Hydroacoustic Array Research Articles

Variability analysis of low-frequency and very low-frequency ambient ocean noise

Much of the information presented was made possible by the presence over the past several years of a collection of deep water hydroacoustic arrays with long-term, high data rate recorders of ocean ambient noise, deployed and maintained under the auspices of the United Nations Comprehensive Nuclear Test Ban Treaty Office (CTBTO). Not only does the long time span of the noise record allow high frequency resolution of annual and multi-year variability, but the high data rate supports as well the analysis of high temporal resolution minute-by-minute variability. The present analysis is directed at acoustic frequencies where the noise level and its variability are governed primarily by natural events : planetary—such as the influence of tides, oceanographic—such as the influence of El Nino, and meteorological—such as the influence of storms and routine weather. The analysis is focused on the fluctuations of a series of narrow band tonals at acoustic frequencies between 0.1 and 10 Hz. Results of the analysis display the time scales of the various processes that drive the fluctuations.

Expanding the Range of Frequencies in Determining the Direction of Sources of Sound Using Randomly Non-Equidistant Antenna Arrays

The possibility is demonstrated of considerably expanding the range of working frequencies of a linear hydroacoustic antenna array by positioning the receiving elements non-equidistantly. The characteristics of an antenna array modeled with allowance for practical limitations are discussed. The results from processing experimental data are presented that show the effectiveness of the proposed technique.

Low-frequency parametric systems in the shallow sea

The theoretical model of acoustic wave propagation in the ice-water layer is developed, with, taking into account the velocity dispersion. New opportunities for monitoring the marine environment on long routes, including those covered with ice, are discussed when using parametric sonar antennas. The results of research demonstrating single-mode excitation of a waveguide by a parametric array in a wide frequency band are presented. The possibility of compression of a broadband signal during its propagation, as a result of waveguide dispersion, which leads to an increasing of signal intensity, is shown. The results of experimental studies of the propagation characteristics of a wide-band acoustic signal of a parametric hydroacoustic array in a shallow sea are presented.

Algorithm for Determining the Coordinates of a Hydroacoustic Source Using the Correlation Function of Its Signal

Abstract—An algorithm for determining the coordinates (distance and depth) of a broadband signal source has been developed, which uses as the input data the parameters of the maxima in the signal correlation function at the output of the receiving hydroacoustic array and takes into account the number of negative factors affecting the algorithm’s accuracy when operating in real conditions. Simulation is used to demonstrate the algorithm’s applicability conditions and obtain the dependences of the accuracy in estimating the coordinates of a signal source on the number of detected maxima in the correlation function, on the signal-to-noise ratio at the output of the receiving array, and on the accuracy in measuring the vertical sound velocity distribution in the observation area.

Observing the Oceans Acoustically

Acoustics play a central role in mankind’s interactions with the ocean and the life within. Passive listening to ocean “soundscapes” informs us about the physical and bio-acoustic environment from earthquakes to communication between fish. Active acoustic probing of the environment informs us about ocean topography, currents and temperature, and abundance and type of marine life vital to fisheries and biodiversity related interests. The two together in a multi-purpose network can lead to discovery and improve understanding of ocean ecosystem health and biodiversity, climate variability and change, and marine hazards and maritime safety. Passive acoustic monitoring (PAM) of sound generated and utilized by marine life as well as other natural (wind, rain, ice, seismics) and anthropogenic (shipping, surveys) sources, has dramatically increased worldwide to enhance understanding of ecological processes. Characterizing ocean soundscapes (the levels and frequency of sound over time and space, and the sources contributing to the sound field), temporal trends in ocean sound at different frequencies, distribution and abundance of marine species that vocalize, and distribution and amount of human activities that generate sound in the sea, all require passive acoustic systems. Acoustic receivers are now routinely acquiring data on a global scale, e.g., Comprehensive Nuclear-Test-Ban Treaty Organization International Monitoring System hydroacoustic arrays, various regional integrated ocean observing systems, and some profiling floats. Judiciously-placed low-frequency acoustic sources transmitting to globally distributed PAM and other systems provide: 1) high temporal resolution measurements of large-scale ocean temperature/heat content variability, taking advantage of the inherent integrating nature of acoustic travel-time data using tomography; and 2) acoustic positioning (“underwater GPS”) and communication services enabling basin-scale undersea navigation and management of floats, gliders, and AUVs. This will be especially valuable in polar regions with ice cover. Routine deployment of sources during repeat global-scale hydrographic ship surveys would provide high spatial coverage snapshots of ocean temperatures. To fully exploit the PAM systems, precise timing and positioning need to be broadly implemented. Ocean sound is now a mature Global Ocean Observing System (GOOS) “essential ocean variable”, which is one crucial step toward providing a fully integrated global multi-purpose ocean acoustic observing system.

