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2010
Kang, T, Song HC, Hodgkiss WS.  2010.  Long-range multi-carrier acoustic communications in shallow water based on iterative sparse channel estimation. Journal of the Acoustical Society of America. 128:EL372-EL377.   10.1121/1.3514157   AbstractWebsite

Long-range orthogonal frequency division multiplexing (OFDM) acoustic communications is demonstrated using data from the Kauai Acomms MURI 2008 (KAM08) experiment carried out in about 106 m deep shallow water west of Kauai, HI, in June 2008. The source bandwidth was 8 kHz (12-20 kHz), and the data were received by a 16-element vertical array at a distance of 8 km. Iterative sparse channel estimation is applied in conjunction with low-density parity-check decoding. In addition, the impact of diversity combining in a highly inhomogeneous underwater environment is investigated. Error-free transmission using 16-quadtrative amplitude modulation is achieved at a data rate of 10 kb/s. (c) 2010 Acoustical Society of America

Song, AJ, Badiey M, Song HC, Hodgkiss WS.  2010.  Impact of source depth on coherent underwater acoustic communications (L). Journal of the Acoustical Society of America. 128:555-558.   10.1121/1.3459843   AbstractWebsite

A recent paper [Porter, and KauaiEx Group, J. Acoust. Soc. Am. 123, 856-865 (2008)] investigated ocean variability impact on coherent underwater acoustic communications (8-16 kHz) for a single near-seafloor transmitter in shallow water during an extended period (27 h). This letter extends that investigation to various source depths and receiver subarrays. Specifically, the middle water column source, which is either in or out of the thermocline, experiences performance variability of 6-7 dB in terms of output signal-to-noise ratio. Further, the source below the thermocline consistently outperforms the source above the thermocline when the receiver subarray is located below the thermocline. (C) 2010 Acoustical Society of America. [DOI: 10.1121/1.3459843]

Siderius, M, Song HC, Gerstoft P, Hodgkiss WS, Hursky P, Harrison C.  2010.  Adaptive passive fathometer processing. Journal of the Acoustical Society of America. 127:2193-2200.   10.1121/1.3303985   AbstractWebsite

Recently, a technique has been developed to image seabed layers using the ocean ambient noise field as the sound source. This so called passive fathometer technique exploits the naturally occurring acoustic sounds generated on the sea-surface, primarily from breaking waves. The method is based on the cross-correlation of noise from the ocean surface with its echo from the seabed, which recovers travel times to significant seabed reflectors. To limit averaging time and make this practical, beamforming is used with a vertical array of hydrophones to reduce interference from horizontally propagating noise. The initial development used conventional beamforming, but significant improvements have been realized using adaptive techniques. In this paper, adaptive methods for this process are described and applied to several data sets to demonstrate improvements possible as compared to conventional processing.

2009
Traer, J, Gerstoft P, Song HC, Hodgkiss WS.  2009.  On the sign of the adaptive passive fathometer impulse response. Journal of the Acoustical Society of America. 126:1657-1658.   10.1121/1.3206696   AbstractWebsite

Harrison [J. Acoust. Soc. Am. 125, 3511-3513 (2009)] presented a mathematical explanation for a sign-inversion induced to the passive fathometer response by minimum variance distortionless response (MVDR) beamforming. Here a concise mathematical formulation is offered, which decomposes the cross-spectral density matrix into coherent and incoherent components and allows the matrix inversion to be obtained exactly by eigendecomposition. This shows that, in the region containing the bottom reflection, the MVDR fathometer response is identical to that obtained with conventional processing multiplied by a negative factor. (C) 2009 Acoustical Society of America. [DOI: 10:1121/1.3206696]

Song, HC, Kuperman WA, Hodgkiss WS.  2009.  Basin-scale time reversal communications. Journal of the Acoustical Society of America. 125:212-217.   10.1121/1.3021435   AbstractWebsite

