Shallow Water Acoustic Experiment Research Articles

Effect of oceanographic fluctuations on geo-acoustic inversions and source localization using ships of opportunity

Several merchant ship-radiated noise events were recorded in two separate seabed characterization experiments in the New England Mudpatch region in 2017 (SBCEX17) and 2022 (SBCEX22). These shallow water acoustic experiments were conducted under different oceanographic conditions, resulting in fluctuations in sound propagation. Several statistical inference approaches are used to invert geo-acoustic parameters such as sound speed and density in a mud over sand sediment (See https://doi.org/10.1121/10.0008419). In this work, we explore the effect of sound speed profile fluctuations in the water column on geo-acoustic inversions in the 360–1100 Hz frequency band. Inversions based on measured and simulated data using acoustic models are provided to support our findings. The results demonstrate that sediment parameters become more sensitive to oceanographic variations as the frequency increases and, therefore, sound speed profile fluctuations in the water column need to be taken into close consideration for more accurate geo-acoustic inversions. [Work supported by ONR].

Deep learning for source localization and seabed classification under dynamic oceanographic environments using explosive sound signals

A deep-learning approach is proposed for simultaneous source localization and seabed classification of underwater signals generated by SUS (Signal, Underwater Sound) MK64 explosives. The charges were deployed in two separate shallow water acoustic experiments on the New England Mudpatch region in 2017 (SBCEX17) and 2022 (SBCEX22) conducted under different oceanographic conditions. The proposed approach uses a multitask learning (MTL) methodology, and convolutional neural networks (CNNs) trained on the spectrogram representation of SUS signals in the 10–199 Hz frequency band where the time-frequency modal dispersion curves exhibit distinct patterns. CNNs were trained on simulated data generated using the normal modes model, ORCA. This synthetic dataset is composed of several sound speed profiles measured in the water column in both experiments and representative seabeds ranging from muddy to sandy sediments. CNNs were tested on the signals measured in the two experiments where the source localization achieved a low mean squared error (0.35–0.8 km), and the seabed classification closely matched existing inversion results in the Mudpatch area. Notably, our method effectively handled oceanographic variations, showcasing the utilization of modal dispersion for training deep learning algorithms. [Work supported by ONR Grant N00014-21-1-2760.]

High frequency source depth localization using deep neural network

Localization of high-frequency sources in underwater is a difficult problem because the propagation model is increasingly sensitive to environmental parameters as the frequency increases. A deep learning approach is proposed to estimate the depth of a high-frequency underwater source using a single hydrophone trained on real data. A residual neural network is trained by a spectrogram of measured signal and estimates the depth of the source as a regression problem. The method is applied to data collected during the shallow water acoustic variability experiment 2015 in the northeastern East China Sea and compared with the results of the frequency difference matched field processing which uses 16 sensors. It was also validated that the trained model can display the generalized results for the signals measured at other times or at similar settings but different source-receiver location. As a result, the neural network is able to more accurately estimate the depth of high-frequency source and shows the features found by the network from real data are effective for localization, while the model often fails to generate accurate replicas.

Performance analysis of an improved modal dispersion compensation receiver for low-frequency and long-range shallow water communication experiment

Low-frequency acoustic signal will suffer from dispersion effect when propagating in long-range shallow water waveguide, which leads to multi-mode and waveform expanding. Longer transmission range and shallower waveguide will result in more serious dispersion effect. In this paper, we propose an improved modal dispersion compensation receiver, and evaluate its performance for a shallow water acoustic communication experiment. The receiver not only exploits time-frequency analysis approach to compensate the waveform expanding effect of each mode simultaneously, but also combines the modified multi-mode energy to enhance symbol estimation accuracy. The experiment is conducted by a differential-binary-phase-shift-keying modulated direct-sequence-spread-spectrum communication system, which is suitable for long-range transmission. With this receiver, we obtain no error demodulation results without coding in the experiment. The experimental data analysis also shows that the proposed receiver can obtain about 3 dB gain than the conventional one, which is of significant importance for long-range communications.

