Mohsen Badiey

Impact of ocean variability on coherent underwater acoustic communications during the Kauai experiment (KauaiEx)

During the July 2003 acoustic communications experiment conducted in 100m deep water off the western side of Kauai, Hawaii, a 10s binary phase shift keying signal with a symbol rate of 4kilosymbol∕s was transmitted every 30min for 27h from a bottom moored source at 12kHz center frequency to a 16 element vertical array spanning the water column at about 3km range. The communications signals are demodulated by time reversal multichannel combining followed by a single channel decision feedback equalizer using two subsets of array elements whose channel characteristics appear distinct: (1) top 10 and (2) bottom 4 elements. Due to rapid channel variations, continuous channel updates along with Doppler tracking are required prior to time reversal combining. This is especially true for the top 10 elements where the received acoustic field involves significant interaction with the dynamic ocean surface. The resulting communications performance in terms of output signal-to-noise ratio exhibits significant change o...

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.

Temporal and azimuthal dependence of sound propagation in shallow water with internal waves

The short time scale (minutes) and azimuthal dependence of sound wave propagation in shallow water regions due to internal waves is examined. Results from the shallow water acoustics in random media (SWARM-95) experiment are presented that reflect these dependencies. Time-dependent internal waves are modeled using the dnoidal solution to the nonlinear internal wave equations, so that the effects of both temporal and spatial variability can be assessed. A full wave parabolic equation model is used to simulate broadband acoustic propagation. It is shown that the short term temporal variability and the azimuthal dependence of the sound field are strongly correlated to the internal wave field.

Signal variability in shallow-water sound channels

Coherence of broad-band acoustic waves for mid-to-high frequencies (0.6-18 kHz) is obtained for a very shallow-water (15-m-deep) waveguide over a wide band of environmental conditions and for a source-receiver range of 387 m. Temporal behavior is sampled at two different rates: one that resolves at fractions of a second over intermittent periods of 40 s and another that resolves at 10 min over periods of several days. Spatial behavior is sampled in the vertical by hydrophones with spacings of the order of meters. The effect of environmental variability on coherence, in particular, soundspeed fluctuations in the water column and wind-induced modulations of the air-sea interface, is noted as a function of acoustic frequency and ray path. Analysis of the acoustic fluctuations over short time scales more accurately resolves the temporal decorrelation of the received signal due to sea surface waves. The vertical sampling of the received signal permits an analysis of arrival-angle fluctuations. The dependence of coherence on the number of surface bounces is studied by comparing arrivals that have zero, one, two, and three surface bounces.

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.

Quantifying the uncertainty of geoacoustic parameter estimates for the New Jersey shelf by inverting air gun data

This paper describes geoacoustic inversion of low frequency air gun data acquired during an experiment on the New Jersey shelf. Hybrid optimization and Bayesian inversion techniques based on matched field processing were applied to multiple shots from three air gun data sets recorded by a vertical line array in a long-range shallow water geometry. For the Bayesian inversions, full data error covariance matrix was estimated from a set of consecutive shots that had high temporal coherence and small spatial variation in source position. The effect of different data error information on the geoacoustic parameter uncertainty estimates was investigated by using the full data error covariance matrix, a diagonalized version of the full error covariance, and a diagonal matrix with identical variances. The comparison demonstrated that inversion using the full data error information provided the most reliable parameter uncertainty estimates. The inversions were highly sensitive to the near sea floor geoacoustic parameters, including sediment attenuation, of a simple single-layer geoacoustic model. The estimated parameter values of the model were consistent with depth averaged values (over wavelength scales) of a high resolution geoacoustic model developed from extensive ground truth information. The interpretation of the frequency dependence of the estimated attenuation is also discussed.

