Shallow Water Experiment Research Articles

Shallow water experiments and results

Shallow water acoustics has flourished over the past fifty years, but most especially over the last 25 years. After World War II, the Cold War between the USSR and the West focused the emphasis in ocean acoustics on deep, “blue water.” After the Cold War waned in 1990, the emphasis changed to “brown water” coastal acoustics studies. In this paper, we will look at the advances over the past half-century in shallow water acoustics, with emphasis on the experimental results from acoustics, oceanography, marine geology, and marine biology.

The exponential decay of underwater acoustic intensity with increasing altitude of low-frequency sound from a Robinson R44 helicopter

A series of underwater acoustic experiments utilizing a Robinson R44 helicopter and an underwater receiver station has been conducted in shallow (16 m) and deep (>100 m) water. The receiver station consisted of an 11-element nested hydrophone array with a 12 m aperture. In the shallow water experiments the array was configured as a horizontal line (HLA) 0.5m above the seabed; whereas in deep water the array was suspended from the surface in a vertical line (VLA). An in-air microphone was located immediately above the surface. In this paper the power spectral density as a function of helicopter altitude will be reported from measurements on a single hydrophone. The main rotor blades of the helicopter produce low-frequency harmonics, the lowest frequency being ~13 Hz. The tail rotor produces a sequence of harmonics approximately six times higher in frequency. Between heights of 30 m to 600 m above the sea surface the underwater intensity was found to decay exponentially with increasing helicopter altitude. Interpretation of the observed low-frequency sound signatures is being facilitated with a numerical three-layer (atmosphere-ocean-sediment) acoustic propagation model in which the source may be moving or stationary. [Research supported by ONR, NAVAIR, and SIO.]

Seabed attenuation inversion from broadband reverberation measurements in the Yellow Sea

The Yellow Sea 1996 experiment was the first joint China-U.S. shallow water experiment conducted in August, 1996. The water depth is 75 m with a deviation of ±1 m at the experimental area. The seabed sound speed and attenuation in this experiment have been successfully estimated using the sound propagation measurements [Wan, Zhou, and Rogers, J. Acoust. Soc. Am. 128(2), 2010]. During this experiment, the monostatic reverberation data produced by 1-kg TNT shots provide a spatial sampling of the acoustic field and contains the information of seabed acoustic properties. Thus, the present paper uses the inverted seabed sound speed from the aforementioned propagation measurements as a constraint condition and analyzes the reverberation data for seabed attenuation inversion. The broadband normal mode filter is applied to the reverberation data received by a 32-element vertical line array. The reverberation level of the first three modes is obtained as a function of range to extract the modal attenuation coefficients. The seabed attenuation is inferred by minimizing the difference between the theoretical and measured modal attenuation coefficients. The inverted seabed attenuation over a frequency range of 400–1000 Hz shows non-linear frequency dependence.

Experimental demonstration of multiband underwater acoustic transmissions based on single carrier communication

In a recent shallow water experiment, multiband transmissions were carried out using a vertical array with six sensors uniformly spaced every 0.5 m. The entire frequency band (2–8 kHz) was divided into two sub-bands, each of which is about 2.55 kHz in width. Time reversal processing is first used to compress severe multipath spread, following by a single channel decision feedback equalizer (DFE) to remove residual inter-symbol-interference (ISI). Using experimental data, this paper demonstrates that multiband transmissions are very beneficial to improve the throughput of underwater acoustic communications, achieving a data rate of 6k bits/s using QPSK modulation with almost error free performance.

Shallow water experiments of flow past two identical square cylinders in tandem

This study investigates experimentally the flow structure between two identical square cylinders as well as in the wake of downstream cylinder that are in-line positioned in tandem. The experiments are performed in a large scale water channel under shallow water conditions. Time-averaged streamline pattern as well as vorticity and turbulence statistics were calculated using Particle Image Velocimetry (PIV) measurement method for different gap ratios (G/D=0.5 to G/D=5, where G is the distance between the cylinders and D is the diameter of cylinder) between the cylinders at a fixed Reynolds number of 4470. The measurements were conducted for both side view and plan view configurations at Froude number Fr = 0.186. The results obtained under shallow water conditions are compared with those obtained under deep water conditions by other researchers and it is found that, as explained in detail in the manuscript, the flow structures are remarkably different especially in the region between the two rectangular cylinders.

