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When using underwater optical wireless communication in areas close to human habitats— such as shallow sea areas—specifications for highly-secure, large-capacity optical transceivers are required. Real-time transmission of 850 nm, direct-current optical orthogonal frequency division multiplexing signals for full-duplex underwater invisible light communication has been achieved. We experimentally confirmed that subcarrier adaptive modulation could transmit at maximum capacity depending on the transmission distance, while changing the transmission distance in shallow seawater channels. We confirmed that there was no disturbing influence due to sunlight by using a honeycomb structure for sunlight shielding. Moreover, we found that the effect of disruption caused by the sea surface vibrating due to the 3 m/s wind speed did not affect the signal quality. 4K video streaming is also done on a 1.2 m underwater channel transmission. To the best of our knowledge, this is the first report of full-duplex transmission of invisible-band underwater optical wireless communication for shallow waters.

Satellite technology is an efficient tool, which can provide valuable observations for coastal areas from space. Compared with conventional bathymetric surveying approaches, remote sensing-based shallow water bathymetry retrieval methods have been widely used in recent years. Various empirical models have been proposed for deriving bathymetry of coastal shallow water, and prior topographic information is required to construct models. Traditional studies tend to select a cloud-free remote sensing image to map the coastal shallow water topography. As a result, in addition to the selection of empirical models, the high-quality remote sensing image and accurate prior topographic data are also of importance. This study aims to propose a method for mapping coastal shallow water bathymetry from multitemporal remote sensing imagery. Here, Sentinel-2 imagery time series are composited to produce a clear image, which can effectively avoid the contamination of clouds, water turbidity and other noises. ICESat-2 lidar altimeter data that contain accurate underwater elevations are used to provide topographic information. Moreover, Sentinel-2-based multispectral information and ICESat-2-based topographic information are combined for the coastal bathymetry retrieval by five empirical models (i.e., linear band model, ratio band model, support vector machine, neural network, and random forest). This proposed method is tested in Dongsha Atoll in South China Sea, and achieve a good performance [training: root mean square error (RMSE): 0.97 m ± 0.76 m, mean absolute percentage error (MAPE): 4.07% ± 0.046%, R-square (R2): 0.90 ± 0.14; validation: RMSE: 1.22 m ± 0.43 m, MAPE: 5.43% ± 0.035%, R2: 0.86 ± 0.089]. The comparison confirms that machine learning methods perform better than traditional methods, and the deep learning techniques can be further introduced in estimating shallow water bathymetry in the future, which is expected to achieve an excellent accuracy in bathymetry inversion.

A few scholars have developed the models for retrieval of water depth from subsurface reflectance of multispectral images to avoid the influences of sun glitter. However, the models are only suitable for case I water. For this reason, this study proposes a bathymetry retrieval model using subsurface reflectance for both case I and case II water. The model first corrects the water surface reflectance image and then converts it into a subsurface reflectance image, and the subsurface reflectance image is used as the water depth retrieval image. Landsat 8 images were taken for experiments in case 1 water and case 2 water, and two water areas, Weizhou Island, Guangxi, China, and Molokai Island, Hawaii, USA, were used to verify the proposed model. The experimental results showed that the proposed model reduced the root-mean-squared error of the retrieved water depth in the Weizhou and Molokai areas from 3.113 to 2.903 m and 4.239 to 3.653 m, respectively, i.e., improve accuracy of water depth at 6.75% and 13.82% for Weizhou and Molokai areas, respectively. Therefore, the results demonstrate that the proposed model using subsurface reflectance can significantly improve the accuracy of bathymetry retrieval via spaceborne multispectral images.

The log-ratio method (LRM) proposed by Stumpf et al. has been widely used to map bathymetry from multispectral imagery for oligotrophic waters, while the selection criteria of bands for the LRM have been subject to tradeoffs between maximum detectable depth and sensitivity. In this article, we first applied a method for global sensitivity analysis to a semianalytical forward model of optically shallow waters with the WorldView-2 band-set. The results show that the sensitive wavelength band in water-leaving reflectance for water depth varies from the longer wavelength band to the shorter wavelength band with increasing water depth. Then, we developed an adaptive blended algorithm approach (ABAA) to seamlessly map bathymetry from the shallower region to the deeper region. The LRM with different band combinations was selected for the sub-algorithms of the ABAA. The subalgorithms and depth range used for each subalgorithm of the ABAA were automatically determined by the proposed applicable depth range analysis that considers logarithmic regression for the LRM. The ABAA was applied to WorldView-2 and Landsat-8 imagery of the Xisha Qundao. When the in situ bathymetry data are available, compared with the LRM with the blue and green bands, the ABAA significantly improves the accuracy of the estimated depth, especially for waters shallower than 6 m (root-mean- square error (RMSE) = 0.31 to 0.94 m for WorldView-2 data, RMSE= 0.25 to 1.42 m for Landsat-8 data). When the in situ bathymetry data are absent, the ABAA performs better than the LRM with a single band ratio and an optimization-based method overall.

