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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.

Sabra, KG, Roux P, Song HC, Hodgkiss WS, Kuperman WA, Akal T, Stevenson JM.  2006.  Experimental demonstration of iterative time-reversed reverberation focusing in a rough waveguide. Application to target detection. Journal of the Acoustical Society of America. 120:1305-1314.   10.1121/1.2227377   AbstractWebsite

For most shallow water waveguides, the backscattered energy measured in a monostatic configuration is dominated by ocean bottom reverberation. A selected time-gated portion of the measured reverberation signal is used to provide a transfer function between a time-reversal array and a corresponding range interval on the bottom. Ultrasonic and at-sea experiments demonstrate the focusing capabilities of a time-reversal array along the rough bottom interface using these reverberation signals only. The iterative time-reversal technique facilitates robust focusing along the ocean bottom, with little signal processing effort involved and a priori information of the environment. This allows for enhanced detection and localization of proud or buried targets in complex shallow water environments. A passive implementation of the iterative time-reversal processing is used to construct reflectivity maps, similar to a sonar map, but with an enhanced contrast for the strongest reflectors (or scatterers), at the water-bottom interface. Ultrasonic and at-sea experiments show that targets on the seafloor located up to 400 wavelengths from a time-reversal array are detectable in the presence of bottom reverberation. (c) 2006 Acoustical Society of America.

Song, HC, Hodgkiss WS, Kuperman WA, Higley WJ, Raghukumar K, Akal T, Stevenson M.  2006.  Spatial diversity in passive time reversal communications. Journal of the Acoustical Society of America. 120:2067-2076.   10.1121/1.2338286   AbstractWebsite

A time reversal mirror exploits spatial diversity to achieve spatial and temporal focusing, a useful property for communications in an environment with significant multipath. Taking advantage of spatial diversity involves using a number of receivers distributed in space. This paper presents the impact of spatial diversity in passive time reversal communications between a probe source (PS) and a vertical receive array using at-sea experimental data, while the PS is either fixed or moving at about 4 knots. The performance of two different approaches is compared in terms of output signal-to-noise ratio versus the number of receiver elements: (1) time reversal alone and (2) time reversal combined with adaptive channel equalization. The time-varying channel response due to source motion requires an adaptive channel equalizer such that approach (2) outperforms approach (1) by up to 13 dB as compared to 5 dB for a fixed source case. Experimental results around 3 kHz with a 1 kHz bandwidth illustrate that as few as two or three receivers (i.e., 2 or 4 in array aperture) can provide reasonable performance at ranges of 4.2 and 10 km in 118 in deep water. (c) 2006 Acoustical Society of America.

Huang, CF, Gerstoft P, Hodgkiss WS.  2006.  Validation of statistical estimation of transmission loss in the presence of geoacoustic inversion uncertainty. Journal of the Acoustical Society of America. 120:1932-1941.   10.1121/1.2261356   AbstractWebsite

Often the ocean acoustic environment is not well known and sonar performance prediction will be affected by this uncertainty. Here, a method for estimating transmission loss (TL) is proposed which incorporates these environmental uncertainties. Specifically, we derive an approach for the statistical estimation of TL based on the posterior probability density of environmental parameters obtained from the geoacoustic inversion process. First, a Markov chain Monte Carlo procedure is employed in the inversion process to sample the posterior probability density of the geoacoustic parameters. Then, these sampled parameters are mapped to the transmission loss domain where a full multidimensional probability distribution of TL as a function of range and depth is obtained. In addition, TL is also characterized by its summary statistics including the median, percentiles, and correlation coefficients. The approach is illustrated using a,data set obtained from the ASIAEX 2001 East China Sea experiment. Based on the geoacoustic inversion results, the predicted TL and its variability are estimated and then compared with the measured TL. In general, there is a good agreement with the percentage of observed number of data points inside the credibility interval. (c) 2006 Acoustical Society of America.

