Bruce H. Pasewark

Acoustic normal mode fluctuation statistics in the 1995 SWARM internal wave scattering experiment

In order to understand the fluctuations imposed upon low frequency (50 to 500 Hz) acoustic signals due to coastal internal waves, a large multilaboratory, multidisciplinary experiment was performed in the Mid-Atlantic Bight in the summer of 1995. This experiment featured the most complete set of environmental measurements (especially physical oceanography and geology) made to date in support of a coastal acoustics study. This support enabled the correlation of acoustic fluctuations to clearly observed ocean processes, especially those associated with the internal wave field. More specifically, a 16 element WHOI vertical line array (WVLA) was moored in 70 m of water off the New Jersey coast. Tomography sources of 224 Hz and 400 Hz were moored 32 km directly shoreward of this array, such that an acoustic path was constructed that was anti-parallel to the primary, onshore propagation direction for shelf generated internal wave solitons. These nonlinear internal waves, produced in packets as the tide shifts from ebb to flood, produce strong semidiurnal effects on the acoustic signals at our measurement location. Specifically, the internal waves in the acoustic waveguide cause significant coupling of energy between the propagating acoustic modes, resulting in broadband fluctuations in modal intensity, travel-time, and temporal coherence. The strong correlations between the environmental parameters and the internal wave field include an interesting sensitivity of the spread of an acoustic pulse to solitons near the receiver.

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.

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 monitoring of the tide height and slope-water intrusion at the New Jersey Shelf in winter conditions

Waveguide invariant theory is used to describe the frequency shifts of constant acoustic intensity level curves in broadband signal spectrograms measured at the New Jersey Shelf during the winter of 2003. The broadband signals (270-330 Hz) were transmitted from a fixed source and received at three fixed receivers, located at 10, 20, and 30 km range along a cross-shelf propagation track. The constant acoustic intensity level curves of the received signals indicate regular frequency shifts that can be well predicted by the change in water depth observed through tens of tidal cycles. A second pattern of frequency shifts is observed at only 30 km range where significant variability of slope-water intrusion was measured. An excellent agreement between observed frequency shifts of the constant acoustic intensity levels and those predicted by the change in tide height and slope water elevations suggests the capability of long-term acoustic monitoring of tide and slope water intrusions in winter conditions.

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.

Stabilized high‐resolution beamforming with horizontal arrays: Two experimental trials in shallow water

High‐resolution algorithms are potentially able to outperform conventional linear methods at the task of beamforming, i.e., spatial‐spectrum estimation, and in controlled experiments or simple computer simulations they often do prove superior. In marine environments, however, high‐resolution techniques frequently fall short of these expectations. Their outputs can take on an unstable appearance characterized by fading of the true target peak and by the occurrence of multiple false targets. In recent years a class of stabilization techniques has evolved to moderate these effects by suppressing contributions from the smaller eigenvalues of the cross‐spectral density matrix. While such techniques generally produce good results in simulations with the maximum likelihood estimator and even with the more sensitive maximum entropy estimator, they have seen little use on sea data. Here, results are presented of two sea trials of a simple eigenvalue stabilization procedure in a relatively demanding class of enviro...

Inversion of range‐dependent geoacoustic properties in South China Sea ASIAEx01 experimental site

Matched‐field inversion of range‐dependent geoacoustic properties is studied using broadband (50–200 Hz, 240–260 Hz, and 550–600 Hz) acoustic data collected in the South China Sea during the ASIAEx01 experiment. Range‐dependent sediment sound‐speed and attenuation profiles are inverted using a global optimization scheme based on genetic algorithms to minimize an objective function defined by the Bartlett processor output. For the forward model, an efficient coupled normal mode model is used to calculate broadband acoustic fields incorporating range‐dependent bathymetry and sediment layers thickness obtained by chirp sonar surveys. Inversions were performed starting from a range‐independent region with a relatively short source/receiver distance. Additional geoacoustic profiles were inverted by incorporating the previously inverted profiles as the source/receiver distance was increased. The results obtained at three different frequency bands are in good agreement, especially when the range‐dependency of ba...

Fluctuations in acoustic propagation seen in the SWARM 95 experiment

Fluctuations in acoustic propagation as seen in data collected during the SWARM 95 experiment are analyzed and related to the oceanographic environment. Acoustic data were collected on a 32 element vertical receiver array which spanned the 88 m water column from depths of 23 to 85 m. The receiver array was located 42 km seaward from two fixed acoustic projectors which transmitted a 224 Hz PRN signal (16 Hz bandwidth) and a 400 Hz PRN signal (100 Hz bandwidth). Intensity statistics, such as scintillation index (SI) and the probability distribution function (PDF), are discussed as a function of depth and time for narrow‐band and broadband (replica correlation) processing. Broadband SI is shown to be less than the narrow‐band SI which has a typical value of 2. The PDF is shown to be neither a Rayleigh nor a log‐normal distribution and evidence is presented that unsaturated scattering processes are involved. [Work supported by the Office of Naval Research.]

Acoustic intensity and phase variability caused by time‐evolving internal wave fields

Relationships between time‐evolving internal wave fields in a shallow‐water environment and both acoustic intensity and phase variability are explored through a computer simulation model developed from experimental data acquired on the continental shelf off the New Jersey coast [Apel et al., IEEE J. Ocean. Eng. 22, 465–500 (1997)]. From an analysis of SWARM (Shallow Water Acoustics in a Random Media) data, single realizations of the acoustic field are computed at an array located approximately 42 km from sources emitting at 224 and 400 Hz. Using a wide‐angle parabolic equation, the acoustic field is computed in 1‐min intervals over a 12‐h time window during which both spatially diffuse and spatially localized internal wave fields propagate through the water column. Results include time‐evolving modal power and scintillation indices, and can be interpreted in terms of mode coupling. The ability to locate a source using phase‐coherent techniques such as matched field processing will also be addressed. [Work...