The local connection characteristics of convolutional neural network (CNN) are linked with the local spatial correlation of image pixels for water depth retrieval in this article. The method has greater advantages and higher precision than traditional retrieval methods. Traditional remote sensing empirical models require manual extraction of retrieval factors and the process is complex. This article proposes a model based on CNN, which uses different remote sensing images in four spectral bands, red, green, blue, and near-infrared, to retrieve the water depth. In general, CNN is mostly used for image recognition and classification tasks, which can make full use of the local spatial correlation between pixels. The method in this article exploits this feature of CNN for water depth retrieval, taking into consideration the nonlinear relationship between the radiance value and water depth value from adjacent and central pixels. In this article, remote sensing image data, measured water depth data, and lidar sounding data are used as input data to build the model. Then, the retrieval error is analyzed and the parameters are adjusted to further optimize the model. Quantitative analysis and experimental results show that the accuracy of the CNN model in shallow sea areas retrieval is improved by more than 50%. The mean absolute error can reach within 0.8 m. Finally, the model is shown to be highly portable and capable of retrieving water depth data with resolution equal to the spatial resolution of the remote sensing image using only a small amount of input water depth data.

The log-ratio method (LRM) proposed by Stumpf et al. has been widely used to map bathymetry from multispectral imagery for oligotrophic waters, while the selection criteria of bands for the LRM have been subject to tradeoffs between maximum detectable depth and sensitivity. In this article, we first applied a method for global sensitivity analysis to a semianalytical forward model of optically shallow waters with the WorldView-2 band-set. The results show that the sensitive wavelength band in water-leaving reflectance for water depth varies from the longer wavelength band to the shorter wavelength band with increasing water depth. Then, we developed an adaptive blended algorithm approach (ABAA) to seamlessly map bathymetry from the shallower region to the deeper region. The LRM with different band combinations was selected for the sub-algorithms of the ABAA. The subalgorithms and depth range used for each subalgorithm of the ABAA were automatically determined by the proposed applicable depth range analysis that considers logarithmic regression for the LRM. The ABAA was applied to WorldView-2 and Landsat-8 imagery of the Xisha Qundao. When the in situ bathymetry data are available, compared with the LRM with the blue and green bands, the ABAA significantly improves the accuracy of the estimated depth, especially for waters shallower than 6 m (root–mean- square error (RMSE) = 0.31 to 0.94 m for WorldView-2 data, RMSE= 0.25 to 1.42 m for Landsat-8 data). When the in situ bathymetry data are absent, the ABAA performs better than the LRM with a single band ratio and an optimization-based method overall. [ABSTRACT FROM AUTHOR]

The output of a vertical linear array is used to infer about the parameters of the normal mode model that describes acoustic propagation in a shallow water. Existing subspace algorithms perform singular vector decomposition of the array data matrix to estimate the sampled model functions. Estimates are exact only if the sensing array is totally covering the water column. We design a new subspace algorithm free from this very restrictive requirement. We use two short hydrophone arrays and activate a monochromatic source at different depths. Estimates of both the modal functions and the wave numbers are obtained in a fully automatic and search-free manner. The algorithm can be qualified as truly high resolution in the sense that, while using short sensing arrays, estimation error becomes arbitrarily low if observation noise is arbitrarily low. This method compares advantageously to existing subspace techniques, as well as transform-domain techniques that require impulsive sources, among other constraints. With two (eigen and singular) vector decompositions, the proposed technique has the complexity of a regular subspace algorithm. [ABSTRACT FROM AUTHOR]