During November 1994, broadband acoustic signals were transmitted from a 75-Hz source to a 20-element, 700-m vertical array at approximately 3250 km range in the eastern North Pacific Ocean as part of the acoustic engineering test (AET) of the acoustic thermometry of ocean climate program [Worcester , J. Acoust. Soc. Am. 105, 3185-3201 (1999)]. The AET tomography signal can be treated as a binary-phase shift-keying communication signal with an information rate of 37.5 bits/s. With the multipath arrivals spanning 5-8 sec, these data represent an extreme case of intersymbol interference. The AET array data are processed using time reversal combined with frequent channel updates to accommodate channel variations over the 20-min long reception, followed by a single channel decision-feedback equalizer. The almost error-free performance using all 20 array elements demonstrates the feasibility of time reversal communications at basin scale. Further, comparable performance of single receive element communications integrating over multiple transmissions indicates that the ocean provided temporal diversity that is as effective as the spatial diversity provided by the array.

Yardim, C, Gerstoft P, Hodgkiss WS.  2009.  Sensitivity analysis and performance estimation of refractivity from clutter techniques. Radio Science. 44   10.1029/2008rs003897   AbstractWebsite

Refractivity from clutter (RFC) refers to techniques that estimate the atmospheric refractivity profile from radar clutter returns. A RFC algorithm works by finding the environment whose simulated clutter pattern matches the radar measured one. This paper introduces a procedure to compute RFC estimator performance. It addresses the major factors such as the radar parameters, the sea surface characteristics, and the environment (region, time of the day, season) that affect the estimator performance and formalizes an error metric combining all of these. This is important for applications such as calculating the optimal radar parameters, selecting the best RFC inversion algorithm under a set of conditions, and creating a regional performance map of a RFC system. The performance metric is used to compute the RFC performance of a non-Bayesian evaporation duct estimator. A Bayesian estimator that incorporates meteorological statistics in the inversion is introduced and compared to the non-Bayesian estimator. The performance metric is used to determine the optimal radar parameters of the evaporation duct estimator for six scenarios. An evaporation duct inversion performance map for a S band radar is created for the larger Mediterranean/Arabian Sea region.

Yardim, C, Gerstoft P, Hodgkiss WS.  2009.  Tracking of geoacoustic parameters using Kalman and particle filters. Journal of the Acoustical Society of America. 125:746-760.   10.1121/1.3050280   AbstractWebsite

This paper incorporates tracking techniques such as the extended Kalman, unscented Kalman, and particle (PF) filters into geoacoustic inversion problems. This enables spatial and temporal tracking of environmental parameters and their underlying probability densities, making geoacoustic tracking a natural extension to geoacoustic inversion techniques. Water column and seabed properties are tracked in simulation for both vertical (VLA) and horizontal (HLA) line arrays using the three tracking filters. Filter performances are compared in terms of filter efficiencies using the posterior Cramer-Rao lower bound. Tracking capabilities of the geoacoustic filters under slowly and quickly changing environments are studied in terms of divergence statistics. Geoacoustic tracking can provide continuously environmental estimates and their uncertainties using only a fraction of the computational power of classical geoacoustic inversion schemes. Interfilter comparison show that while a high-particle-number PF outperforms the Kalman filters, there are many cases where all three filters perform equally well depending on the inversion configuration (such as the HLA versus VLA and frequency) and the tracked parameters. (C) 2009 Acoustical Society of America. [DOI: 10.1121/1.3050280]

Song, HC, Hodgkiss WS, Kuperman WA, Akal T, Stevenson M.  2009.  High-rate synthetic aperture communications in shallow water. Journal of the Acoustical Society of America. 126:3057-3061.   10.1121/1.3257184   AbstractWebsite