Song, HC, Roux P, Hodgkiss WS, Kuperman WA, Akal T, Stevenson M.  2006.  Multiple-input-multiple-output coherent time reversal communications in a shallow-water acoustic channel. IEEE Journal of Oceanic Engineering. 31:170-178.   10.1109/joe.2005.850911   AbstractWebsite

A recent time reversal (TR) experiment demonstrated that multiple foci can be projected from an array of sources to the same range but at different depths. This multiple input/multiple output process can potentially increase the information data rate. This paper presents experimental results of coherent TR communications (binary phase-shift keying, quaternary phase-shift keying, and 8-quadratic-amplitude modulation) at 3.5 kHz with a 1-kHz bandwidth where different messages were sent simultaneously to either two or three different depths at an 8.6-km range in a 105-m-deep water.

Huang, CF, Gerstoft P, Hodgkiss WS.  2006.  Uncertainty analysis in matched-field geoacoustic inversions. Journal of the Acoustical Society of America. 119:197-207.   10.1121/1.2139075   AbstractWebsite

Quantifying uncertainty for parameter estimates obtained from matched-field geoacoustic inversions using a Bayesian approach requires estimation of the uncertainties in the data due to ambient noise as well as modeling errors. In this study, the variance parameter of the Gaussian error model, hereafter called error variance, is assumed to describe the data uncertainty. In practice, this parameter is not known a priori, and choosing a particular value is often difficult. Hence, to account for the uncertainty in error variance, several methods are introduced for implementing both the full and empirical Bayesian approaches. A full Bayesian approach that permits uncertainty of the error variance to propagate through the parameter estimation processes is a natural way of incorporating the uncertainty of error variance. Due to the large number of unknown parameters in the full Bayesian uncertainty analysis, an alternative, the empirical Bayesian approach is developed, in which the posterior distributions of model parameters are conditioned on a point estimate of the error variance. Comparisons between the full and empirical Bayesian inferences of model parameters are presented using both synthetic and experimental data. (c) 2006 Acoustical Society of America.

Yardim, C, Gerstoft P, Hodgkiss WS.  2006.  Estimation of radio refractivity from radar clutter using Bayesian Monte Carlo analysis. IEEE Transactions on Antennas and Propagation. 54:1318-1327.   10.1109/tap.2006.872673   AbstractWebsite

This paper describes a Markov chain Monte Carlo (MCMC) sampling approach for the estimation of not only the radio refractivity profiles from radar clutter but also the uncertainties in these estimates. This is done by treating the refractivity from clutter (RFC) problem in a Bayesian framework. It uses unbiased MCMC sampling techniques, such as Metropolis and Gibbs sampling algorithms, to gather more accurate information about the uncertainties. Application of these sampling techniques using an electromagnetic split-step fast Fourier transform parabolic equation propagation model within a Bayesian inversion framework can provide accurate posterior probability distributions of the estimated refractivity parameters. Then these distributions can be used to estimate the uncertainties in the parameters of interest. Two different MCMC samplers (Metropolis and Gibbs) are analyzed and the results compared not only with the exhaustive search results but also with the genetic algorithm results and helicopter refractivity profile measurements. Although it is slower than global optimizers, the probability densities obtained by this method are closer to the true distributions.

Gerstoft, P, Huang CF, Hodgkiss WS.  2006.  Estimation of transmission loss in the presence of geoacoustic inversion uncertainty. IEEE Journal of Oceanic Engineering. 31:299-307.   10.1109/joe.2006.875104   AbstractWebsite

A common problem in sonar system prediction is that the ocean environment is not well known. Utilizing probabilistic based results from geoacoustic inversions we characterize parameters relevant to sonar performance. This paper describes the estimation of transmission loss and its statistical properties based on posterior parameter probabilities obtained from inversion of ocean acoustic array data. This problem is solved by first finding an ensemble of relevant environmental model parameters and the associated posterior probability using a likelihood based inversion of the acoustic array data. In a second step, each realization of these model parameters is weighted with their posterior probability to map into the transmission loss domain. This approach is illustrated using vertical-array data from a recent benchmark data set and from data acquired during the Asian Seas International Acoustics Experiment (ASIAEX) 2001 in the East China Sea. The environmental parameters are first estimated using a probabilistic-based geoacoustic inversion technique. Based on the posterior probability that each of these environmental models fits the ocean acoustic array data, each model is mapped into transmission loss. This enables us to compute a full probability distribution for the transmission loss at selected frequencies, ranges, and depths, which potentially could be used for sonar performance prediction.