A study on the ambient noise field at a hydroacoustic array near Robinson Crusoe Island

Continuous long-term sound sources are recorded at hydroacoustic station H03S, a threeelement hydrophone array south of Robinson Crusoe Island between 2014 April 23 and 2017 August 20. The origin of the signal between 3 and 20 Hz is investigated by using cross-correlation, array processing using plane wave beamforming and spectral analysis. Onebit normalization is successfully applied as a cross-correlation pre-processing step in order to suppress undesired earthquake events in the data. Traveltimes retrieved from averaged cross-correlations do not yield a coherent array direction of arrival. Averaged envelopes of the cross-correlations, however, indicate a coherent signal approaching H03S from a south. southwest direction. Beamforming indicates two dominant backazimuth directions: 172°-224° (Antarctica) and 242° (Monowai Volcanic Seamount). This continuous source field creates possibilities to investigate the applicability of acoustic thermometry at hydrophones H03 S1. S2. Cross-correlation and array processing indicate significant directional variation in local propagation, most likely related to the steep slope in the bathymetry near H03S. In addition, it is demonstrated that the ambient noise field is not sufficiently equipartitioned. It is shown that this causes a large error in the estimated temperature, primarily due to the short receiver spacing. These large errors have not been addressed in previous studies on deep-ocean acoustic thermometry. Hence, it is shown that acoustic thermometry does not perform well on small arrays such as H03S. The power spectral density yields a strong broadband signal in January. March, most likely related to iceberg noise. A narrow banded signal around 17 Hz between April and September corresponds to whale calls. The best-beam sound pressure levels towards Antarctica are compared to ERA5 climatologies for sea ice cover and normalized stress into the ocean, supporting the hypothesis of iceberg noise.

Modeling of Processing Algorithms for Processes at the Output of a Combined Receiver and Combined Array Channels

Using a computer-based simulation method, we consider the noise immunity of an individual combined receiver consisting of sound pressure and vibration velocity receivers, as well as the noise immunity of a plane acoustically transparent array consisting of such receivers, are considered. The computer simulation results verify the earlier developed analytic method of calculating the noise immunity coefficient of hydroacoustic arrays for the multiplicative processing algorithm for processes in their channels. It is shown that if the anisotropy of the noise field acting on the combined receiver and array varies in a very wide range, the maximum noise immunity is ensured not by multiplicative but by additive processing.

Locating fish bomb blasts in real-time using a networked acoustic system

Results are presented of a demonstration of real-time fish blast location in Sabah, Malaysia using a networked hydroacoustic array based on the ShotSpotter gunshot location system. A total of six acoustic sensors - some fixed and others mobile - were deployed at ranges from 1 to 9 km to detect signals from controlled test blasts. This allowed the blast locations to be determined to within 60 m accuracy, and for the calculated locations to be displayed on a map on designated internet-connected computers within 10 s. A smaller three-sensor system was then installed near Semporna in Eastern Sabah that determined the locations of uncontrolled blasts set off by local fishermen. The success of these demonstrations shows that existing technology can be used to protect reefs and permit more effective management of blast fishing activity through improved detection and enforcement measures and enhanced community engagement.

Seismic and hydroacoustic analysis of four large underwater explosions off the east coast of Florida

During the summer of 2016, the US Navy detonated four large underwater explosions off the east coast of Florida as part of shock wave testing for the Littoral Combat Ships USS Jackson and USS Milwaukee. The water depth at the explosion site is 900 m. Videos of the events provide further insight into the sources. Originally listed as magnitude 3.7 earthquakes in USGS bulletins, the source types were changed to “Experimental Explosion” as the Navy began releasing information to local news stations. These explosions were recorded on US seismic stations to distances of ~2000 km and on the hydroacoustic array at Ascension Island at a distance of ~8100 km. Seismic signals are complex but cepstral analysis reveals a clear bubble pulse period of 1.15 sec, corresponding to source depths of 50 meters. At Ascension the signals arrive at an RMS level of 136 dB at an SNR of 30 dB. We modeled the propagation loss from the source to Ascension using broadband PE and find the propagation loss to be 126 dB. Backing out this loss, we estimate source levels for the two explosions at 262 dB corresponding to explosion yields of 11,000 lbs.