Estimating low-frequency (<100 Hz) sound speed in fine to very-fine sand sediment from the horizontal coherence of the head wave excited by a light helicopter

A series of shallow-water acoustic experiments has been conducted off La Jolla, California using a Robinson R44 helicopter as a low-frequency (≈13–2500 Hz) sound source. The sound speed of the sand sediment was recovered from the horizontal coherence of the head wave excited in the water column by the helicopter. Previously, on 14 December 2016, with a horizontal aperture of 3 m, this technique returned the sound speed of the sediment, from 800–2300 Hz, as 1682 ± 16 m/s, consistent with the known sediment type. An additional experiment was conducted on 6 February with larger horizontal apertures of 9, 12, and 15 m. The hydrophones, situated 0.5 m above the seabed, received the head-wave signals, allowing the coherence function to be formed over the bandwidth of the airborne source. The sediment sound speed was recovered by matching the zero crossings of the measured coherence function to those predicted by the theory of head-wave generation in shallow water. The dispersion in the sediment sound speed was estimated over a frequency range extending between 27 Hz, the lowest zero crossing of the coherence function, and 2.5 kHz, the bandwidth of the source. [Research supported by ONR, SMART(DOD), NAVAIR and SIO.]

The preliminary study on spatial correlation of ocean sound field

The sound field temporal correlation and spatial correlation, which are the foundation of the investigation of underwater signal space-time high-order characteristics, have important value in the underwater acoustic application. The spatial correlation is studied based on the shallow water acoustic propagation experiment data acquired in the northern South China Sea in 2017, and the deep water acoustic propagation experiment data acquired in the western Pacific in 2013. As for the explosive sound signals in shallow water, time domain waveform cross-correlation coefficients between signals from different propagation distance are calculated. In contrast, the linear frequency modulated signals in deep water need additional matched filtering. The signal processing results shows that, the overall spatial correlation is poor and the correlation radius is relatively small in shallow water, the convergence zone has an obviously better spatial correlation than the shadow zone for the deep water situation. The processing result is verified by simulation and analysis.

Exploiting time varying sparsity for underwater acoustic communication via dynamic compressed sensing

While it has been recognized that the multipath structure of the underwater acoustic (UWA) channel offers the potential for compressed sensing (CS) sparsity exploitation, the rapidly time varying arrivals induced by highly dynamic surfaces unfortunately pose significant difficulties to channel estimation. From the viewpoint of underwater acoustic propagation, with the exception of the highly time varying arrivals caused by dynamic surface, generally there exist relatively stationary or slowly changing arrivals caused by direct path or bottom reflection, which imply the adoption of a discriminate estimation method to handle sparse components with different time variation scale. By modeling the time varying UWA channels as a sparse set consisting of constant and time-varying supports, in this paper, estimation of time varying UWA channel is transformed into a problem of dynamic compressed sensing sparse recovery. The combination of a Kalman filter and compressed sensing is adopted to pursue the solution of it. Numerical simulations demonstrate the superiority of the proposed algorithm. In the form of a channel-estimation-based decision-feedback equalizer, the experimental results with the field data obtained in a shallow water acoustic communication experiment indicate that the proposed dynamic compressed sensing algorithm outperforms classic algorithms as well as CS algorithms.

The Airy phase of explosive sounds in shallow water.

The Airy phase is identified in the received signals from explosive charges deployed in a shallow water acoustic experiment conducted in the New England Mudpatch region during the spring of 2017. Measured and modeled time-frequency dispersion curves are compared and a geoacoustic sensitivity study utilizing marginal probability distributions for the sound speed in five sediment layers is performed. The analysis suggests that inclusion of the Airy phase frequency and arrival time in a geoacoustic-inversion method could lower the uncertainty of sound speed parameter estimation in a multi-layer sediment as compared to methods that do not include the Airy phase structure.