Acoustic emissions from cargo ships transiting coastal waterways, measured by a single hydrophone, can be exploited to estimate both the time-varying source–receiver range and receiver location. In this paper, two parameter-search-based maximum-likelihood estimators are presented: one for $\beta$ , the waveguide invariant parameter, and one for source range. $\beta$ characterizes the interference structure inherent to ducted acoustic propagation and is central to the spectral analysis involved in the range estimation. The source ranging method extends prior work by the authors and focuses on the strong narrowband tonals that typically dominate the acoustic spectra of cargo ships. Source ranging results are presented using real data from the SWellEx-96 experiment and are shown to be in close agreement with simulation results obtained using the KRAKEN normal mode program. A technique for estimating the position of a receiver, which can be stationary or moving, is also proposed. The localization technique requires knowledge of the source track, hydrophone data, and an initial estimate of the receiver's position. An application to autonomous underwater vehicle (AUV) navigation is included in which the waveguide-invariant-based range estimates are used to mitigate the position estimation error due to drift in inertial measurement units on submerged AUVs. A specific scenario is examined, using SWellEx-96 data, in which an initial position error of 3 km is reduced to under 1 km using the proposed technique. [ABSTRACT FROM AUTHOR]

This study investigates the information content of ship noise received on a drifting volumetric array of hydrophones in shallow water marine environments for the purposes of conducting acoustic thermometry or other environmental inversions. Passive inversions for physical oceanographic parameters are conducted using travel time differences, determined by cross-correlating ship noise received on hydrophones suspended beneath drifting buoys. Ships are tracked using the automatic identification system. Information content gained from the inversion is assessed using traditional a posteriori error analysis. Numerical simulations using a standard normal mode propagation model are used to test limitations of the proposed approach with respect to frequency band, drifting receiver configuration, precision, and accuracy of the inversion results, along with sensitivity to environmental and position mismatch. Performance predictions using this model are compared with results from a field experiment using at-sea data collected off the coast of New London, CT, USA, in Long Island Sound. Information gathered using passive acoustic inversion methods on drifting arrays can be used to constrain general circulation models, in coastal environments, where ship noise is ubiquitous, environmental data is sparse, and the oceanography is dynamic and important for understanding large-scale ocean processes. [ABSTRACT FROM AUTHOR]

In this work, we use the acoustic ray model to study passive source localization problems using a single hydrophone for shallow water waveguides. It has been found that conventional matched field processing, which requires a multi-hydrophone array, is no longer effective. In this paper, we introduce matched feature localization to achieve passive source localization using a single hydrophone. The eigenray that contains the arrival structure is the core of the acoustic ray model; however, it is difficult to estimate the relative arrival delay of the eigenray for passive localization, especially in shallow water. Therefore, we propose a novel peak extraction passive source localization method based on the autocorrelation function, which does not require estimation of the relative arrival delay of the eigenray. According to a computer simulation, the proposed method can achieve good performance using only one hydrophone. Further, its performance is validated using wideband data collected on a vertical line array during the Shallow Water 2006 experiment. [ABSTRACT FROM AUTHOR]

Lagrangian field data in tidal shallow waters are rare, but valuable for the understanding of the spatiotemporal structure of flow and particle transport. The response of drifters to the wind and water flow in tidal shallow water was examined using correlation, spectral, and coherence analyses. Under moderate wind conditions (0–4 m/s), floating drifter motions in bounded sheltered water are affected by wind through low-frequency induced wind current rather than direct wind drag, when only a small portion of the drifter is unsubmerged. The field validation of both high- and low-resolution drifters with surface measured velocity from a fixed acoustic Doppler current profiler is good in the streamwise direction. The correlation between the drifter and fixed instrument velocities is low in the cross-stream direction due to strong spatial variability of the flow field. The evaluation shows that drifters are applicable to studying the flow dynamics of tidal water bodies in relation to small-scale processes. [ABSTRACT FROM AUTHOR]

Sound-source localization in shallow water is a difficult task due to the complicated environment, e.g., complex sound-speed profile and irregular water bottom reflections. Full-wave numerical techniques are currently able to accurately simulate the propagation of sound waves in such complex environments. However, the source localization problem, which generally involves a large number of sound propagation calculations, still requires a fast computation of the wave equation, and thus a simplified model is well advised. In this paper, a four-layer model is considered, which is able to approximate a wide range of shallow-water environments, particularly those in summer conditions. More specifically, the medium is assumed to be horizontally stratified and vertically divided into four layers, and the sound speed in each layer is assumed to be constant or varying linearly. Under this assumption, the wave propagation can be rapidly computed via a classical wave number integration method. The main contribution of this paper is to show the suitability of the four-layer model in terms of source localization in a complex (range-dependent) environment. The sound-speed profile is assumed to be vertically irregular and horizontally slowly varying and the bottom is nonflat. In the forward problem, sound propagation in complex underwater environments is simulated via a time-domain full-wave simulation approach called the spectral-element method. The source localization error due to model imprecision is analyzed. [ABSTRACT FROM AUTHOR]