Comparison of shallow water mode transport theory with acoustic transmissions during Shallow Water Experiment 2006

Coupled-mode transport theory appears to have put on solid theoretical ground acoustical scattering by internal waves in both deep and shallow water, over a range of low, medium, and high frequencies [Raghukumar and Colosi (2014) and Colosi and Morozov (2009)]. Here, full-field theoretical calculations of the acoustic field moments are compared against experimental data gathered during the Shallow Water Experiment 2006. Transport theory at low frequency is validated using data gathered by the WHOI Shark array at 175 Hz with a source towed by R/V Sharp at several different speeds over distances of 1.5–5.5 km. At high frequencies, comparisons are made at 1.2 kHz using data received by a WHOI bottom-mounted single hydrophone unit with a source towed by CFAV Quest over distances of 0–20 km. Acoustic observables include the mean and variance of intensity. The effect of a range dependent stochastic internal wave field is examined in the context of data-model comparison, along with the effect of random surface waves. In addition, further insights into mode coupling are presented using the shallow water hybrid transport theory.

From Swarm 1995 to Shallow Water Experiment 2006: What have we learned in shallow waveguide acoustics?

Last 20 years have witnessed some very interesting scientific discoveries in shallow water waveguide physics. While before1995 SWARM experiment most studies in shallow water waveguides considered two-dimensional (2D) slices of the ocean waveguide to describe the behavior of sound wave propagation, many studies in the recent years have focused on the three dimensionality of the acoustic wave propagation in the presence of water column variability. This paper summarizes the highlights of what has been done with reference to the published literature and some ongoing work. [Work supported by ONR322OA.]

Statistical Bit Error Trace Modeling of Acoustic Communication Links Using Decision Feedback Equalization

Underwater network simulation and performance analysis require accurate packet error models. The packet error probability depends on the packet length and the temporal distribution of bit errors. We analyze error traces from decision-feedback-equalized single-carrier acoustic communication links from several shallow-water experiments and show that clustering of bit errors occurs at several timescales. We propose a two-part statistical error model consisting of a generalized Pareto fractal renewal parent process that drives Bernoulli daughter processes with generalized extreme value distributed lifetimes. We present an algorithm to simulate communication errors using this error process model and show that the simulated packet loss probability accurately matches experimental observations.

Efficient use of bandwidth for underwater acoustic communication

In a recent shallow water experiment, acoustic communication transmissions were carried out over the 10 to 32 kHz band in ~100 m deep water over a 3 km range. A natural question is how best to utilize that bandwidth. In one multiband approach discussed previously, the band was divided into four smaller subbands that were processed independently using time reversal decision-feedback equalizers (TR-DFEs). This letter presents a complementary wideband approach using data from the same experiment achieving a data rate of up to 60 kbits/s with 32 quadrature amplitude modulation. These results suggest that a wideband approach can be beneficial in terms of spectral efficiency with modest computational complexity using a TR-DFE.

Energy fluctuations of high-frequency sound signals in a shallow water in the presence of nonlinear internal waves

The paper presents an analysis of energy fluctuations of high-frequency (2–4.5 kHz) sound signals propagating in a shallow water in the presence of nonlinear (soliton-like) internal waves (2006 Shallow Water experiment, US Atlantic shelf). Signals were received by three single hydrophones in different directions at distances of ∼4, ∼12, and ∼5 km from the source. The angle between the first two acoustic tracks was ∼15°. The third track was almost an extension of the first and was on the other side of the source. A relatively short (one to two solitons) nonlinear internal wave packet first moved approximately along the first two tracks and then along the third track. It is demonstrated that in the presence of solitons on the track in the frequency spectrum of energy fluctuations, there is an isolated frequency that depends, in particular, on the angle between the soliton front and the acoustic track. The experimental results agree well with the theory previously proposed by the authors, where the occurrence mechanism of fluctuations is explained using the ray approach.

Dynamics of wood fall colonization in relation to sulfide concentration in a mangrove swamp

Wood debris are an important component of mangrove marine environments. Current knowledge of the ecological role of wood falls is limited by the absence of information on metazoan colonization processes over time. The aim of this study was to provide insights to their temporal dynamics of wood eukaryotic colonization from a shallow water experiment in a mangrove swamp. Combined in situ chemical monitoring and biological surveys revealed that the succession of colonizers in the mangrove swamp relates with the rapid evolution of sulfide concentration on the wood surface. Sulfide-tolerant species are among the first colonizers and dominate over several weeks when the sulfide content is at its maximum, followed by less tolerant opportunistic species when sulfide decreases. This study supports the idea that woody debris can sustain chemosynthetic symbioses over short time-scale in tropical shallow waters.