Time reversal communication exploits spatial diversity to achieve spatial and temporal focusing in complex ocean environments. Spatial diversity can be provided easily by a vertical array in a waveguide. Alternatively, spatial diversity can be obtained from a virtual horizontal array generated by two elements, a transmitter and a receiver, due to relative motion between them, referred to as a synthetic aperture. This paper presents coherent synthetic aperture communication results from at-sea experiments conducted in two different frequency bands: (1) 2-4 kHz and (2) 8-20 kHz. Case (1) employs binary-phase shift-keying modulation, while case (2) involves up to eight-phase shift keying modulation with a data rate of 30 kbits/s divided by the number of transmissions (diversity) to be accumulated. The receiver utilizes time reversal diversity combining followed by a single channel equalizer, with frequent channel updates to accommodate the time-varying channel due to coupling of space and time in the presence of motion. Two to five consecutive transmissions from a source moving at 4 kts over 3-6 km range in shallow water are combined successfully after Doppler compensation, confirming the feasibility of coherent synthetic aperture communications using time reversal.

Song, HC, Hodgkiss WS, Kuperman WA, Akal T, Stevenson M.  2009.  High-frequency acoustic communications achieving high bandwidth efficiency. Journal of the Acoustical Society of America. 126:561-563.   10.1121/1.3160284   AbstractWebsite

A recent communications experiment was conducted in a shallow water environment at high-frequency permitting the use of a large bandwidth (11-19 kHz). This paper investigates the communication performance versus various symbol rates (or bandwidths) in terms of output signal-to-noise ratio with, an assortment of constellations, illustrating a trade-off between performance and data rate. A high bandwidth efficiency of 4 bits/s Hz is demonstrated using 32 quadrature amplitude modulation with a data rate of 31.25 kbits/s over a 2.2-km range. (C) 2009 Acoustical Society of America. [DOI: 10.1121/1.3160284]

Huang, CF, Gerstoft P, Hodgkiss WS.  2009.  Statistical estimation of source location in presence of geoacoustic inversion uncertainty. Journal of the Acoustical Society of America. 125:EL171-EL176.   10.1121/1.3097690   AbstractWebsite

A statistical estimation of source location incorporating uncertainty in ocean environmental model parameters is derived using a Bayesian approach. From a previous geoacoustic inversion, a posterior probability distribution of the environmental parameters that reflects uncertainty in the ocean environment is obtained. This geoacoustic uncertainty then is mapped into uncertainty in the acoustic pressure field and is propagated through the Bartlett matched-field processor for source localization. Using data from the ASIAEX 2001 East China Sea experiment, the estimated source location and variability over time are compared with the known source positions. (C) 2009 Acoustical Society of America

Jasso, H, Hodgkiss W, Baru C, Fountain T, Reich D, Warner K.  2009.  Using 9-1-1 call data and the space-time permutation scan statistic for emergency event detection. Government Information Quarterly. 26:265-274.   10.1016/j.giq.2008.12.005   AbstractWebsite

During medium to large-scale emergency events such as fires or earthquakes, emergency response teams must be deployed to the location of the event in a timely fashion. Information gathered from 9-1-1 call makers is useful for this, but is not always accurate. In this paper, we show how the space-time permutation scan statistic can be used to detect the presence and location of such events by monitoring the overall spatiotemporal pattern of 9-1-1 emergency calls instead. In an analysis of collected 9-1-1 call data, top detected clusters were found to correlate with emergency events as reported on the news, showing the algorithm's usefulness for automatically estimating their location and temporal extent. We show how the detection procedure works in cases where the emergency event generates a small but statistically significant increase in the number of 9-1-1 calls, as well as cases where events must be detected against a large background activity of 9-1-1 calls. We discuss the possible adoption of the proposed algorithm within the next generation digital government framework. Published by Elsevier Inc.

