Ocean Acoustic Channel Research Articles

Radiated power and acoustic convergence zone properties of the vibration of the shaft-hull system in an ocean acoustic channel

To compute the vibroacoustic radiation of the shaft-hull system in an ocean acoustic channel, we apply the finite element method/boundary element method. For the convergence zone effect, we use the beam displacement ray normal mode to examine the profile of the speed of sound in seawater. To evaluate the radiation property of the system, we propose the concept of effective radiated acoustic power, where only reversing acoustic rays and seabed totally reflected rays are included. We reveal that the far-field radiation can be approximately inferred from the effective radiated acoustic power. Effective radiated acoustic power can be utilized to extract the main radiating modes, which dominate the far-field radiation. The axial force induces vibration, including breathing and bending modes, resulting in the highest peak of the radiated sound pressure level (SPL). The lateral and vertical excitation results in horizontal and vertical bending modes, respectively, and the SPL in convergence zones induced laterally are higher than those induced vertically.

A hybrid algorithm of underwater structure vibration and acoustic radiation-propagation in ocean acoustic channel

In ocean environment, the sound speed gradient of seawater has an important influence on far field sound propagation. The FEM/BEM is used to decouple the vibroacoustic radiation of the spherical shell, and the Green function of the virtual source chain is adopted for decoupling. For far field radiated Sound Pressure Level (SPL), the Beam Displacement Ray normal Mode (BDRM) is employed. The vibration and near-/far-field radiated SPL of spherical shell is analyzed in shallow sea uniform layer, negative/positive gradient, negative thermocline environment, and deep-sea sound channel. Results show that the vibroacoustic radiation of spherical shell acted at 300Hz can be analogous to dipole. When the radiated field of the spherical shell is dominated by large-grazing-angle waves, it can be analogous to vertically distributed dipole, and the far field radiated SPL is lower; while similar to horizontally distributed dipole if dominated by small-grazing-angle waves, and the far field SPL is high.

Characterizing ocean acoustic channel through databases: Issues, requirements, and methods

There is a growing interest in underwater acoustic telemetry. This calls for methods to evaluate the performance of various (proposed) modulation and equalization algorithms under the same channel conditions. An ideal solution is to develop acoustic channel simulators. However, this effort has not been fruitful since acoustic channel simulations (at practical communication frequencies) so far have failed to capture the characteristics of real ocean channels. Currently, acoustic communications still rely on field experiments for evaluation of the algorithm performance. The problem with this approach is that each experiment is usually tailored to a particular modulation scheme and the cost prohibits testing of many different algorithms under the same ocean conditions. Some data-based channel simulators have been proposed where realizations of the channel impulse response (CIR) functions are generated based on the scattering function estimated from real data but it is still not adequate. A scheme to build ocean channel database is proposed in this paper, where CIR is deduced from data on a fractional symbol-by-symbol basis with negligibly small channel estimation error (using the surrogate signal predication error as an indicator) so that the developers can test the performance of different algorithms under same, realistic ocean conditions.

Team formation and steering algorithms for underwater gliders using acoustic communications

In order to take measurements in space and time from the undersampled vast ocean, it is necessary to employ multiple autonomous underwater vehicles, such as gliders, that communicate and coordinate with each other. These vehicles need to form a team in a specific formation, steer through the 3D region of interest, and take application-dependent measurements such as temperature and salinity. In this article, team formation and steering algorithms relying on underwater acoustic communications are proposed in order to enable glider swarming that is robust against ocean currents and acoustic channel impairments (e.g., high propagation and transmission delay, and low communication reliability). Performance of the proposed algorithms is evaluated and compared against existing solutions, which do not rely on underwater communications, using different ocean current models.

Understanding the feedback effect through the physics of the diffraction-based sensitivity kernel for a target in shallow water: A small-scale experimental demonstration

Using the feedback effect, we experimentally detect a wavelength-sized target in a shallow ultrasonic waveguide between two source-receiver transducers on acoustic feedback. The waveguide represents a 1-km-long, 50-m-deep ocean acoustic channel at the 1/1000 scale. The feedback phenomenon, or Larsen effect, occurs when a source and a receiver are connected both acoustically through the propagation medium and electrically through an amplifier in such a way that the received signal is simultaneously and continuously added to the emitted signal. A resonance is obtained when the emitter and the receiver are in phase. This resonance is very sensitive to any change in the medium which makes it a good observable for target detection. In presence of a target in the waveguide, the numerical gain of the feedback effect has to increase in order to compensate the scattering of the acoustic field from the target. In a separate experiment, the scattered field may also be recorded in a transmission configuration from the same couple of emitter/receiver with an impulse as a source signal. A comparison is made between the two different approaches.