浅海水平相关系数的频率特性研究

It was observed in a shallow water experiment that the horizontal correlation coefficients oscillated with frequency. The relationship among the longitudinal correlation, the intensity and frequency is given according to the normal mode theory and verified by the experimental data. The transverse correlation was also studied. In theory, the relationship among the transverse correlation coefficient, the intensity and frequency is obtained from the sound speed fluctuation.

Algorithmic improvements for schemes using the ADER time discretization

We detail several algorithmic changes to the ADER Multi-Moment Finite-Volume Methods (ADER+MMFV) of Norman and Finkel (2012) [2]. The DT recurrence relations are improved, flux and source term differential transforms are saved, each are expanded as polynomials, quadrature is removed from the algorithm, integration is performed analytically, and a different Riemann solver admitting direct use of time-averaged fluxes is used. These algorithmic changes were implemented and tested, and smooth 1-D shallow water experiments confirm the same or slightly better accuracy as well as 2–3× lower runtimes compared to Norman and Finkel (2012) [2].

Growth of surface wind-waves in water of finite depth. A theoretical approach

The Miles' theory of wave amplification by wind is extended to the case of finite depth. A depth-dependent wave growth rate is derived from the dispersion relation of the wind/water interface. For different values of the dimensionless water depth parameter δ=gh/U12, with h the depth, g the gravity and U1 a characteristic wind speed, a family of wave growth curves is plotted as a function the dimensionless parameter θfd=cU1, with c the wave phase velocity. The model provides a fair agreement with the data and empirical relationships obtained from the Lake George experiment, as well as with the data from the Australian Shallow Water Experiment. Two major results are obtained: (i) for small θfd the wave growth rates are comparable to those of deep water and (ii) for large θfd a finite-depth limited growth is reached, with wave growth rates going to zero.

Multiuser acoustic communications with mobile users

A multiuser receiver is developed that is capable of separating receptions from independent, mobile users whose transmissions overlap in both time and frequency. With respect to any one user's Doppler corrected signal, the other communication signals appear as multiple-access interference distributed across the Doppler dimension. A previously developed receiver composed of an adaptive time-reversal processor embedded within a successive interference cancellation framework is limited to stationary users. This paper extends the receiver to properly remove the interference from moving sources by modeling the effects of Doppler through the interference cancellation receiver. The combined receiver has the ability to remove interference in both the temporal and spatial domains, and this property is shown to be preserved even when users are in motion. When applied to data collected during a recent shallow water experiment (KAM11), the receiver is shown to be capable of separating packets in a two user system where one user is moving while the other is stationary.

The effects of mode structure on coherence in shallow water propagation

In an ideal shallow water propagation channel the sound field is accurately described by normal modes and the mode structure is predictable with clean separated modes. However the real ocean environment is rarely ideal, and variations in bottom bathymetry and water column sound speed are usually present. When fluctuations are small and on the order of a fraction of an acoustic wavelength the sound field propagation is deterministic and any mode pattern deviations are small and slow and the phase response is linear. Here the propagation is considered phase coherent and spatial and/or temporal averaging will produce gain. When the fluctuations increase to the order of ½ to 1 acoustic wavelength the mode patterns becomes distorted such that slow linear fluctuations in phase can no longer describe the propagation. Here coherence is reduced and in some cases completely lost. A unifying theory will be presented linking mode pattern deviations and coherence. PE models will be used to predict mode structure and individual mode correlation will be computed with the ideal case. These mode correlations will be used to estimate temporal and spatial coherence and the results will be compared with data from three shallow water experiments.