2008
Traer, J, Gerstoft P, Bromirski PD, Hodgkiss WS, Brooks LA.  2008.  Shallow-water seismoacoustic noise generated by tropical storms Ernesto and Florence. Journal of the Acoustical Society of America. 124:EL170-EL176.   10.1121/1.2968296   AbstractWebsite

Land-based seismic observations of double frequency (DF) microseisms generated during tropical storms Ernesto and Florence are dominated by signals in the 0.15-0.5 Hz band. In contrast, data from sea floor hydrophones in shallow water (70 m depth, 130 km off the New Jersey coast) show dominant signals in the ocean gravity-wave frequency band, 0.02-0.18 Hz, and low amplitudes from 0.18 to 0.3 Hz, suggesting significant opposing wave components necessary for DF microseism generation were negligible at the site. Florence produced large waves. over deep water while Ernesto only generated waves in coastal regions, yet both storms produced similar spectra. This suggests near-coastal shallow water as the dominant region for observed microseism generation. (C) 2008 Acoustical Society of America.

Gerstoft, P, Hodgkiss WS, Siderius M, Huang CF, Harrison CH.  2008.  Passive fathometer processing. Journal of the Acoustical Society of America. 123:1297-1305.   10.1121/1.2831930   AbstractWebsite

Ocean acoustic noise can be processed efficiently to extract Green's function information between two receivers. By using noise array-processing techniques, it has been demonstrated that a passive array can be used as a fathometer [Siderius, et al., J. Acoust. Soc. Am. 120, 1315-1323 (2006)]. Here, this approach is derived in both frequency and time domains and the output corresponds to the reflection sequence. From this reflection sequence, it is possible to extract seabed layering. In the ocean waveguide, most of the energy is horizontally propagating, whereas the bottom information is contained in the vertically propagating noise. Extracting the seabed information requires a dense array, since the resolution of the bottom layer is about half the array spacing. If velocity sensors are used instead of pressure sensors, the array spacing requirement can be relaxed and simulations show that just one vertical velocity sensor is sufficient. (C) 2008 Acoustical Society of America.

Huang, CF, Gerstoft P, Hodgkiss WS.  2008.  Effect of ocean sound speed uncertainty on matched-field geoacoustic inversion. Journal of the Acoustical Society of America. 123:EL162-EL168.   10.1121/1.2908406   AbstractWebsite

The effect of ocean sound speed uncertainty on matched-field geoacoustic inversion is investigated using data from the SW06 experiment along a nearly range-independent bathymetric track. Significant sound speed differences were observed at the source and receiving array and several environmental parameterizations were investigated for the inversion including representing the ocean sound speed at both source and receivers with empirical orthogonal function (EOF) coefficients. A genetic algorithm-based global optimization method was applied to the candidate environmental models. Then, a Bayesian inversion technique was used to quantify uncertainty in the environmental parameters for the best environmental model, which included an EOF description of the ocean sound speed. (C) 2008 Acoustical Society of America.

Raghukumar, K, Cornuelle BD, Hodgkiss WS, Kuperman WA.  2008.  Pressure sensitivity kernels applied to time-reversal acoustics. Journal of the Acoustical Society of America. 124:98-112.   10.1121/1.2924130   AbstractWebsite

Sensitivity kernels for receptions of broadband sound transmissions are used to study the effect of the transmitted signal on the sensitivity of the reception to environmental perturbations. A first-order Born approximation is used to obtain the pressure sensitivity of the received signal to small changes in medium sound speed. The pressure perturbation to the received signal caused by medium sound speed changes is expressed as a linear combination of single-frequency sensitivity kernels weighted by the signal in the frequency domain. This formulation can be used to predict the response of a source transmission to sound speed perturbations. The stability of time-reversal is studied and compared to that of a one-way transmission using sensitivity kernels. In the absence of multipath, a reduction in pressure sensitivity using time reversal is only obtained with multiple sources. This can be attributed both to the presence of independent paths and to cancellations that occur due to the overlap of sensitivity kernels for different source-receiver paths. The sensitivity kernel is then optimized to give a new source transmission scheme that takes into account knowledge of the medium statistics and is related to the regularized inverse filter. (c) 2008 Acoustical Society of America.