Dynamic tomography of a gravity wave at the interface of an ultrasonic fluid waveguide

Based on single-scattering effects, the diffraction-based sensitivity kernels which make the link between the acoustic perturbations and the medium fluctuations have been extensively studied. This research has been recently extended to perturbations at the air–water interface in an ultrasonic waveguide that scales down with a 1-km-long, 50-m-deep ocean acoustic channel in the kilohertz regime. Using array processing between two source-receiver arrays, the sensitivity kernel for both time and amplitude has been experimentally measured and theoretically calculated for each acoustic echo reverberated in the waveguide in response to a small and local surface elevation. In the present work, an ultrasonic experiment is performed during which the waveguide transfer matrix between the source-receiver arrays is recorded 50 times per second while a low-amplitude gravity wave is generated at the water–air interface. The acoustic inversion performed from the surface sensitivity kernels permits to follow the gravity wave dynamics at any point of the surface between the source and the receiver arrays.

Travel-time tomography in shallow water: Experimental demonstration at an ultrasonic scale

Acoustic tomography in a shallow ultrasonic waveguide is demonstrated at the laboratory scale between two source-receiver arrays. At a 1/1,000 scale, the waveguide represents a 1.1-km-long, 52-m-deep ocean acoustic channel in the kilohertz frequency range. Two coplanar arrays record the transfer matrix in the time domain of the waveguide between each pair of source-receiver transducers. A time-domain, double-beamforming algorithm is simultaneously performed on the source and receiver arrays that projects the multi-reflected acoustic echoes into an equivalent set of eigenrays, which are characterized by their travel times and their launch and arrival angles. Travel-time differences are measured for each eigenray every 0.1 s when a thermal plume is generated at a given location in the waveguide. Travel-time tomography inversion is then performed using two forward models based either on ray theory or on the diffraction-based sensitivity kernel. The spatially resolved range and depth inversion data confirm the feasibility of acoustic tomography in shallow water. Comparisons are made between inversion results at 1 and 3 MHz with the inversion procedure using ray theory or the finite-frequency approach. The influence of surface fluctuations at the air-water interface is shown and discussed in the framework of shallow-water ocean tomography.

Target detection and localization in shallow water: An experimental demonstration of the acoustic barrier problem at the laboratory scale

This study demonstrates experimentally at the laboratory scale the detection and localization of a wavelength-sized target in a shallow ultrasonic waveguide between two source-receiver arrays at 3 MHz. In the framework of the acoustic barrier problem, at the 1/1000 scale, the waveguide represents a 1.1-km-long, 52-m-deep ocean acoustic channel in the kilohertz frequency range. The two coplanar arrays record in the time-domain the transfer matrix of the waveguide between each pair of source-receiver transducers. Invoking the reciprocity principle, a time-domain double-beamforming algorithm is simultaneously performed on the source and receiver arrays. This array processing projects the multireverberated acoustic echoes into an equivalent set of eigenrays, which are defined by their launch and arrival angles. Comparison is made between the intensity of each eigenray without and with a target for detection in the waveguide. Localization is performed through tomography inversion of the acoustic impedance of the target, using all of the eigenrays extracted from double beamforming. The use of the diffraction-based sensitivity kernel for each eigenray provides both the localization and the signature of the target. Experimental results are shown in the presence of surface waves, and methodological issues are discussed for detection and localization.

Detection of a target in shallow water using the feedback phenomenon: A small‐scale experimental demonstration.

We experimentally demonstrate the detection of a wavelength‐size target in a shallow ultrasonic waveguide between two source‐receiver transducers using acoustic feedback. The waveguide represents a 1‐km‐long, 50‐m‐deep ocean acoustic channel at the 1/1000 scale. People are familiar with the feedback phenomenon that results in the loud sound heard when a musician plays an electric instrument directly into a speaker. Feedback occurs when a source and a receiver are connected both acoustically through the propagation medium and electrically through an amplifier in such way that the received signal is simultaneously and continuously added to the emitted signal. A resonance is then obtained when the emitter and the receiver are in phase. The resonant frequency appears to be highly sensitive to fluctuations of the propagation medium. During the crossing of the target trough the waveguide, fluctuations inferred by the target allow the detection. Experimental results are presented with target of different sizes in the presence of surface fluctuations.

Tomography of the surface elevation in shallow water between two source‐receiver ultrasonic arrays: A small scale experiment demonstration.

Based on single‐scattering effects, the diffraction‐based sensitivity kernels which make the link between the acoustic perturbations and the medium fluctuations have been extensively studied. We propose a similar study for perturbations at the air‐water interface in an ultrasonic waveguide that scales down with a 1‐km‐long, 50‐m‐deep ocean acoustic channel in the kilohertz regime. Using array processing between two source‐receiver arrays, the sensitivity kernel for both time and amplitude has been experimentally inferred for each acoustic echo reverberated in the waveguide to a small surface elevation. We demonstrate a good agreement between experimental and theoretical sensitivity kernels. Finally, using theoretical sensivity kernels for both amplitude and time ultrasonic fluctuations, we give a real‐time estimation of the surface elevation at any point between the source and receiver arrays.

Time coherence of acoustic arrivals during the PhilSea09 experiment.

