a pulse resolution of 0.03 s. Signals from a 20 min transmission are collected on a towed array. After correcting the data for what likely appears to be acceleration of the receiver, the probability distribution for multipath coherence time is very similar to that obtained from Monte Carlo simulations of the impulse response. The most likely coherence time is 20 min, the longest that can be measured with a 20 min transmission. Predictions of multipath temporal coherence and amplitude fluctuations appear accurate enough to make useful predictions of channel capacity

Finding the right cross-correlation peak for locating sounds in multipath environments with a fourth-moment function

To locate calling animals in reverberant environments from recordings on widely separated receivers, a fourth-moment Augmented-Template Correlation Function (ATCF) helps identify which of many peaks in each cross-correlation function is that corresponding to the difference in travel times for the first arrivals (reference-lag). This peak may not be the largest. The ATCF, by providing an approximate correlation between auto- and cross-correlation functions, can be orders of magnitude more efficient in selecting the reference-lag than the alternative of randomly selecting peaks. The ATCF\u27s efficacy increases with the number of paths and their signal-to-noise ratios

Geometry of locating sounds from differences in travel time: Isodiachrons

Calling animals may be located from measurements of the differences in acoustic travel time at pairs of receivers. For inhomogeneous fields of speed, locations can be made with better accuracy when the location algorithm allows the speed to vary from path to path. A new geometrical shape, called an isodiachron, is described. It is the locus of points corresponding to a constant difference in travel time along straight paths between the animal and two receivers. Its properties allow an interpretation for locations when the speed differs from path to path. An algorithm has been developed for finding the location of calling animals by intersecting isodiachrons from data collected at pairs of receivers. When the sound speed field is spatially homogeneous, isodiachrons become hyperboloids. Unlike a hyperboloid that extends to infinity, an isodiachron is confined to a finite region of space when the speeds differ between the animal and each of two receivers. Its shape is significantly different than a hyperboloid for cases of practical interest. Isodiachrons can be used to better understand locations of calling animals and other sounds in the sea, Earth, and air

Probability density functions for hyperbolic and isodiachronic locations

Animal locations are sometimes estimated with hyperbolic techniques by estimating the difference in distances of their sounds between pairs of receivers. Each pair specifies the animal\u27s location to a hyperboloid because the speed of sound is assumed to be spatially homogeneous. Sufficient numbers of intersecting hyperboloids specify the location. A nonlinear method is developed for computing probability density functions for location. The method incorporates a priori probability density functions for the receiver locations, the speed of sound, winds, and the errors in the differences in travel time. The traditional linear approximation method overestimates bounds for probability density functions by one or two orders of magnitude compared with the more accurate nonlinear method. The nonlinear method incorporates a generalization of hyperbolic methods because the average speed of sound is allowed to vary between different receivers and the source. The resulting isodiachronic surface is the locus of points on which the difference in travel time is constant. Isodiachronic locations yield correct location errors in situations where hyperbolic methods yield incorrect results, particularly when the speed of propagation varies significantly between a source and different receivers