Roux, P, Cornuelle BD, Kuperman WA, Hodgkiss WS.  2008.  The structure of raylike arrivals in a shallow-water waveguide. Journal of the Acoustical Society of America. 124:3430-3439.   10.1121/1.2996330   AbstractWebsite

Acoustic remote sensing of the oceans requires a detailed understanding of the acoustic forward problem. The results of a shallow-water transmission experiment between a vertical array of sources and a vertical array of receivers are reported. The source array is used to provide additional degrees of freedom to isolate and track raylike arrivals by beamforming over both source and receiver arrays. The coordinated source-receiver array processing procedure is presented and its effectiveness in an example of tracking raylike arrivals in a fluctuating ocean environment is shown. Many of these arrivals can be tracked over an hour or more and show slowly varying amplitude and phase. The use of a double-beamforming algorithm lays the foundation for shallow-water acoustic remote sensing using travel time and source and receive angles of selected eigenrays. (C) 2008 Acoustical Society of America. [DOI: 10.1121/1.2996330]

Sifferlen, JF, Song HC, Hodgkiss WS, Kuperman WA, Stevenson JM.  2008.  An iterative equalization and decoding approach for underwater acoustic communication. IEEE Journal of Oceanic Engineering. 33:182-197.   10.1109/joe.2008.923552   AbstractWebsite

In this paper, we present an iterative approach for recovering information sent over a shallow underwater acoustic (UWA) communication channel. The procedure has three main tasks: estimation of channel model parameters (CMPs), channel equalization, and decoding. These tasks are performed cyclicly until the algorithm converges. Information bits are convolutionally encoded, punctured and permuted, mapped into quaternary phase-shift keying (QPSK) symbols, linearly modulated, and transmitted through a downward-refracting ocean waveguide. Training symbols are prepended to the transmitted sequence for initial estimation of CMPs. Our algorithm processes data from a single receive sensor. Data are received on a vertical array and the performance of the algorithm for each sensor in the array is examined. There is negligible Doppler spread in the received data. However, difference between transmitter and receiver clocks as well as slight motion of the receive array produce a nonnegligible compression of the received signals. Consequently, there is observable Doppler "shift." Nonuniform resampling of the data produces time series we model as the output of a linear time-invariant system. Resampling and CMP estimation are done iteratively, in conjunction with equalization and decoding. The algorithm successfully processes the data to yield few or no information bit errors.

Yardim, C, Gerstoft P, Hodgkiss WS.  2008.  Tracking refractivity from clutter using Kalman and particle filters. IEEE Transactions on Antennas and Propagation. 56:1058-1070.   10.1109/tap.2008.919205   AbstractWebsite

We address the problem of tracking the spatial and temporal lower atmospheric variations in maritime environments. The evolution of the range and height-dependent index of refraction is tracked using the sea clutter return from sea-borne radars operating in the region. A split-step fast Fourier transform based parabolic equation approximation to the wave equation is used to compute the clutter return in complex environments with varying index of refraction. In addition, regional statistics are incorporated as prior densities, resulting in a highly nonlinear and non-Gaussian tracking problem. Tracking algorithms such as the extended Kalman, unscented Kalman and particle filters are used for tracking both evaporative and surface-based electromagnetic ducts frequently encountered in marine environments. The tracking performances and applicability of these techniques to different types of refractivity-from-clutter problems are studied using the posterior Cramer-Rao lower bound. Track divergence statistics are analyzed. The results show that while the tracking performance of the Kalman filters is comparable to the particle filters in evaporative duct tracking, it is limited by the high non-linearity of the parabolic equation for the surface-based duct case. Particle filters, on the other hand, prove to be very promising in tracking a wide range of environments including the abruptly changing ones.