This paper investigates the capability of X-band radar systems to estimate nearshore bathymetry fields by considering both simulated and measured radar data. For the first time, a sensitivity analysis is performed to evaluate how sea-state conditions affect bathymetric estimates. For this purpose, sea wave fields generated by means of a numerical model, based on a nonlinear shallow-water equation solver, are used. Starting from the synthetic radar data, which represent the input of the bathymetric estimation algorithm, the bathymetric reconstruction is performed through the normalized scalar product (NSP) estimation strategy, exploiting a spatial partitioning of the radar data. In this way, it is possible to improve the accuracy of the estimates in nearshore areas, where the space-varying behavior of the sea depth and the presence of coastlines or coastal structures typically leads to a spatial inhomogeneity of the wave motion. In this regard, it is shown how the choice of the partitioning settings affects the bathymetric estimates obtained from high-resolution X-band radar images by using the NSP strategy. In addition, an adaptive partitioning strategy that takes into account the wave evolution in nearshore shallow waters is devised. Based on both simulated and measured radar data, the accuracy of the bathymetric estimates achievable through the proposed adaptive partitioning process and that obtained by exploiting the approach using uniform spatial partitioning are compared. The results obtained confirm the robustness of the NSP technique with respect to sea conditions and, moreover, demonstrate that the proposed adaptive partitioning strategy provides more accurate bathymetric estimates than those obtained with the space-invariant partitioning procedure. [ABSTRACT FROM PUBLISHER]

The 2013 Target and Reverberation Experiment (TREX13) sea trial was held just off the coast of Panama City, FL, USA, in May 2013. One of Defence Research and Development Canada's primary objectives was comparing the performance of continuous active sonar (CAS) and pulsed active sonar (PAS) in shallow water, where reverberation and clutter challenge sonar systems. The only option for a moving target during the trial was an echo repeater towed from a ship; however, this posed a serious challenge because before the trial there were no known echo repeater solutions that were compatible with CAS. Existing echo repeaters required short-duration waveforms as used in PAS, and even then they introduced range errors that caused a mismatch in reverberation background and made localization problematic. This paper presents three echo repeater techniques developed for TREX13 and tested on Canadian Forces Auxiliary research vessel CFAV Quest during the trial. Experimental results presented in this paper demonstrate detection of the echo repeater by a monostatic CAS system on R/V Hugh R Sharp. [ABSTRACT FROM AUTHOR]

A new real-time underwater object detection method adopting geophysical direct-current resistivity techniques is proposed. Our final goals are real-time detection and tracking of small submarines in water depths less than 100 m, under acoustically noisy conditions. The main features of our method are as follows: 1) a detection line, several kilometers long and semipermanently buried in the seabed, consisting of two current electrodes and multiple potential electrodes; 2) fixed current electrode configurations for high-speed real-time detection of the target object; 3) measurement of extremely low-level electric field signals, made possible by the shielding effect of the conductive seawater overburden; and 4) signals from a moving object such as a submarine are extracted by differential data analysis because the target object passes through the detection area for only a short time compared with the long-term drift change of the electrical properties of seawater. We verify our method and confirm real-time detection feasibility through numerical and physical scale experiments. For this purpose, a dedicated instrument and a background data update algorithm are developed, and a differential data analysis method is used. The background data update algorithm is used for counteracting the time-varying electrical characteristics of seawater. The differential data analysis method is exploited to extract disturbed signals by objects from complex background data. Through numerical experiments, we find that the $x$-coordinate value of a target object corresponds to the position of a peak data point, and this result is consistent with 1:200 downscaled water tank experiments. We also confirm real-time detection feasibility using our instrument through an offshore experiment. We expect that our proposed method will complement conventional detection methods for harbor defense and surveillance systems in acoustically noisy environments. [ABSTRACT FROM PUBLISHER]

