Stephen N. Wolf

Acoustic propagation through an internal wave field in a shallow water waveguide

This paper addresses the problem of predicting and interpreting acoustic wave field properties in a stochastic ocean waveguide, for which the sound-speed variability within the water column is treated explicitly as a random process. It is assumed that the sound-speed distribution is composed of three components: a deterministic, time-independent profile and two stochastic components induced by internal wave activity. One random contribution represents a spatially diffuse Garrett–Munk field whose spectrum is constrained by the shallow water waveguide, while the second corresponds to spatially localized soliton packets. A high-angle elastic parabolic equation method is applied to compute single frequency realizations of the pressure field using this three-component representation of the sound-speed distribution. Ensemble-averaged transmission loss and scintillation index measures for the full pressure field and its modal components are estimated for different source depths and for both flat and sloping bott...

Coherence of acoustic modes propagating through shallow water internal waves

The 1995 Shallow Water Acoustics in a Random Medium (SWARM) experiment [Apel et al., IEEE J. Ocean. Eng. 22, 445-464 (1997)] was conducted off the New Jersey coast. The experiment featured two well-populated vertical receiving arrays, which permitted the measured acoustic field to be decomposed into its normal modes. The decomposition was repeated for successive transmissions allowing the amplitude of each mode to be tracked. The modal amplitudes were observed to decorrelate with time scales on the order of 100 s [Headrick et al., J. Acoust. Soc. Am. 107(1), 201-220 (2000)]. In the present work, a theoretical model is proposed to explain the observed decorrelation. Packets of intense internal waves are modeled as coherent structures moving along the acoustic propagation path without changing shape. The packets cause mode coupling and their motion results in a changing acoustic interference pattern. The model is consistent with the rapid decorrelation observed in SWARM. The model also predicts the observed partial recorrelation of the field at longer time scales. The model is first tested in simple continuous-wave simulations using canonical representations for the internal waves. More detailed time-domain simulations are presented mimicking the situation in SWARM. Modeling results are compared to experimental data.

Measurements of Acoustic Ambient Noise in Shallow Water Due to Breaking Surf.

Horizontal directionality of ambient noise was measured at ranges up to 15 km from the southeastern shore of Monterey Bay, California. Water depths at the sites ranged from 8–175 m. A steerable cardioid receiving pattern was formed using signals telemetered from dipole and omnidirectional hydrophones suspended from tethered buoys. With no nearby shipping, whenever the maximum of the cardioid pattern was directed toward the beach, noise levels in the frequency range 20–700 Hz were greater than those obtained when the maximum was directed seaward. This difference or anisotropy (seaward versus shoreward), which depends on range from the beach, on frequency, and on surf intensity, was 10 dB at 300 Hz at the 9‐km site during very heavy surf. Surf beat was clearly audible when the cardioid maximum was steered shoreward at ranges as great as 2 km. The anisotropy effects diminish both in magnitude and frequency range with lower wave height but are still observable during light surf. During heavy surf, the omnidir...

Acoustic field variability induced by time evolving internal wave fields

A space- and time-dependent internal wave model was developed for a shallow water area on the New Jersey continental shelf and combined with a propagation algorithm to perform numerical simulations of acoustic field variability. This data-constrained environmental model links the oceanographic field, dominated by internal waves, to the random sound speed distribution that drives acoustic field fluctuations in this region. Working with a suite of environmental measurements along a 42-km track, a parameter set was developed that characterized the influence of the internal wave field on sound speed perturbations in the water column. The acoustic propagation environment was reconstructed from this set in conjunction with bottom parameters extracted by use of acoustic inversion techniques. The resulting space- and time-varying sound speed field was synthesized from an internal wave field composed of both a spatially diffuse (linear) contribution and a spatially localized (nonlinear) component, the latter consisting of solitary waves propagating with the internal tide. Acoustic simulation results at 224 and 400 Hz were obtained from a solution to an elastic parabolic equation and are presented as examples of propagation through this evolving environment. Modal decomposition of the acoustic field received at a vertical line array was used to clarify the effects of both internal wave contributions to the complex structure of the received signals.

