Michael A. Wolfson

Ray dynamics in a long-range acoustic propagation experiment.

A ray-based wave-field description is employed in the interpretation of broadband basin-scale acoustic propagation measurements obtained during the Acoustic Thermometry of Ocean Climate programs 1994 Acoustic Engineering Test. Acoustic observables of interest are wavefront time spread, probability density function (PDF) of intensity, vertical extension of acoustic energy in the reception finale, and the transition region between temporally resolved and unresolved wavefronts. Ray-based numerical simulation results that include both mesoscale and internal-wave-induced sound-speed perturbations are shown to be consistent with measurements of all the aforementioned observables, even though the underlying ray trajectories are predominantly chaotic, that is, exponentially sensitive to initial and environmental conditions. Much of the analysis exploits results that relate to the subject of ray chaos; these results follow from the Hamiltonian structure of the ray equations. Further, it is shown that the collection of the many eigenrays that form one of the resolved arrivals is nonlocal, both spatially and as a function of launch angle, which places severe restrictions on theories that are based on a perturbation expansion about a background ray.

Ray dynamics in long-range deep ocean sound propagation.

Recent results relating to ray dynamics in ocean acoustics are reviewed. Attention is focused on long-range propagation in deep ocean environments. For this class of problems, the ray equations may be simplified by making use of a one-way formulation in which the range variable appears as the independent (timelike) variable. Topics discussed include integrable and nonintegrable ray systems, action-angle variables, nonlinear resonances and the KAM theorem, ray chaos, Lyapunov exponents, predictability, nondegeneracy violation, ray intensity statistics, semiclassical breakdown, wave chaos, and the connection between ray chaos and mode coupling. The Hamiltonian structure of the ray equations plays an important role in all of these topics.

Full-wave simulation of the forward scattering of sound in a structured ocean: A comparison with observations

The propagation of a 133±16‐Hz acoustic signal through the eastern North Pacific Ocean is simulated over basin‐scale ranges. A high angle c0‐insensitive parabolic equation model is used for the simulations. The ocean environment was constructed using CTD data or climatologically averaged data from the Levitus database, along the geodesic from Kaneohe, near Oahu to the receiver, located 3709 km to the northeast. A projection of a three‐dimensional frozen internal wave field is superposed. The stable earlier arrivals measured from experimental data were used as a reference anchor, and it is demonstrated that internal wave energy in addition to mesoscale structure is responsible for vertically scattering the late ‘‘axial’’ acoustic energy over 1 km.

The Long‐range Ocean Acoustic Propagation EXperiment (LOAPEX): An overview

The Long‐range Ocean Acoustic Propagation EXperiment (LOAPEX), conducted in the NE Pacific Ocean, provided acoustic transmissions from a ship‐suspended source at eight widely separated stations, and from a cabled acoustic source near the Island of Kauai, HI. The transmissions were received on several bottom‐mounted horizontal hydrophone arrays distributed about the NE Pacific Ocean Basin and two, nearly colocated, vertical hydrophone line arrays spanning roughly 3500 m of the water column. Ranges varied from 50 km to several Mm. The goals of the experiment are (i) to study the evolution, with distance (range), of the acoustic arrival pattern and in particular the dependence of the spatial and temporal coherence; (ii) to investigate the nature of the deep caustics and the associated arrivals well below their turning depths; (iii) to analyze the effects of the ocean bottom near the bottom‐mounted acoustic source cabled to Kauai; and (iv) to produce a thermal snapshot of the NE Pacific Ocean. The experiment ...

Ray dynamics in the AET experiment

A ray‐based wave field description is employed in the analysis of measurements made during the November 1994 Acoustic Engineering Test (the AET experiment). In this experiment phase‐coded pulse‐like signals with 75‐Hz center frequency and 37.5‐Hz bandwidth were transmitted near the sound channel axis in the eastern North Pacific Ocean. The resulting acoustic signals were recorded on a moored vertical receiving array at a range of 3252 km. In our analysis both mesoscale and internal‐wave‐induced sound speed perturbations are taken into account. Much of this analysis exploits results that relate to the subject of ray chaos; these results follow from the Hamiltonian structure of the ray equations. It is argued that all of the important features of the measured AET wave fields are consistent with a ray‐based wave field description in which ray trajectories are predominantly chaotic. [Work supported by ONR.]