Song, HC, Hodgkiss WS, Kuperman WA, Stevenson M, Akal T.  2006.  Improvement of time-reversal communications using adaptive channel equalizers. IEEE Journal of Oceanic Engineering. 31:487-496.   10.1109/joe.2006.876139   AbstractWebsite

The spatial and temporal focusing properties of time-reversal methods can be exploited for undersea acoustic communications. Spatial focusing mitigates channel fading and produces a high signal-to-noise ratio (SNR) at the intended receivers along with a low probability of interception elsewhere. While temporal focusing (compression) reduces significantly intersymbol interference (ISI), there always is some residual ISI depending upon the number of transmitters, their spatial distribution (spatial diversity), and the complexity of the channel. Moreover, a slight change in the environment over the two-way propagation interval introduces additional ISI. Using multilevel quadrature amplitude modulation (M-QAM) in shallow water, we demonstrate that the performance of time-reversal communications can be improved significantly by cascading the received time series with an adaptive channel equalizer to remove the residual ISI.

Song, HC, Hodgkiss WS, Kuperman WA, Roux P, Akal T, Stevenson M.  2005.  Experimental demonstration of adaptive reverberation nulling using time reversal. Journal of the Acoustical Society of America. 118:1381-1387.   10.1121/1.1984990   AbstractWebsite

The concept of environmentally adaptive reverberation nulling using a time reversal mirror (TRM) recently has been described [Song et al., J. Acoust. Soc. Am. 116, 762-768 (2004)]. In this paper, monostatic reverberation nulling is demonstrated experimentally at 850 and 3500 Hz using data from a shallow water experiment conducted off the west coast of Italy in April 2003. The active transmission of a seafloor spatial null from a vertical source array is shown to result in the attenuation by 3-5 dB of prominent reverberation features with their levels being reduced to that of the more diffuse reverberation background. 0 2005 Acoustical Society of America.

Higley, WJ, Roux P, Kuperman WA, Hodgkiss WS, Song HC, Akal T, Stevenson M.  2005.  Synthetic aperture time-reversal communications in shallow water: Experimental demonstration at sea. Journal of the Acoustical Society of America. 118:2365-2372.   10.1121/1.2011147   AbstractWebsite

Time reversal has been shown as an effective way to focus in both time and space. The temporal focusing propel-Lies have been used extensively in underwater acoustics communications. Typical time-reversal communication experiments use vertical transducer arrays both to increase the signal-to-noise ratio and decrease the temporal sidelobes created in the time reversal process. Comparable temporal focusing is achieved using a horizontal array. In this paper, synthetic aperture time-reversal communications are accomplished, requiring only two transducers (one transmitter and one receiver). Deriving results from all at-sea experiment, this work confirms the viability of synthetic aperture time-reversal communications. (c) 2005 Acoustical Society of America.

Edelmann, GF, Song HC, Kim S, Hodgkiss WS, Kuperman WA, Akal T.  2005.  Underwater acoustic communications using time reversal. IEEE Journal of Oceanic Engineering. 30:852-864.   10.1109/joe.2005.862137   AbstractWebsite

This paper contains theoretical and experimental results on the application of the time-reversal process to acoustic communications in order to improve data telemetry in the ocean. A coherent underwater acoustic communication system must deal with the inter-symbol interference caused by the time-varying, dispersive, shallow-water ocean environment. An approach is demonstrated that takes advantage of the focal properties of time reversal. The spatial and temporal compression available at the time-reversal focus mitigates channel fading, reduces the dispersion caused by the channel, and increases the signal strength. Thus, a time-reversal communication system does not require spatial diversity at the receiver, i.e., an array of receiving sensors, but takes advantage of spatial diversity at the transmitter. The time-reversal communications system concept is demonstrated using experimental data collected in shallow water. Data telemetry bit rates of 500 bps (BPSK) and 1000 bps (QPSK) with bit error rates of 0 out of 4976 bits and 254 out of 9953 bits, respectively, were obtained when transmitting to a receiver at a distance of 10 km, with a carrier frequency of 3500 Hz, and a 500 Hz; bandwidth. In a shallow-water upslope region, bit error rates of 15 out of 4976 bits and 14 out of 4976 bits were achieved over the same distance. In neither case was complex processing at the receiver used (i.e., channel equalization, error correction coding). Time-reversal transmissions are intercompared with single source and broadside transmissions and shown to have superior results in both range independent and dependent bathymetries. The time-reversal performance appears limited by self-generated inter-symbol interference. In addition, an initial look at the application of a single channel adaptive channel equalizer to received time-reversal communication sequences is presented. The same properties that are beneficial to a single channel receiver are also beneficial to adaptive channel equalization. A single channel RLS DFE equalizer is cascaded with the received time-reversal sequences and shown to further reduce scatter in the I/Q plane. The bit error rate decreased in all but one of the cases.