Park, G, Rosing T, Todd M, Farrar C, Hodgkiss W.  2008.  Energy harvesting for structural health monitoring sensor nNetworks. Journal of Infrastructure Systems. 14:64-79.   10.1061/(ASCE)1076-0342(2008)14:1(64)   AbstractWebsite

This paper reviews the development of energy harvesting for low-power embedded structural health monitoring (SHM) sensing systems. A statistical pattern recognition paradigm for SHM is first presented and the concept of energy harvesting for embedded sensing systems is addressed with respect to the data acquisition portion of this paradigm. Next, various existing and emerging sensing modalities used for SHM and their respective power requirements are summarized followed by a discussion of SHM sensor network paradigms, power requirements for these networks, and power optimization strategies. Various approaches to energy harvesting and energy storage are discussed and limitations associated with the current technology are addressed. The paper concludes by defining some future research directions that are aimed at transitioning the concept of energy harvesting for embedded SHM sensing systems from laboratory research to field-deployed engineering prototypes. Finally, it is noted that many of the technologies discussed herein are applicable to powering any type of low-power embedded sensing system regardless of the application.

2007
Song, HC, Hodgkiss WS, Kuperman WA, Akal T, Stevenson M.  2007.  Multiuser communications using passive time reversal. IEEE Journal of Oceanic Engineering. 32:915-926.   10.1109/joe.2007.904311   AbstractWebsite

A recent paper (Song et al, IEEE Journal of Oceanic Engineering, vol. 31, no. 2, pp. 170 - 178, 2006) demonstrated multiple-input-multiple-output (MIMO) communications in shallow water using active time reversal where the time reversal array (i.e., base station) sent different messages to multiple users simultaneously over a common bandwidth channel. Passive time reversal essentially is equivalent to active time reversal with the communications link being in the opposite direction. This paper describes passive time reversal communications which enables multiple users to send information simultaneously to the time reversal array. Experimental results at 3.5 kHz with a 1-kHz bandwidth demonstrate that as many as six users can transmit information over a 4-km range in a 120-m-deep water using quaternary phase-shift keying (QPSK) modulation, achieving an aggregate data rate of 6 kb/s. Moreover, the same data rate has been achieved at 20-km range by three users using 16 quadrature amplitude modulation (16-QAM).

Song, HC, Hodgkiss WS, Kim SM.  2007.  Performance prediction of passive time reversal communications (L). Journal of the Acoustical Society of America. 122:2517-2518.   10.1121/1.2782820   AbstractWebsite

A recent paper [M. Stojanovic J. Acoust. Soc. Am. 117, 1173-1185 (2005)] presented theoretical performance bounds of time reversal communications. In this letter, the performance of time reversal communications is evaluated using at-sea experimental data and compared to the theory for two different approaches; (1) time reversal alone and (2) time reversal combined with channel equalization. It is found that approach (1) shows a good agreement between theory and data. On the other hand, approach (2) indicates that the data is about 3-5 dB below the theory which assumes perfect knowledge of the channel, an infinite number of taps to remove the intersymbol interference, and no need for phase tracking. Taking these into consideration, the theoretical performance can provide a useful upper bound for predicting performance of time reversal communications. (c) 2007 Acoustical Society of America.

Goh, YH, Gerstoft P, Hodgkiss WS, Huang CF.  2007.  Statistical estimation of transmission loss from geoacoustic inversion using a towed array. Journal of the Acoustical Society of America. 122:2571-2579.   10.1121/1.2782915   AbstractWebsite

Geoacoustic inversion estimates environmental parameters from measured acoustic fields (e.g., received on a towed array). The inversion results have some uncertainty due to noise in the data and modeling errors. Based on the posterior probability density of environmental parameters obtained from inversion, a statistical estimation of transmission loss (TL) can be performed and a credibility level envelope or uncertainty band for the TL generated. This uncertainty band accounts for the inherent variability of the environment not usually contained in sonar performance prediction model inputs.. The approach follows [Gerstoft et al. IEEE J. Ocean. Eng. 31, 299-307 (2006)] and is demonstrated with data obtained from the MAPEX2000 experiment conducted by the NATO Undersea Research Center using a towed array and a moored source in the Mediterranean Sea in November 2000. Based on the geoacoustic inversion results, the TL and its variability are estimated and compared with the measured TL. (c) 2007 Acoustical Society of America.