Estimating low-frequency sediment sound speed dispersion from the horizontal coherence of the head wave excited by a light helicopter

A series of shallow-water acoustic experiments has been conducted off the coast of southern California using a Robinson R44 helicopter as a low-frequency (≈ 13–3500 Hz) sound source. The aim of the experiments was to recover the sound speed of a fine to very-fine sand sediment from the horizontal coherence of the head wave excited in the water column by the helicopter. Two hydrophones, separated horizontally by approximately 15 m and situated 0.5 m above the seabed, received the head-wave signals, allowing the coherence function to be formed over the bandwidth of the airborne source. The sediment sound speed was recovered by matching the zero crossings of the measured coherence function to those predicted from a recently developed theory of head-wave generation in shallow water. Using this technique, the dispersion in the sediment sound speed can be estimated over a frequency range extending between 27 Hz, the lowest zero crossing of the coherence function, and 3.5 kHz, the bandwidth of the source. In the middle of the frequency band, the sound speed of the sediment was estimated to be 1682 ± 16 m/s, consistent with the known sediment type. [Research supported by ONR, SMART(DOD), NAVAIR, and SIO.]A series of shallow-water acoustic experiments has been conducted off the coast of southern California using a Robinson R44 helicopter as a low-frequency (≈ 13–3500 Hz) sound source. The aim of the experiments was to recover the sound speed of a fine to very-fine sand sediment from the horizontal coherence of the head wave excited in the water column by the helicopter. Two hydrophones, separated horizontally by approximately 15 m and situated 0.5 m above the seabed, received the head-wave signals, allowing the coherence function to be formed over the bandwidth of the airborne source. The sediment sound speed was recovered by matching the zero crossings of the measured coherence function to those predicted from a recently developed theory of head-wave generation in shallow water. Using this technique, the dispersion in the sediment sound speed can be estimated over a frequency range extending between 27 Hz, the lowest zero crossing of the coherence function, and 3.5 kHz, the bandwidth of the source. In the...

Shallow water acoustics and oceanography at the Woods Hole Oceanographic Institution over the last quarter century

Shallow water (coastal) acoustics has been a topic of great interest over quarter century, both from the point of view of learning coastal oceanography and of learning coastal acoustics. Indeed, the two are difficult to separate. In this talk, the research done at the Woods Hole Oceanographic Institution (very often in collaboration with other institutions) over the past 25 years will be discussed, with an emphasis on the major experimental efforts that have been fielded, but also including theoretical and computational efforts. The Barents Sea Polar Front experiment, the Shallow Water Acoustic Random Medium experiment, the Shelfbreak PRIMER experiment, the Asian Seas International Acoustics Experiment, the Shallow Water 2006 experiment, and the Quantifying, Predicting and Exploiting Uncertainty program are some of the at-sea efforts to be discussed. Theoretical advances in shallow water acoustics, as well as computational efforts, such as the Integrated Ocean Dynamics and Acoustics project, will also be ...

Statistics of Nonlinear Internal Waves during the Shallow Water 2006 Experiment

AbstractDuring the Shallow Water Acoustic Experiment 2006 (SW06) conducted on the New Jersey continental shelf in the summer of 2006, detailed measurements of the ocean environment were made along a fixed reference track that was parallel to the continental shelf. The time-varying environment induced by nonlinear internal waves (NLIWs) was recorded by an array of moored thermistor chains and by X-band radars from the attending research vessels. Using a mapping technique, the three-dimensional (3D) temperature field for over a month of NLIW events is reconstructed and analyzed to provide a statistical summary of important NLIW parameters, such as the NLIW propagation speed, direction, and amplitude. The results in this paper can be used as a database for studying the NLIW generation, propagation, and fidelity of nonlinear internal wave models.