Understanding the characteristics of channel propagation plays an important role in designing communication systems. Underwater acoustic channel modeling is commonly done by the direct measurement result from a specific location and time. This paper presents an evaluation of channel impulse response (CIR) and power delay profile (PDP) measurements in shallow water environment using three types of probe signals, i.e., pseudonoise (PN)-sequence, exponentially sine swept (ESS), and amplitude-modulated ESS (AM-ESS). The measurements have been carried out in a laboratory-scale towing tank. The ESS signal has been found to produce CIR and PDP that are close to those of the ray tracing model. The multipath pattern of CIR obtained from ESS signal shows an 80% similarity; it was closer to the ray tracing model compared with other results, made using PN-sequence and AM-ESS signals. The estimated value of rms delay spread obtained by the ray tracing model is 1.023 ms and that obtained by the ESS signal is 1.502 ms. Meanwhile, the values obtained by the PN-sequence and AM-ESS signals are 0.769 and 1.410 ms, respectively. In the frequency domain, the spaced-frequency correlation function, obtained using the PN-sequence and ESS signals, is more similar to the ray tracing model compared with the AM-ESS signal. [ABSTRACT FROM PUBLISHER]

A realistic 2-D time-evolving ocean surface model has been combined with an existing acoustic ray-based model to simulate the effects of sea surface roughness on acoustic wave propagation in coastal regions. Rough sea surface realizations are generated and used as sea surface boundaries in the acoustic model. An approach to achieve high resolution and accurate results while maintaining computational efficiency of a ray-based model is applied. The results are then compared against a unique set of experimental data collected in 15-m water depth in Delaware Bay. These data include simultaneous environment and acoustic propagation (1–18 kHz) measurements. Modeled arrival time-angle fluctuations compare well with data and suggest that there are physical processes which need to be included to improve the model, such as bubbles and turbulence as well as 3-D scattering effects. [ABSTRACT FROM PUBLISHER]

Inverse problems in ocean acoustics are based on 2-D modeling of sound propagation, hence ignoring the effects of horizontal refraction, referred to as 3-D propagation effects. However, the acoustic propagation in shallow-water environments, such as the continental shelf, may be affected by 3-D effects requiring 3-D modeling to be accounted for. The aim of this work is to investigate the importance of the 3-D effects with respect to the performance and reliability of typical 2-D-model-based inversion procedures of ocean acoustics. The study is carried out on a well-established synthetic test case which exhibits well-known 3-D effects. A matched-field inversion procedure is implemented based on the exhaustive search over the parameter space. The feasibility and the limits of inverting low-frequency noisy 3-D synthetic data for some parameters describing the test case by matching replica from 2-D computations are explored. Both synthetic data and replica are generated using a parabolic-equation-based code. This approach highlights the relevance of using 2-D propagation models when inversions are performed at relatively short ranges from the source. On the other hand, important mismatch occurs when inverting at farther ranges, demonstrating that the use of fully 3-D forward models is required. [ABSTRACT FROM PUBLISHER]

There is often a need for making a high-resolution or a complete bathymetric map based on sparse point measurements of water depth. Well-known feasible methods for this problem include spatial interpolation and passive remote sensing using readily available multispectral imagery, whose accuracies depend strongly on geometric and optical conditions, respectively. For a more accurate and robust water-depth mapping, in this letter, the two methods are combined into a new method in a statistically reasonable and beneficial manner. The new method is based on a semiparametric regression model that consists of a parametric imagery-based term and a nonparametric spatial interpolation term that complement one another. An accuracy comparison in a test site shows that the new method is more accurate than either of the existing methods when sufficient training data are available and far more accurate than the spatial interpolation method when the training data are scarce. [ABSTRACT FROM PUBLISHER]

Target tracking in multistatic active sonar systems is often limited in shallow-water environments due to the high level of bottom reverberation that produces false detections. Past research has shown that these false alarms may be mitigated when complete knowledge of the environment is available for discrimination, but these methods are not robust to environmental uncertainty. Recent work has demonstrated the existence of a waveguide invariant for active sonar geometries. Since this parameter is independent of specifics of the environment, it may be used when the environment is poorly known. In this paper, the invariance extended Kalman filter (IEKF) is proposed as a new tracking algorithm that incorporates dynamic frequency information in the state vector and uses the invariance relation to improve tracker discrimination. IEKF performance is quantified with both simulated and experimental sonar data and results show that the IEKF tracks the target better than the conventional extended Kalman filter (CEKF) in the presence of false detections. [ABSTRACT FROM AUTHOR]