Sabra, KG, Roux P, Thode AM, D'Spain GL, Hodgkiss WS, Kuperman WA.  2005.  Using ocean ambient noise for array self-localization and self-synchronization. IEEE Journal of Oceanic Engineering. 30:338-347.   10.1109/joe.2005.850908   AbstractWebsite

Estimates of the travel times between the elements of a bottom hydrophone array can be extracted from the time-averaged ambient noise cross-correlation function (NCF). This is confirmed using 11-min-long data blocks of ambient noise recordings that were collected in May 1995 near the southern California coast at an average depth of 21 m in the 150-700 Hz frequency range. Coherent horizontal wavefronts emerging from the time derivative of the NCF are obtained across the array's aperture and are related to the direct arrival time of the time-domain Green's function (TDGF). These coherent wavefronts are used for array element self-localization (AESL) and array element self-synchronization (AESS). The estimated array element locations are used to beamform on a towed source.

Battle, DJ, Gerstoft P, Hodgkiss WS, Kuperman WA, Nielsen PL.  2004.  Bayesian model selection applied to self-noise geoacoustic inversion. Journal of the Acoustical Society of America. 116:2043-2056.   10.1121/1.17855671   AbstractWebsite

Self-noise geoacoustic inversion involves the estimation of bottom parameters such as sound speeds and densities by analyzing towed-array signals whose origin is the tow platform itself. As well as forming inputs to more detailed assessments of seabed geology, these parameters enable performance predictions for sonar systems operating in shallow-water environments. In this paper, Gibbs sampling is used to obtain joint and marginal posterior probability distributions for seabed parameters. The advantages of viewing parameter estimation problems from such a probabilistic perspective include better quantified uncertainties for inverted parameters as well as the ability to compute Bayesian evidence for a range of competing geoacoustic models in order to judge which model explains the data most efficiently. (C) 2004 Acoustical Society of America.

Huang, CF, Hodgkiss WS.  2004.  Matched-field geoacoustic inversion of low-frequency source tow data from the ASIAEX East China Sea experiment. IEEE Journal of Oceanic Engineering. 29:952-963.   10.1109/joe.2004.836989   AbstractWebsite

Geoacoustic inversion results based on data obtained during the Asian Seas International Acoustics Experiment (ASIAEX) 2001 East China Sea experiment are reported. The inversion process uses a genetic-algorithm-based matched-field-processing approach to optimize the search procedure for the unknown parameters. Inversion results include both geometric and geoacoustic variables. To gauge the quality of the inversion, two different analyses are employed. First, the inversion results based upon discrete source-receiver ranges are confirmed by continuous source localization over an interval of time. Second, separate inversions at many different ranges are carried out and the uncertainties of the parameter estimation are analyzed. The analysis shows that both methods yield consistent results, ensuring the reliability of inversion in this study.

Gerstoft, P, Hodgkiss WS, Rogers LT, Jablecki M.  2004.  Probability distribution of low-altitude propagation loss from radar sea clutter data. Radio Science. 39   10.1029/2004rs003077   AbstractWebsite

[1] This paper describes the estimation of propagation loss and its statistical properties on the basis of observations of radar sea clutter data. This problem is solved by first finding an ensemble of relevant refractivity model parameters, and each refractivity model is weighted according to its data likelihood function. A parabolic equation propagation model is used both in mapping from environmental model to radar clutter data and also when mapping to propagation loss. Two different methods are then used for mapping from a statistical description of refractivity parameters to a statistical description of propagation loss. In the first approach, all of the sampled models explored in the inversion are used to give a statistical description of propagation loss. Alternatively, the environmental model is sampled from the probability for the refractivity model parameters and then mapped into propagation loss. This can be done efficiently if we are using the one-dimensional marginal distributions instead of the full distribution for the environmental parameters.