Experimental investigation of the thin fiber-optic hydrophone array based on fiber Bragg gratings

Abstract The paper presents the results of experimental investigations of the fiber optic hydrophone array consisting of six sensors, placed in one thin sensitive cable. Sensors were formed by pairs of Bragg gratings spaced 1.5 m apart and recorded in a birefringent optical fiber with the elliptical stressed coating. To form an extended sensor array the optical fiber was additionally covered with a silicone material RTV655 and protective coatings. Experimental investigations of the array showed that fiber-optic sensors pressure sensitivity increases as the acoustic frequency decreases at average value from −169.4 dB re rad/uPa at 495 Hz to −143.7 dB re rad/uPa at 40 Hz. The minimum detectable pressure was at average value from 53 mPa/√Hz at 495 Hz to 8.3 mPa/√Hz at 40 Hz. The obtained results might be used for developing and producing long thin hydroacoustic arrays for geophysical investigations and other hydroacoustic applications.

Estimation of interference of piezoelectric ceramic transducers in a hydroacoustic array

A method for experimental estimation of intrinsic and mutual impedances of transducers in a shipborne multielement hydroacoustic array is proposed and investigated. It is shown that consideration of the transducer interactions increases the radiated power and suppresses the lateral field.

2011년 토호쿠 대지진의 수중음향 관측

2011년 3월 11일에 일본 토호쿠 앞바다에서 규모 9.0의 대지진이 발생하였고 이는 전 세계 지진관측망뿐 아니라 IMS(International Monitoring System)에서 운영하는 하와이 수중음향 관측망에도 기록되었다. 우리는 이 대지진을 비롯한 두개의 규모 7 이상의 여진의 수중음향 자료에 대하여 방위각 및 스펙트럼 분석을 통해 복잡한 지진원에서 발생한 T 파의 특성을 파악하고자 연구를 수행하였다. 이를 통해 복잡한 단층 파열 현상이 직접적으로 T 파의 형태에 영향을 미치는 것을 관측하였고, 빠른 단층 파열 과정에 의하여 발생한 T 파가 중첩되는 구간에서 큰 에너지가 기록된 것을 확인할 수 있었다. 두 여진에서 발생한 T 파를 비교한 결과 단층 종류와 깊이에 의해 T 파의 형태와 주파수 특성이 변하는 것을 확인하였다. The <TEX>$M_W$</TEX> 9.0 thrust-fault earthquake has occurred in the Pacific coast of Tohoku, Japan, on March 11, 2011. We present the detection of the great earthquake and analyze T-waves associated with the main event and two other big aftershocks (<TEX>$M_W$</TEX> > 7) recorded in a hydroacoustic array (H11N) in the Pacific Ocean by performing array and spectral analysis to examine characteristics of T-waves generated from the big events. The complex rupture process of the main event directly influences on the shape of the T-waves, and the peak locates on where T-waves excited from fast rupturing process arrive. We compare the two aftershocks with different fault type and show that the fault type and the source depth change shape and spectral contents of T-waves.

Experiment on constructing and studying a combined sensor array

The paper presents the results of studying a breadboard model of a vector combined hydroacoustic array created in the mid-1980s. The combined sensor array was developed on the basis of molecular-electronic hydroacoustic pressure-gradient sensors. According to the experimental data, the gain from joint processing of the total pressure and pressure-gradient channels exceeded 20 dB. The paper discusses the parameters of molecular-electronic converters, which ensured the qualitative characteristics of the combined array.

Synthesis of frequency-independent directional patterns for hydroacoustic arrays of arbitrary configurations

Synthesis of directional patterns varying only slightly in a wide frequency band is considered for continuous linear arrays of arbitrary configurations. It is assumed that a linear array lies in a plane, has a convex shape, and is described by a single-valued function y(x). It is shown that, for such an array, stabilization of the width of its directional pattern is possible in a frequency band exceeding several octaves, provided that the required directional pattern is broader than the directional pattern of the array with phase compensation.

The power pattern of a hydroacoustic array in a waveguide

A definition of the directional pattern of a transmitting hydroacoustic array operating in a waveguide is introduced as the normalized azimuth dependence of the emitted power. Directional patterns of a horizontal linear array and a planar rectangular vertical array are considered as examples.