As with traditional sonar, synthetic aperture sonar (SAS) is susceptible to multipath contamination, reducing the quality and also modifying the statistics of the image. Such multipath contaminants may either be environmentally induced, as is often the case when attempting to image ranges greater than the water depth resulting in returns from the boundaries, or may be induced by the system's supporting structure itself. A clear understanding of such statistical impact is necessary to advance synthetic aperture formation algorithms and for predicting system performance. Broadband acoustic data suitable for SAS processing collected with a rail-mounted mobile-tower as part of the U.S. Office of Naval Research (ONR)-funded Sediment Acoustics eXperiment 2004 (SAX04) are analyzed in this paper. Analysis focused on both system structure and environmentally induced multipath using the K-distribution shape parameter as a metric. High-resolution sonar imagery often exhibited significantly non-Rayleigh, heavy-tailed envelope statistics, characterized by a low equivalent K-distribution shape parameter. Analysis showed a clear and significant increase in the estimated shape parameter in the presence of multipath, representing a trend toward a Rayleigh-distributed envelope. A model for reverberation is presented to provide bounds of the statistical impact using observable image intensity level increases in synthetic-aperture-formed images caused by multipath contamination. This model further shows potential for statistical impact when multipath arrivals are of similar level as the direct path even when not observable in the image (e.g., within 10 dB). [ABSTRACT FROM AUTHOR]

Sonar performance predictions in shallow water are strongly dependent on good knowledge of the geoacoustic and scattering properties of the seabed. One technique to extract information about the bottom is to use a towed source and a towed horizontal array. This towed system has been shown to be applicable for characterizing the bottom properties locally by inversion of the acoustic signals received directly on the towed array at short ranges. The same towed system has also been applied extract bottom properties from long-range reverberation data providing effective bottom properties over a large area. However, independent geoacoustic inversion of the short-range propagation and long-range reverberation data can introduce low sensitivity and uncertainty in the extracted bottom properties. An attempt to resolve this low sensitivity and ambiguity is made by a simultaneous geoacoustic inversion of short-range propagation and long-range reverberation data with the intention of constraining the possible solutions of the bottom properties. [ABSTRACT FROM AUTHOR]

A multistatic active sonar system with several widely distributed sensors must share information between the sensors. If acoustic communication is the means used to share information, it can consume a significant fraction of the system's total energy budget. In this communication, an energy consumption model is developed for a shallow-water acoustic communications network. The model includes the environmental factors like the sound-speed profile in the water column and the composition of the seabed. The model uses the waveguide invariant concept to incorporate efficiently the broadband nature of the communications signals. Numerical results demonstrate how relaying messages between intermediate sensors can save substantial energy compared to direct communications. The calculations also show that energy consumption can vary by more than an order of magnitude depending on the seabed composition. [ABSTRACT FROM AUTHOR]

In this paper, we present results from a joint oceanographic-acoustic study of solitary, waves and their effects during the 1997 winter PRIMER4 experiment on the shelfbreak south of Cape Cod, MA. The study addresses the acoustic effects induced by solitary waves and associated oceanographic phenomena. Solitary wave generation and propagation simulations are produced by the Lamb model [J. Geophys. Res., vat. 99, pp. 848-864, 1994]. The model is nonhydrostatic and is formulated in 2.5 dimensions using terrain following coordinates. Acoustic field calculations are performed with a parabolic equation acoustic model along the path of solitary wave train propagation. The oceanographic model is initialized from density profiles derived from conductivity-temperature-depth (CTD) casts using analytical functions. The model is forced with a prescribed semidiurnal tidal velocity. An ocean background current is introduced. Simulations based on parameters derived from measurements show the following: 1) internal solitary waves of elevation propagate onto the shelfbreak region; 2) opposing ocean currents enhance the formation of solitary waves at the shelf break; 3) deepening of the winter mixed layer results in less penetration of the solitary waves on to the shelf; 4) density structure, mixed-layer depth, tidal forcing, and ocean currents control the formation of solitary waves of elevation at the shelfbreak: 5) energy conversion, from semidiurnal barotropic to semidiurnal barcoclinic tides and to internal solitary waves, occurs: 6) amplitudes and periods of modeled solitary waves are in the range of thermistor chain measurements; and 7) lower mixed-layer densities increase the phase speed of simulated solitary waves. Acoustic field calculations are coupled to the propagation of the solitary wave packets through the sound-speed changes that are derived from the oceanographic simulations. Acoustic model predictions show signal intensity fluctuations similar to the anomalous loses in acoustic energy observed in the Yellow Sea data taken by Zhou et aL [J. Acoust. Soc. Amer., vol. 90, pp. 2042-2054, 1991]. In some cases, the presence of solitary waves on the shelf enhances the propagation of acoustic energy onto the shelf. This was observed for acoustic simulations where the acoustic source was located beyond the shelfbreak and at a depth greater than the shelf depth. [ABSTRACT FROM AUTHOR]