Modeling transmission loss over stratified muddy sediments based on chirp sonar imagery

The Office of Naval Research is sponsoring a shallow water acoustics experiment offshore New Jersey where the sediment consists nominally of a layer of mud over sand, and where the layer thickness is about 10 m. A preliminary chirp sonar survey shows that there exists fine layering within the mud layer. Nearby core data also reveal the existence of fine layers within the mud. One of the interesting acoustic questions is what impact such fine layering within the muddy sediments has on acoustic propagation. Based on the chirp sonar imagery, various plausible models of sediment layering and material composition are proposed and the transmission loss is estimated for those models for the purpose of assisting field experiment planning.

Horizontal coherence of sound propagation in the presence of internal waves on the New Jersey continental shelf

The knowledge of spatial coherence of sound field is required for sonar application and array design. In shallow water waveguides, the spatial and temporal evolution of the temperature field induced by internal waves will cause signal fluctuations and variations in spatial coherence. During the Shallow Water Acoustic Experiment 2006, the three dimensional temperature field of internal waves was measured and reconstructed while acoustic signals were simultaneously transmitted between various sources and an L-array. These internal wave events have been used to investigate the internal wave effect on temporal coherence and vertical structures of acoustic normal modes [Wan and Badiey, J. Acoust. Soc. Am. 135, 2014]. In this paper, the horizontal structures of acoustic normal modes are examined. The arrival angles of normal modes are analyzed. The horizontal coherence of sound field is obtained at different frequencies for these internal wave events. The relationship between horizontal coherence and internal wave parameters, such as propagation direction, speed, and amplitude, is discussed. [Work supported by ONR322OA.]

Coherence measurements of acoustic normal modes during one month of internal wave events on the New Jersey continental shelf

During the Shallow Water Acoustic Experiment 2006 (SW06) conducted on the New Jersey continental shelf, the three-dimensional (3D) temperature field for one month of internal wave (IW) events has been reconstructed by Badiey et al. [J. Acoust. Soc. Am. El. 134(5), 4035 (2013)]. The aforementioned IW events, with the angle between the acoustic track and the IW front varying from -8° to 83°, were measured while simultaneously acoustic signals were transmitted from fixed sources at an along-shelf distance of about 20 km with frequencies at 87.5–112.5 Hz (m-sequence), 175–225 Hz (m-sequence), 270–330 Hz (chirp), and 470–530 Hz (chirp), respectively. The acoustic signals were recorded by an L-shaped hydrophone array moored inside the area with IW measurements. The main goal of this paper is to analyze the coherence of acoustic normal modes accompany with the simultaneously measured IW events. The coherence of acoustic normal modes decomposed from the measured acoustic field is obtained as a function of frequency and IW parameters, such as the IW propagation direction, amplitude, coherence length, etc. The relationship between the modal coherence and IW parameters is discussed. [Work supported by ONR322OA.]

Statistics of internal waves measured during the Shallow Water 2006 experiment

During the Shallow Water Acoustic Experiment 2006 (SW06), detailed measurements of the time-varying ocean environment were made while simultaneously acoustic signals were transmitted between various source and receiver pairs. The time-varying environment induced by internal waves (IW) was recorded by an array of moored thermistor chains, as well as by the attending research vessels. Using a mapping technique described by Badiey et al. [J. Acoust. Soc. Am. EL. 134 (2013)], the three-dimensional (3D) temperature field for over a month of IW events was reconstructed. The results of this mapping are used for the statistical analysis of the IW parameters, such as the IW propagation speed, direction, amplitude, coherence length, etc. This paper provides a summary of these results and also examines the implications of the detailed statistics as regards to the acoustic field. The results in this paper could be used as a database for studying the IW generation, propagation, and its impact on the 3D acoustic propagation in waveguides. [Work supported by ONR322OA.]