Hursky, P, Porter MB, Cornuelle BD, Hodgkiss WS, Kuperman WA.  2004.  Adjoint modeling for acoustic inversion. Journal of the Acoustical Society of America. 115:607-619.   10.1121/1.1636760   AbstractWebsite

The use of adjoint modeling for acoustic inversion is investigated. An adjoint model is derived from a linearized forward propagation model to propagate data-model misfit at the observation points back through the medium to the medium perturbations not being accounted for in the model. This adjoint model can be used to aid in inverting for these unaccounted medium perturbations. Adjoint methods are being applied to a variety of inversion problems, but have not drawn much attention from the underwater acoustic community. This paper presents an application of adjoint methods to acoustic inversion. Inversions are demonstrated in simulation for both range-independent and range-dependent sound speed profiles using the adjoint of a parabolic equation model. Sensitivity and error analyses are discussed showing how the adjoint model enables calculations to be performed in the space of observations, rather than the often much larger space of model parameters. Using an adjoint model enables directions of steepest descent in the model parameters (what we invert for) to be calculated using far fewer modeling runs than if a forward model only were used. (C) 2004 Acoustical Society of America.

Song, HC, Kim S, Hodgkiss WS, Kuperman WA.  2004.  Environmentally adaptive reverberation nulling using a time reversal mirror. Journal of the Acoustical Society of America. 116:762-768.   10.1121/1.1765194   AbstractWebsite

Backscattering from the rough water-bottom interface can serve as a surrogate probe source in time reversal. A time-gated portion of the reverberation then is refocused to the bottom interface at the corresponding range [Lingevitch et al., J. Acoust. Soc. Am. 111, 2609-2614 (2002)]. In this paper, reverberation nulling is investigated to enhance active target detection without a priori knowledge of the environment. The basic idea is to minimize the acoustic energy incident on the corresponding scattering interface by applying an excitation weight vector on the time reversal mirror which is in the complementary subspace orthogonal to the focusing vector. Numerical simulations illustrate the potential of reverberation nulling using a time reversal mirror. (C) 2004 Acoustical Society of America.

Roux, P, Kuperman WA, Hodgkiss WS, Song HC, Akal T, Stevenson M.  2004.  A nonreciprocal implementation of time reversal in the ocean. Journal of the Acoustical Society of America. 116:1009-1015.   10.1121/01.1707089   AbstractWebsite

The performance of a time reversal mirror (TRM) in complex ocean scenarios can be evaluated without invoking spatial reciprocity in the experimental procedure. The experimental implementation requires connectivity between a source array and a receiver array but eliminates the requirement of actually having a probe source collocated with the receiver array. It is shown with data taken in a recent experiment that this streamlined, nonreciprocity-based time reversal procedure yields results potentially better than the classical time reversal method. Further, it provides a more versatile method to study a TRM in a fluctuating medium. (C) 2004 Acoustical Society of America.

Kim, S, Kuperman WA, Hodgkiss WS, Song HC, Edelmann G, Akal T.  2004.  Echo-to-reverberation enhancement using a time reversal mirror. Journal of the Acoustical Society of America. 115:1525-1531.   10.1121/1.1649737   AbstractWebsite

Reverberation from rough ocean boundaries often degrades the performance of active sonar systems in the ocean. The focusing capability of the time-reversal method provides a new approach to this problem. A time-reversal mirror (TRM) focuses acoustic energy on a target enhancing the target echo while shadowing the boundaries below and above the focus in a waveguide, thereby reducing reverberation. The resulting echo-to-reverberation enhancement has been demonstrated experimentally using a time-reversal mirror in the 3-4 kHz band in shallow water. (C) 2004 Acoustical Society of America.