Low-frequency acoustic data acquired in the central East China Sea basin at two locations are analyzed for the purpose of making inferences on seabed acoustics. Previous geophysical studies indicate that the first sediment layer is composed of a fine to medium sand. The current analysis employs octave-averaged transmission loss (TL) versus range data and pressure time series generated from explosive sources. The TL and time series data were collected in locations separated by about 65 km during the same month of the year. Both locations are near the same longitude, with water depths of 100-120 m. A linear frequency dependence of the attenuation in the 25-800 Hz band, with or without sound speed dispersion, leads to a geoacoustic solution using the TL data consistent with a soft clay, and thus inconsistent with the existing geophysical data. However, seabed representations that allow for a nonlinear frequency dependence of the attenuation, such as a Kramers-Kronig dispersion relationship, a simplified six-parameter Biot description, and an empirical frequency power law of the attenuation, all give similar values of the attenuation as a function of frequency and sediment sound speeds that are consistent with the previous geophysical studies in the area. Geoacoustic solutions obtained with the TL inversions produce reasonably good fits to the measured time series data. Inversions of the time series indicate that the sound speed at the top of the sediment is lower as compared to the values estimated from the location where the TL data were acquired. While the data have significant limitations as to the information they contain on the properties of the seabed, the analysis aids in quantifying the sensitivity of geoacoustic inversion of acoustic data in shallow water littoral regions to assumptions about the frequency dependence of attenuation and sound speed. [ABSTRACT FROM AUTHOR]

Temporal coherence is closely related to single path phase rate. The single path phase rates in a shallow water environment were obtained by applying two different multipath models [Dyson, Munk, and Zetler, J. Acoust. Soc. Am., vol. 59, pp. 1121-1133, 1976 and Mikhalevsky, J. Acoust. Soc. Am., vol. 66, pp. 751-762, 1979] on simulated data with internal waves. The estimated phase rates were consistent between the two models. In contrast, previous application of the same models on deep water data showed a factor of 8 difference in their phase rates. The phase rates derived from the models become independent of range after a uniform phase distribution is reached in the (-π, π) region. The shallow water coherence time varies considerably with depth at a given range. The depth dependence is explained with a correlation function constructed with mode depth functions. [ABSTRACT FROM AUTHOR]

Matched-field processors can suffer degradation in dynamic environments due to mismatch between data and replica vectors. One measure of such degradation is the autocorrelation time scale of the data vectors. Matched-field autocorrelation times for 300- and 500-Hz narrow-band signals have been extracted from data acquired during the 2001 Asian Seas International Acoustics Experiment (ASIAEX). The acoustic signals were received on a vertical hydrophone array located near the South China Sea (SCS) shelf break at a range of 18.9 km from the acoustic sources. The water depth along the propagation path ranged from 100 to 125 m. For acoustic signal propagation through a diurnal internal tide that takes the form of a uniform depression of the thermocline, the matched-field correlation time at 300 Hz is approximately τ½ = 15 min or, nondimensionally, n½ = 200 000 cycles. For acoustic signal propagation through packets of nonlinear internal waves, the correlation time for the 300-Hz signal is approximately τ½ 2 min or n½ = 30 000 cycles. A nonlinear internal wave packet entering the acoustic propagation path at 11:40 UTC on May 7 was observed to be unambiguously associated with a drop in the signal correlation time. A two-dimensional advective, frozen-ocean acoustic propagation model produces matched-field correlation times that are qualitatively similar to these observations. It is concluded that, in this environment, nonlinear internal wave packets propagating in the acoustic path shorten the lifetime of replica vectors and cause matched-field processor degradation. [ABSTRACT FROM AUTHOR]