The long‐range ocean acoustic propagation experiment (LOAPEX) observable: Modal content as a function of arrival time. Part A: Relevance

It is well known that for gradual enough changes in the sound speed structure, the mode number is nearly constant. Thus, changes in mode number are the most direct measure of scattering by smaller structures, such as internal waves. The mode number changes can be determined from data by separating the signal by arrival time. It is shown how the separation happens in an ideal case without scattering, allowing an optimum choice of the time window. The modal content in a small time window can also be estimated from the depth dependence of the intensity in the region of the turning depths of the modes. Each mode has a rapid falloff near its turning depth, and the extension beyond that depth measures the higher mode content. The deep vertical line array at LOAPEX measured this depth dependence. In addition, for certain times, all of the arrival is in the deep array, allowing unambiguous projections onto modes, especially if the source is above the sound axis. [Work supported by the Office of Naval Research.]

Comparing the effects of internal waves, mode coupling, and change in bandwidth of a radiated signal on low mode energy propagation.

Wideband signal propagation modeling is carried out for the actual space‐time configuration realized during the long‐range ocean acoustic propagation experiment (LOAPEX) conducted in the North Pacific in 2004. In particular, the experiment used the Kauai transmit station that was located at a range of 2432 km from a vertical line array (VLA), M‐sequence at 75 Hz, and a source depth of 800 m close to the depth of a sound‐channel axis. Two sound speed profiles were utilized to get the smooth two‐dimensional sound speed field. The first one was the profile obtained from the conductivity‐temperature‐depth (CTD) measurements at the Kauai station. The second sound speed profile from the VLA location was based on a CTD cast taken at station T50 (50 km from the VLA). To take into account the acoustic fluctuations due to internal waves, the buoyancy frequency profile based on LOAPEX CTD measurements made at station T50 was used. For simulations, 512 values of the horizontal wave number were utilized with the maxim...

Computational Predictions of sonar performance based on full wave acoustic propagation modeling

For fishery acoustics, sonars offer the advantage of inferring the biomass of a given ocean volume at a lower operational cost of traditional echo‐sounders, but wave guide effects which induce multi‐pathing yield unknown variances in bio‐mass estimates. A sonar performance model has recently been developed to assist in understanding this variance based on the para‐axial approximation to the acoustic wave equation. The model takes into account the geometry of the sonar deployment (source depth, transmit/receive beam patterns, pulse shape) and the acoustic environment (water column and geo‐acoustic bottom) and creates a data base of two‐way transfer functions for a chosen set of range‐depth cells. Then, given a hypothetical aggregation or school of fish, each with its own target strength, as well as noise and reverberation levels, one can create intensity maps which can be used to perform sensitivity studies to ascertain the performance characteristics of a fishery sonar. This talk will discuss the theoreti...

Effect of ocean internal waves on the interference component of the acoustic field in the Long‐range Ocean Acoustic Propagation Experiment

The propagation of energy along the sound‐channel axis cannot be described in terms of geometrical acoustics because of the presence of cusped caustics repeatedly along the axis. In neighborhoods of these cusped caustics a very complicated interference pattern is observed. Neighborhoods of interference grow with range and at long ranges they overlap. This results in the formation of a complex interference wave‐the axial wave‐that propagates along the sound‐channel axis like a wave belonging to a crescendo of near‐axial arrivals. In this paper, the axial wave is simulated for the LOAPEX CTD data measured at seven different ranges from the vertical line array. A signal with the center frequency of 75 Hz and 37.5‐Hz bandwidth is used for computations. This signal well approximates one transmitted m‐sequence in the LOAPEX experiment. The effect of environmental variability, induced by internal waves, on the axial wave is studied. The sound‐speed fluctuations caused by ocean internal waves are obtained with the use of the buoyancy frequency profile measured in the LOAPEX. Calculations are based on the integral representation of the axial wave in a local coordinate system introduced in the vicinity of the range‐variable sound‐channel axis. [Work supported by ONR.]

Long‐range ocean acoustic propagation experiment (LOAPEX) acoustic modeling analysis using measured environmental data

One objective of LOAPEX concerns the range dependence of multiple forward scattering for broadband, long‐range acoustic transmissions. Using environmental data extracted during the experiment, a sound speed field is constructed, which includes sound speed fluctuations due to neutrally buoyant intrusions as well as internal waves. This sound speed field is used as input to a full wave simulation model. Range‐dependent results from simulations regarding the coherent field and depth scattering below lower turning point caustics are presented.