Effect of underwater currents on hydroacoustic arrays in monitoring shallow sea

The problem of monitoring the environment in a shallow sea using linear hydroacoustic arrays deflected by underwater currents is considered.

Underwater noise as source of information on conditions and dynamics of ocean environments

Leonid Brekhovskikh wrote in his book The Ocean and the Human (1987): ‘‘Ocean noise is as important oceanographic parameter as temperature, current, and wind.’’ Brekhovskikh created and headed the Laboratory of Acoustic Methods of Ocean Research in 1956. One of the scientific directions of the Laboratory was investigation of underwater noise both as interference for sound reception and a source of environmental information. Long-term studies on the unique acoustic research vessels created under the initiative of Brekhovskikh resulted in numerous important findings, including ambient noise spectra and envelopes of acoustic fluctuations, depth dependence of noise directivity, and mechanisms of ambient noise generation. Brekhovskikh was always eager to find practical applications of scientific results. Different methods of ensuring noise immunity of hydroacoustic arrays were developed under his supervision. Passive methods of acoustic navigation based on use of natural noise were suggested. Techniques for underwater acoustic monitoring of the ocean based either on ambient noise analysis or reemission of noise from a point away from the receiving system have been developed. The success of the team of scientists headed by Brekhovskikh was determined by the creative atmosphere around him: there was neither competition nor commercial interests. The common goal was knowledge of the ocean.

THE CONVERTED HYDROACOUSTIC ARRAY "MG-10M" – A BASIC MODULE FOR A DEEP WATER ν-TELESCOPE

The possibility of converted hydroacoustic arrays to create a module of deep water ν-Telescope is discussed. The array MC-10M is good tool to start the first real ocean experiments to investigate the problem of detecting neutrinos with energy 1018 eV. Technical parameters and systems which are necessary to create the module are presented. The Mediterranean Sea is an appropriate area for the first experiments due to existing of a deep underwater sound channel and stable high temperature. Some estimation of possible detecting volumes is presented. This project is continuation of works of authors in this direction, started in 1997.

Sumatra tsunamis recorded on hydophones and seismic stations in the Indian Ocean

Tsunamis generated by the great Indonesian earthquakes of 26 December 2004 (Mw 9.3) and 28 March 2005 (Mw 8.6) produced high‐frequency dispersed signals (5 to 60 MHz) recorded by hydrophone and seismic stations in and around the Indian Ocean. For the first and greater earthquake, the dispersed energy is frequently observed to 30 MHz and in one case to 60 MHz. The high‐frequency signals are consistent with pointlike sources. The earliest‐arriving tsunami signal is located to a source at 4.3N, 93.8E using event‐to‐station distances estimated by measuring dispersion and azimuth estimates using an array of hydrophones. Fine structure in the tsunami signal indicates a second high‐frequency source just south of Great Nicobar Island. The dispersion of later‐arriving energy matches predictions for reflections of the tsunami from bathymetric features in the Indian Ocean basin. For the 28 March 2005 tsunami, the high‐frequency dispersion is observed at the Diego Garcia hydrophone station and the AIS seismic station, and tsunami signals without apparent dispersion are seen at four other seismic stations. Phase velocities are estimated at hydroacoustic arrays and agree with linear dispersion theory at frequencies above 12 MHz.

Hydroacoustic observations of the 2004 megathrust earthquake and other events

Hydroacoustic signals (T-waves) generated by the 2004 Great Sumatra earthquake were recorded by a network of 5 small hydroacoustic arrays located in the Indian Ocean at distances of 2800 to 7000 km from the epicenter. The array configurations allow for accurate determination of the receiver-to-source azimuth given coherent arrivals. Analysis of a series of 10-s windows within the T-wave coda shows that the receiver-to-source azimuth varies smoothly as a function of time, suggesting a nonstationary T-wave source. Apparent motion in the T-wave source has also been observed for many small-scale earthquakes and, for those events, reflects T-wave excitation over a broad expanse of the seafloor. For small-scale earthquakes, it is shown that the variations in the source azimuth are predictable using a seafloor scattering model. The azimuthal variations for the great Sumatra event are shown to be inconsistent with a small-scale source and are thus indicative of the rupture velocity. The data indicate that the rupture proceeded in two distinct phases; initially it progressed northwest at approximately 2.4 km/s along the Sunda trench. At 600 km from the epicenter, the rupture slowed to approximately 1.5 km/s.