During the spring of 2009, 275‐Hz signals were transmitted over a distance of nearly 200 km from an axial source to a vertical line array with 30 elements. The source transmitted a 100‐Hz bandwidth signal every 5 min over a period of several days. This time interval is fast enough to capture the slightly slower changes in the ocean acoustic sound channel which are due to internal waves. The internal waves cause a loss of coherence over time, frequency, and space. The unique sampling of this experiment allows us to examine all three domains at once. The time coherence has not been captured in this detail in any previous experiment. Loss of coherence in these domains cause the received signal to have variations in time of arrival, phase, and intensity. These signal variations are measurable over both geometric and non‐geometric arrivals.

Detection and localization of a target in shallow water between two source-receive ultrasonic arrays: A small-scale experiment demonstration.

We experimentally demonstrate the detection and localization of a wavelength‐size target in a shallow ultrasonic waveguide between two source‐receive arrays. The waveguide represents a 1.5‐km‐long, 50‐m‐deep ocean acoustic channel at the 1/1000 scale, in which two coplanar arrays would record the broadband transfer matrix around 3 kHz. Invoking reciprocity principle, we perform a time‐domain double beamforming algorithm simultaneously on the source and receive array. This array processing projects the multi‐reverberated acoustic signals recorded between each source and each receiver into an equivalent set of eigenrays defined by their launch and receive angles. Comparison is made between the intensity of each eigenray with and without the target in the ultrasonic waveguide. When detection is achieved, localization is performed through the back projection of each eigenray into the acoustic waveguide weighted by the intensity fluctuation induced by the target. The use of the diffraction‐based sensitivity kernel for each eigenray improves the localization of the target. Detection and localization experimental results will be shown in the presence of surface waves for ultrasonic arrays at 1 and 3 MHz.

Blind Estimation of the Ocean Acoustic Channel by Time–Frequency Processing

A blind estimator of the ocean acoustic channel impulse response envelope is presented. The signal model is characterized by a deterministic multipath channel excited by a highly nonstationary deterministic source signal. The time-frequency (TF) representation of the received signal allows for the separation between the channel and the source signal. The proposed estimator proceeds in two steps: First, the unstable initial arrivals allow for the estimation of the source signal instantaneous frequency (IF) by maximization of the radially Gaussian kernel distribution; then, the Wigner-Ville distribution (WV) is sequentially windowed and integrated, where the window is defined by the previously estimated IF. The integral gives the channel impulse response envelope, which turns to be an approximation to the blind conventional matched filter (MF). The blind channel estimator (CE) is applicable upon the following conditions: that the multipath channel contains at least one dominant arrival well separated from the others, and that the IF of the source signal is a one-to-one function. Results obtained on real data from the INternal TIde Measurements with Acoustic Tomography Experiments (INTIMATE'96), where the acoustic channel was driven by an linear frequency modulation signal, show that the channel's envelope detailed structure could be accurately and consistently recovered, with the correlation of the estimates ranging from 0.796 to 0.973, as compared to the MF result

Arctic abyssal T-phase coupling to the ocean acoustic channel

Five T-phase events were detected in April 1980 on a horizontal hydrophone array suspended from the arctic icecap at a distance of 300 km from the mid-Arctic ridge. The hydrophone array localized the bearing of the event source and was able to detect P-wave arrivals with 5000- to 8000-m/s phase velocities in addition to the dominant T-phase signature with 1400- to 1700-m/s phase velocities. Scattering from the ice surface is considered as a mechanism to convert the near-vertical seismic energy into propagating acoustic energy. At 5 Hz, the scattering losses are about 40 dB, and at 15 Hz they are about 30 dB. The received acoustic levels of the T-phase spectral peaks ranged from 103–119 dB re: 1 μPa, 1 Hz, and the propagation losses over the 300-km distance between source and receiver are on the order of 90–100 dB. The earthquake source level in the water column is estimated to be about 250 dB re: 1 μPa, 1 Hz at 5 Hz, and 235 dBre : 1 μPa, 1 Hz at 15 Hz. These levels are consistent with refraction measurements of crustal attenuation of 0.52 dB/km at 5 Hz [R. E. Keenan and L. R. L. Merriam, J. Acoust. Soc. Am. 89, 1128–1133 (1991)].

Simulations of an adaptive equalizer applied to high-speed ocean acoustic data transmission'

Computer simulations are carried out to study the feasibility of an adaptive equalizer applied to an hydroacoustic data-transmission channel. The channel is examined with a comprehensive acoustical model to acquire worst-case examples of the ocean acoustic transmission channel. The equalizer performance is investigated by simulations with a computer-generated channel response. Equalizer behavior in a mobile time-variant environment is also studied by use of a simplified, time-discrete multipath channel model. A stochastic gradient lattice equalizer is simulated for a channel which varies due to movement of the transmitter platform. The equalizer was able to track a velocity of up to 0.4 m/s for a favorable transmission geometry, using a transmitter beamwidth of 10 degrees . The results demonstrate the feasibility of coherent modulation schemes for medium-distance ocean acoustic telemetry. It was found that small beamwidths are imperative in maintaining signal coherence and in facilitating adaptive equalization. In particular, narrow-beam transducers will reduce equalizer complexity as well as the frequency spread.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Ray Propagation in an Ocean Acoustic Channel with Slowly Time-Varying Stratification

Ray Propagation in an Ocean Acoustic Channel with Slowly Time-Varying Stratification