From geology to geoacoustics—Evaluation of Biot–Stoll sound speed and attenuation for shallow water acoustics

A procedure for estimating acoustic wave velocity and attenuation in ocean sediment using a minimum amount of geological and geotechnical data is demonstrated. First, the Biot–Stoll theory is presented. Next, various asymptotic formulae for the attenuation coefficient are derived for high, low, and intermediate frequencies. These expressions clearly isolate the effects of intergranular Coulomb friction and fluid viscous dissipation on the attenuation of shear and compressional waves. Under the constraint of a minimum amount of geological and geotechnical information, a sequence of empirical equations is compiled to convert basic data, such as blow count number from a Standard Penetration Test or shipboard density, into sediment geoacoustic properties. As a demonstration, two well-known field cases, the Atlantic Generating Station (AGS) site and the Atlantic Margin Coring (AMCOR 6010) site, are examined. By incorporating the uncertainty involved in the data collection, the estimated geoacoustical parameter...

Passive Time Reversal Acoustic Communications Through Shallow-Water Internal Waves

During a 12-h period in the 2006 Shallow Water Experiment (SW06), binary phase shift keying (BPSK) signals at the carrier frequencies of 813 and 1627 Hz were propagated over a 19.8-km source-receiver range when a packet of strong internal waves passed through the acoustic track. The communication data are analyzed by time reversal processing followed by a single-channel decision feedback equalizer. Two types of internal wave effects are investigated in the context of acoustic communications. One is the rapid channel fluctuation within 90-s data packets. It can be characterized as decreased channel coherence, which was the result of fast sound-speed perturbations during the internal wave passage. We show its effect on the time reversal receiver performance and apply channel tracking in the receiver to counteract such fluctuation. The other one is the long-term (in the scale of hours) performance degradation in the depressed waveguide when the internal waves passed through the acoustic track. Even with channel tracking, the time reversal receiver experiences average 3-4-dB decrease in the output signal-to-noise ratio (SNR). Such long-term performance degradation is explained by the ray approximation in the depressed waveguide.

Acoustic multipath arrivals in the horizontal plane due to approaching nonlinear internal waves

Simultaneous measurements of acoustic wave transmissions and a nonlinear internal wave packet approaching an along-shelf acoustic path during the Shallow Water 2006 experiment are reported. The incoming internal wave packet acts as a moving frontal layer reflecting (or refracting) sound in the horizontal plane. Received acoustic signals are filtered into acoustic normal mode arrivals. It is shown that a horizontal multipath interference is produced. This has previously been called a horizontal Lloyds mirror. The interference between the direct path and the refracted path depends on the mode number and frequency of the acoustic signal. A mechanism for the multipath interference is shown. Preliminary modeling results of this dynamic interaction using vertical modes and horizontal parabolic equation models are in good agreement with the observed data.

Time Reversal Receivers for High Data Rate Acoustic Multiple-Input–Multiple-Output Communication

A low-complexity receiver is proposed for high-frequency underwater acoustic multiple-input-multiple-output (MIMO) channels. The receiver uses time reversal combining followed by a single-channel decision feedback equalizer (DFE) to deal with the intersymbol interference. Periodical MIMO channel estimation is employed to track fast channel fluctuations. Both serial and parallel interference cancellation techniques are integrated with time reversal DFE to address the cochannel interference (CoI) in underwater MIMO systems. Two channel estimation algorithms are also implemented. It was demonstrated through the experiment conducted at Kauai, HI in 2005 that the proposed receiver can deal with the fast-fluctuating, dispersive MIMO channel at the carrier frequency of 37.5 kHz. Parallel interference cancellation combined with matching pursuit channel estimation was shown to provide significant performance improvements, indicating the receiver algorithm can effectively suppress the CoI. Four streams of binary phase-shift keying (BPSK) sequences at an aggregate rate of 16 kb/s and quadrature phase-shift keying (QPSK) sequences at a rate of 32 kb/s were demodulated at low bit error rates. These data rates corresponded to bandwidth efficiencies of 2.29 b/s/Hz or 4.57 b/s/Hz in a dynamic underwater environment, where the source and the receiver were drifting at a 2-km range.