An operational ocean circulation prediction system for the western North Atlantic: Hindcasting during July–September of 2006

A real-time ocean prediction system for the western North Atlantic (WNA) has been implemented operationally for the Mid-Atlantic Regional Association Coastal Ocean Observing System (MARACOOS). Based on the physical circulation model component of the Harvard Ocean Prediction System (HOPS), this system is built to capture the mesoscale dynamics of the Gulf Stream (GS), its meanders and rings, and its interaction with the shelf circulation, the mid-Atlantic shelf (MAS), and the buoyancy-driven circulation in the Gulf of Maine (GOM). To accomplish this, the multiscale velocity-based feature models for the GS region are melded with the water-mass-based feature model for the GOM and with shelf climatology across the shelf-slope front for a multiscale synoptic initialization. A ring and front analysis is fed into the feature modeling scheme for generating a three-dimensional initial field. The initialization field is dynamically adjusted with wind-forcing and used in an Sea Surface Temperature (SST)-assimilative forecast mode. For this study, 8 weekly forecasts are compared with GS paths and drifter trajectories during the 2006 Shallow Water Experiment (SW06) conducted during the months of July and September. GS path and ring formation/absorption events were compared and presented by Taylor diagrams (TD). For all of 8 weeks, the forecast skills were comparable or higher than that of the persistence. The model shows reasonable skill in simulating drifter trajectories, especially over days 1–3 of simulation.

Investigation of the horizontal directivity of underwater ambient noise with the circular hydrophone array

The ambient noise field in shallow water is highly variable in time and space. The horizontal directivity pattern of ambient noise is anisotropic and can be exploited for source localization in sonar signal processing. In the present work, the 12-element circular hydrophone array with radius of 1 meter was designed and used to observe the ambient noise in the shallow water experiment in the Yellow Sea. The MVDR beamformer with diagonal loading was designed to get the horizontal directivity pattern in frequency band of 1kHz to 3kHz. The conventional beamformer was used to analyze the horizontal directivity of ambient noise above 3 kHz. The noise data were sampled at 25kHz, bandpass filtered between 100Hz and 5kHz in the experiment. The horizontal directivity patterns at the experiment location at different frequencies were obtained based on above methods and the noise data. This work was supported by the National Natural Science Foundation of China(10734030)

Source Localization Using Matched-Phase Matched-Field Processing With Phase Descent Search

The matched-phase coherent broadband matched-field (MF) processor has been previously proposed and shown to outperform other advanced broadband MF processors. It has been previously proposed to search the matched phases using the simulated annealing, which is well known for its ability of solving global optimization problems while having high computational complexity. This prevents simultaneous processing of many frequencies, and thus, limits the processor performance. We propose to use a novel iterative technique, the phase descent search (PDS), for searching the matched phases. This technique is based on coordinate descent optimization which is mainly applicable to solving convex problems. In this work, we investigate its application to the phase search problem, which is a nonconvex problem. We show that the PDS algorithm obtains matched phases similar to that obtained by the simulated annealing, and has significantly lower complexity. Therefore, it enables to search phases for a large number of frequencies and significantly improves the processor performance. The proposed processor is applied to real data from the 1996 Shallow Water Experiment (SWellEx-96) for locating a moving acoustic source at distances between 1 and 9 km with a step of about 150 m. At each distance, one 1-s snapshot with 13 frequencies is enough to provide accurate localization of the source well matched to global positioning system (GPS) measurements.

Doubly Selective Underwater Acoustic Channel Model for a Moving Transmitter/Receiver

We propose a new method of modeling the signal transmission in underwater acoustic communications when the transmitter and receiver are moving. The motion-induced channel time variations can be modeled by sampling the transmitter/receiver trajectory at the signal sampling rate and calculating, for each position, the channel impulse response from the acoustic-field computation. This approach, however, would result in high complexity. To reduce the complexity, the channel impulse response is calculated for fewer (waymark ) positions and then interpolated by local splines to recover it at the signal sampling rate. To allow higher distances between waymarks and, thus, further reduction in the complexity, the multipath delays are appropriately adjusted before the interpolation. Because, for every time instant, this method only requires local information from the trajectory, the impulse response can recursively be computed, and therefore, the signal transmission can be modeled for arbitrarily long trajectories. An approach for setting the waymark sampling interval is suggested and investigated. The proposed method is verified by comparing the simulated data with data from real ocean experiments. For a low-frequency shallow-water experiment with a moving source that transmits a tone set, we show that the Doppler spectrum of the received tones is similar in the simulation and experiment. For a higher frequency deep-water experiment with a fast-moving source that transmits orthogonal frequency-division multiplexing (OFDM) communication signals, we investigate the detection performance of a receiver and show that it is similar in the simulation and experiment.