South China Sea internal tide/internal waves-impact on the temporal variability of horizontal array gain at 276 Hz

The temporal variability of the spatial coherence of an acoustic signal received on a bottomed horizontal array has been calculated for 276-Hz narrow-band signals. A conventional plane wave beamformer was applied to the received signals. The temporal variability of the arrays omnipower, beam power, and array gain are related to variability in the sound-speed field. The spectral characteristics of array omnipower are nonstationary and changed as the spectral characteristics of the temperature field varied. The array omnipower and beam-power variability tracked each other in time and varied by as much as 15 dB over time intervals as short as 7 min. Array gain varied up to 5 dB and usually tracked the omnipower variability. A contiguous 24-h section of data is discussed in detail. This data section is from a time period during which the high-frequency fluid dynamic perturbation of the sound-speed field was of smaller amplitude than other sections of the 16-d data set. Consequently, this section of data sets an upper bound for the realizable array gain. The temporal variability of array gain and spatial coherence at times appears to be correlated with environmental perturbation of the sound-speed field, but are also correlated with changes in the signal-to-noise ratio. The data was acquired during the Office of Naval Researchs South China Sea Asian Seas International Acoustics Experiment. The 465-m 32-channel horizontal array was placed on the bottom in 120 m of water at the South China Sea shelf break. The acoustic source was moored in 114 m of water /spl sim/19 km from the receiving array.

Acoustic propagation in shallow water overlying a consolidated bottom

An experiment designed to measure normal mode amplitude functions and attenuation coefficients was conducted in shallow water on Campeche Bank off the Yucatan Peninsula. Measurements were made at two locations on the bank in water of about 30 m in depth over a bottom consisting of consolidated limestone having a measured and sound velocity of 1900 m/sec. Pulsed cw signals with frequencies of 400, 750, and 1500 Hz were used. Theoretical calculations of the mode amplitude functions using a fluid model of the bottom were found to agree well with the measurements. In order to reconcile the measured mode attenuation coefficients with theory, it was necessary to assume that the shear velocity of the bottom was 1000 m/sec. The latter is lower than the minimum sound velocity in the water column so that the generation of propagating shear waves in the bottom was the dominant attenuation mechanism. Significant differences in the measured mode attenuation coefficients at the two stations were explained by the deepen...

Acoustic Intensity Variability in a Shallow Water Environment

Acoustic signals with center frequencies 224 and 400 Hz were recorded for 63-hours during an experiment on the New Jersey Shelf, USA (SWARM95). Acoustic energy statistics have been extracted for both narrowband and broadband signals at a fixed range of 42 km. The statistics have been found to be non-stationary and depth dependent. There is frequency and bandwidth dependence to the signal properties and no unique probability distribution representation.

Experimental determination of modal depth functions from covariance matrix eigenfunction analysis

A low‐frequency sound field in a shallow‐water duct is usually represented by a set of discrete normal modes, each of which is characterized by a unique vertical pressure amplitude depth function. In many applications, such as matched‐field signal processing, it is important to have accurate determinations of these depth functions, which are usually calculated from normal‐mode models. By using a densely populated vertical array spanning the water column and a large time‐bandwidth test signal, in some circumstances the normal‐mode functions of the duct may easily be obtained by diagonalizing the cross‐spectral density function of the received signal. The condition required for using this technique is that, over the bandwidth used, the fields of individual modes must be mutually incoherent so that the cross‐spectral density matrix is the sum of individual modal diads, making the modal depth functions the same as the matrix eigenfunctions. Experimentally determined mode functions of orders 1 through 4 are compared with theoretical results.

Matched‐field inversion of seabed geoacoustic properties complemented by chirp sonar surveys

Matched‐field inversion of range‐dependent seabed parameters is studied using broadband (525–725 Hz) acoustic data collected on the New Jersey Shelf during the SWARM95 experiment. An efficient global optimization scheme is used to minimize an objective function defined by the Bartlett processor output in both beam and mode space. With the input from chirp sonar bathymetry and sediment thickness data, a broadband acoustic field is calculated by a coupled normal mode model and range‐dependent geoacoustic properties are inverted. An independent chirp sonar inversion of acoustic impedance profiles was also performed along the same track by using chirp sonar data. The inversion results indicate good agreement between two inversion methods and show their effectiveness if they are used as complementary to each other. The effect of a strongly range‐dependent water column on the inversion performance and possible broadband inversion using single hydrophone is also studied. [Work supported by ONR.]

Range-Dependent, Normal-Mode Reverberation Model for Bistatic Geometries

An acoustic normal-mode reverberation model for bistatic source and receiver configurations in multi-layered, range-dependent underwater environments has been developed. Adiabatic normal-mode theory is used to propagate the signal field (with dispersion) to and from the scattering regions. The model is designed such that scattering from the interfaces and volume inhomogeneities may be easily included. Currently, scattering from the ocean bottom is treated by using a three-dimensional Lambert’s-law/facet reflection model. Sources and receivers may be either omnidirectional or vertical/horizontal line arrays. An explicit description of the coupling between incident and scattered modes and their contribution to reverberation is obtained.