Yardim, C, Gerstoft P, Hodgkiss WS.  2007.  Statistical maritime radar duct estimation using hybrid genetic algorithm-Markov chain Monte Carlo method. Radio Science. 42   10.1029/2006rs003561   AbstractWebsite

[1] This paper addresses the problem of estimating the lower atmospheric refractivity ( M profile) under nonstandard propagation conditions frequently encountered in low-altitude maritime radar applications. This is done by statistically estimating the duct strength (range- and height-dependent atmospheric index of refraction) from the sea surface reflected radar clutter. These environmental statistics can then be used to predict the radar performance. In previous work, genetic algorithms (GA) and Markov chain Monte Carlo (MCMC) samplers were used to calculate the atmospheric refractivity from returned radar clutter. Although GA is fast and estimates the maximum a posteriori ( MAP) solution well, it poorly calculates the multidimensional integrals required to obtain the means, variances, and underlying posterior probability distribution functions of the estimated parameters. More accurate distributions and integral calculations can be obtained using MCMC samplers, such as the Metropolis-Hastings and Gibbs sampling (GS) algorithms. Their drawback is that they require a large number of samples relative to the global optimization techniques such as GA and become impractical with an increasing number of unknowns. A hybrid GA-MCMC method based on the nearest neighborhood algorithm is implemented in this paper. It is an improved GA method which improves integral calculation accuracy through hybridization with a MCMC sampler. Since the number of forward models is determined by GA, it requires fewer forward model samples than a MCMC, enabling inversion of atmospheric models with a larger number of unknowns.

Huang, CF, Gerstoft P, Hodgkiss WS.  2007.  On the effect of error correlation on matched-field geoacoustic inversion. Journal of the Acoustical Society of America. 121:EL64-EL69.   10.1121/1.2424267   AbstractWebsite

The effect of correlated data errors on matched-field geoacoustic inversion for vertical array data is examined. The correlated errors stem from the inability to model the inhomogeneities in the environment resulting in an additional error term beyond ambient noise. Simulated data with these correlated errors are generated and then inverted with or without using the proper covariance matrix. Results show that the correlated error has a negative impact on geoacoustic parameter estimation if not accounted for properly. (c) 2007 Acoustical Society of America.

Song, HC, Hodgkiss WS, Kuperman WA, Sabra KG, Akal T, Stevenson M.  2007.  Passive reverberation nulling for target enhancement. Journal of the Acoustical Society of America. 122:3296-3303.   10.1121/1.2799508   AbstractWebsite

Echo-to-reverberation enhancement previously has been demonstrated using time reversal focusing when knowledge of the channel response between a target and the source array elements is available. In the absence of this knowledge, direct focusing is not possible. However, active reverberation nulling still is feasible given observations of reverberation from conventional source array transmissions. For a given range of interest, the response between the source array elements and the dominant sources of boundary reverberation is provided by the corresponding reverberation from this range. Thus, an active transmission can be projected from the source array which minimizes the energy interacting with the boundaries at a given range while still ensonifying the waveguide between the boundaries. As an alternative, here a passive reverberation nulling concept is proposed. In a similar fashion, the observed reverberation defines the response between the source array elements and the dominant sources of boundary reverberation at each range and this is used to drive a range-dependent sequence of projection operators. When these projection operators subsequently are. applied to the received data vectors, reverberation can be diminished. The improvement in target delectability is demonstrated using experimental data with an echo repeater simulating the presence of a target. (c) 2007 Acoustical Society of America.