Sequential analysis for underwater communications

The state-of-the-art in high-rate, single-carrier wideband signaling for acoustic communications is represented by the decision feedback equalizer (DFE). Though often effective, its performance is far from the optimal obtained from soft decision maximum a posteriori probability (MAP) and maximum-likelihood sequence detectors (MLSD). While these algorithms have optimal performance, their complexity increases exponentially with the duration of the channel impulse response. In practice, such methods are only used for multicarrier modulation, after mode filtering or other form of channel shortening, time-spatial pre-processing equalization. The optimal joint channel estimation and data decoding algorithm is derived and analyzed. The combination of pre-processing, channel response shortening equalization, and joint channel and data recovery have shown excellent performance in shallow water acoustic communications experiments. A sequential estimation alternative to MLSD-based decoding is “almost” as effective in a probability sense, given a modest increase in signal-to-noise ratio (SNR). That approach combines very high performance with a small computational burden relative to the MLSD approach. The practical result of the paper is the investigation of the replacement of the DFE with a sequential implementation (FANO) of a likelihood sequence estimator. Comparative performance of the two using at-sea experiments in very shallow water is provided.

Time reversal acoustic communication for multiband transmission

In this letter, multiband acoustic communication is proposed to access a relatively wide frequency band. The entire frequency band is divided into multiple separated sub-bands, each of which is several kilohertz in width. Time reversal decision feedback equalizers are used to compensate for inter-symbol interference at each sub-band. The communication scheme was demonstrated in a shallow water acoustic experiment conducted in Kauai, Hawaii during the summer of 2011. Using quadrature phase-shift keying signaling at four sub-bands over the frequency band of 10-32 kHz, a data rate of 32 k bits/s was achieved over a 3 km communication range.

Investigation of joint channel estimation and data decoding for underwater acoustic communications with multi‐carrier modulation.

The current state‐of‐the‐art in high‐rate acoustic communications is represented by adaptive multi‐channel equalization of single‐carrier wideband signals. An alternative technique with significant potential for achieving high bit rates over multipath‐distorted (frequency selective) channels is multi‐carrier modulation (MCM). There are obvious technical advantages of MCM. Each frequency channel of MCM can be transmitted using a relatively narrow‐band, high‐efficiency transducer, and low‐power amplifier, which allows achieving a very broad bandwidth. Use of narrow‐band channels increases the tolerance for time synchronization errors in multipath array processing. Each frequency channel has a relatively low delay spread when measured in symbol units and can be equalized with low‐complexity algorithms and simplified array processing. The reduction of the ISI span in each sub‐channel allows for the application of soft decision decoders optimal for joint channel estimation and data recovery such as maximum‐likelihood sequence detectors (MLSDs) and adaptive BCJR MAP algorithm. The adaptive form of the MAP algorithm is considered. The combination of channel shortening equalization and a practical MLSD algorithm with dual loop feedback and pre‐survivor processing were tested in shallow water acoustic communications experiments and shown a very good performance. [Work supported by ONR Grant N00014‐07‐1‐0229.]

Frequency Dependence of Transverse Correlation Coefficient in the Yellow Sea

Horizontal correlation is one of the most important characteristic parameters in ocean acoustics. In a shallow water acoustic experiment, we observed that the transverse correlation coefficient is an oscillation function of frequency. A model based on adiabatic normal mode theory in a random inhomogeneous waveguide which is statistically isotropic in a horizontal plane is developed. The theory indicates that the oscillation of the transverse correlation coefficient is mainly due to the normal mode interference.

Inversion for Sound Speed Profile by Using a Bottom Mounted Horizontal Line Array in Shallow Water

Ocean acoustic tomography is an appealing technique for remote monitoring of the ocean environment. In shallow water, matched field processing (MFP) with a vertical line array is one of the widely used methods for inverting the sound speed profile (SSP) of water column. The approach adopted is to invert the SSP with a bottom mounted horizontal line array (HLA) based on MFP. Empirical orthonormal functions are used to express the SSP, and perturbation theory is used in the forward sound field calculation. This inversion method is applied to the data measured in a shallow water acoustic experiment performed in 2003. Successful results show that the bottom mounted HLA is able to estimate the SSP. One of the most important advantages of the inversion method with bottom mounted HLA is that the bottom mounted HLA can keep a stable array shape and is safe in a relatively long period.