Akal, M, Kuperman WA, Hodgkiss WS, Song HC, Edelmann GF, Kim S, Roux P, Stevenson M, Guerrini P, Boni PA.  2003.  Potential applications of ocean acoustic time-reversal mirrors - The focusing of acoustic energy in shallow water for active sonar and underwater communications. Sea Technology. 44:25-+. AbstractWebsite
Battle, DJ, Gerstoft P, Kuperman WA, Hodgkiss WS, Siderius M.  2003.  Geoacoustic inversion of tow-ship noise via near-field-matched-field processing. IEEE Journal of Oceanic Engineering. 28:454-467.   10.1109/joe.2003.816679   AbstractWebsite

This paper discusses geoacoustic inversion from tow-ship noise data acquired via a horizontal towed array. Through simulations and experimental results, it is shown that even very quiet ships radiate sufficient noise power to enable self-noise inversion of basic geoacoustic parameters such as effective bottom velocity. The experimental results presented are particularly encouraging in view of the high level of interference shown to be tolerated from nearby shipping.

Gerstoft, P, Hodgkiss WS, Kuperman WA, Song HC.  2003.  Phenomenological and global optimization inversion. IEEE Journal of Oceanic Engineering. 28:342-354.   10.1109/joe.2003.816681   AbstractWebsite

This paper discusses geoacoustic inversion results based on benchmark range-dependent data using SAGA, a global inversion package, and using phenomenological inversions. In phenomenological inversions, physical and signal-processing approaches are used to enhance the data to extract specific features. The global optimization approach is carried out on complex-valued vertical array data, transmission loss data, and reverberation data. The importance of checking the solution is emphasized by inspecting the match with the data and the error estimates and by checking the solution using data that has not been used in constructing the solution. The results show that we are able to estimate the geoacoustic parameters and that these parameters could be used to predict the field for different frequencies and/or source-receiver geometry than used in the inversion.

Gerstoft, P, Rogers LT, Hodgkiss WS, Wagner LJ.  2003.  Refractivity estimation using multiple elevation angles. IEEE Journal of Oceanic Engineering. 28:513-525.   10.1109/joe.2003.816680   AbstractWebsite

Estimation of the atmospheric refractivity is important for the prediction of radar performance. Surface or elevated trapping layers formed by the outflow of relatively dry and warm air over a cooler body of water often result in the refractive structure-supporting-convergence-zone-like behavior and multimodal effects. The propagation under such conditions can be very sensitive to even small changes in the vertical and horizontal structure of refractivity. Obtaining in situ measurements of sufficient fidelity to estimate where intensifications in the electromagnetic field will occur is difficult. The authors previously have demonstrated the ability to infer refractivity parameters from grazing-incidence radar sea-clutter data. The radar system was the 2.8-GHz space range radar that overlooks the Atlantic Ocean in the vicinity of Wallops Island, VA. The forward modeling consisted of the mapping of an 11-parameter environmental model via an electromagnetic propagation model into the space of the radar clutter observations. A genetic algorithm was. employed to optimize the objective function. Ground truth data were atmospheric soundings obtained by a helicopter flying a saw-tooth pattern. The overall result was that the ability to estimate the propagation within the duct itself was comparable to that of in situ measurements. However, the ability to characterize the region above the duct was quite poor. Modern three-dimensional radars, however, have relatively narrow beams. Using these narrow beams at multiple elevations might resolve the ambiguity leading to the poor characterization in the region above the duct. Using radar data from the SPANDAR radar, it is demonstrated that such an approach is feasible and that more-robust estimates can be obtained by using two elevation angles and/or by constraining the solution to contain realistic refractivity profiles.

Kim, S, Kuperman WA, Hodgkiss WS, Song HC, Edelmann GF, Akal T.  2003.  Robust time reversal focusing in the ocean. Journal of the Acoustical Society of America. 114:145-157.   10.1121/1.1582450   AbstractWebsite

Recent time-reversal experiments with high-frequency transmissions (3.5 kHz) show that stable focusing is severely limited by the time-dependent ocean environments. The vertical focal structure displays dynamic variations associated with focal splitting and remerging resulting in large changes in focal intensity. Numerical simulations verify that the intensity variation is linked to the focal shift induced by phase changes in acoustic Waves resulting from sound speed fluctuations due to internal waves. A relationship between focal range shift, frequency shift, or channel depth changes is illustrated using waveguide-invariant theory. Based on the analysis of experimental data and numerical simulations, methods for robust time-reversal focusing are developed to extend the period of stable focusing. (C) 2003 Acoustical Society of America.