During the recent Asian Seas International Acoustics Experiment (ASIAEX), extensive current meter moorings were deployed around the continental shelf-break area in the northeastern South China Sea. Thirteen RADARSAT SAR images were collected during the field test to integrate with the in situ measurements from the moorings, ship-board sensors, and conductivity/temperatire/depth (CTD) casts. Besides providing a synoptic view of the entire region, satellite imagery is very useful for tracking the internal waves, locating surface fronts, and identifying mesoscale features. During ASIAEX in May 2001, many large internal waves were observed at the test area and were the major oceanic features studied for acoustic volume interaction. Based on the internal wave distribution maps compiled from satellite data, the wave crests can be as long as 200 km with an amplitude of 100 m. Environmental parameters have been calculated based on extensive CTD casts data near the ASIAEX area. Nonlinear internal wave models have been applied to integrate and assimilate both synthetic aperture radar (SAR) and mooring data. Using SAR data in deep water as an initial condition, numerical simulations produced the wave evolution on the continental shelf and compared reasonably well with the mooring measurements at the downstream station. The shoaling, turning, and dissipation of large internal waves at the shelf break have been studied and are very important issues for acoustic propagation. [ABSTRACT FROM AUTHOR]

An adaptive technique for underwater acoustic communication using passive phase conjugation (PPC) is developed. Multipath channel-parameter identification is accomplished by decision-directed model building and finite-window block-updated least squares computed by LSQR (an iterative linear systems solver). The resulting channel estimates are then used by the PPC processor to generate decisions for use in the next processing block. This architecture effectively accomplishes array equalization with low computation cost in shallow-water environments that exhibit rapidly fluctuating multipath scattering. The performance on shallow-water acoustic communications channels is demonstrated at ranges of 0.9-4.6 km under windy surface conditions and shipping noise, using measured wide-band telemetry data with binary phase-shift keying signaling. The algorithm is evaluated with sparse receiver apertures using subsets of a 14-element array. [ABSTRACT FROM AUTHOR]

Traditionally, matched-field processing (MFP) has been used to localize low-frequency sources (e.g., <300 Hz) from their acoustic signals received on long vertical arrays. However, some sources emit acoustic signals of much higher frequency. Applying MFP to signals in the mid-frequency range (e.g., 1-4 kHz) is a very challenging problem because MFP's sensitivity to environmental parameter mismatch becomes more severe with increasing frequency. Robust MFP techniques are required to process signals in the mid-frequency range. As a practical issue, short vertical arrays are more convenient to work with than are long vertical arrays; they are easier to deploy and are less prone to large amounts of deformation. However, short vertical arrays undersample the water column, which can result in severely degraded MFP performance. In this paper, we present experimental data results for this nonconventional paradigm. Using the environmentally robust broad-band L∞-norm estimator, MFP results are given using shallow-water experimental data. This data consisted of broad-band signals in the 3-4-kHz band collected on an eight-element 2.13-m vertical array. These results serve to demonstrate that good localization performance can be attained for this difficult problem. Guidelines on the appropriate use of ray and normal-mode propagation models are also presented. [ABSTRACT FROM AUTHOR]

A computational case study of coupled-mode 400-Hz acoustic propagation over the distance 27 km on the continental shelf is presented. The mode coupling reported here is caused by lateral gradients of sound-speed within packets of nonlinear internal waves, often referred to as solitary wave packets. In a waveguide having unequal attenuation of modes, directional exchange of energy between low- and high-loss modes, via mode coupling, can become time dependent by the movement of waves and can cause temporally variable loss of acoustic energy into the bottom. Here, that bottom interaction effect is shown to be sensitive to stratification conditions, which determine waveguide properties and, in turn, determine modal attenuation coefficients. In particular, time-dependent energy loss due to the presence of moving internal wave packets is compared for waveguides with and without a frontal feature similar to that found at the shelfbreak south of New England. The mean and variability of acoustic energy level 27 km distant from a source are shown to be altered in a first order way by the presence of the frontal feature. The effects of the front are also shown to be functions of source depth. [ABSTRACT FROM AUTHOR]

An inversion technique (IT) is developed and preliminarily tested using data from the 2001 IT Workshop. This technique was developed using TL versus range data collected by the harsh environments program (HEP) and provided through the workshop. However, the IT developed here applies to all sonar systems, active and passive. The sonar-independent portion of the IT consists of a simulated annealing algorithm to be developed by Neumann et al. constrained by an expert system called the Hamilton-Bachman Smart Rules (HBSR), which was developed by the authors. This expert system constrains the geoacoustic model being inverted to realistic Hamilton-Bachman-type values, curve shapes, etc. The sonar-specific module of the IT is chosen based on the sonar frequency and models available to run at those frequencies. Two measured data cases from the workshop are presented and, due to the HBSR, good solutions were acquired in less than 50 iterations. [ABSTRACT FROM AUTHOR]