Acoustic telemetry for ocean acoustic tomography

Very low data rate burst telemetry for long-range deep-ocean applications is discussed. Energy efficiency, propagation coherence, and waveform coding, together with transmitter constraints, influenced the design of proposed buoy-to-buoy and buoy-to-shore systems.     

A high-frequency, shallow-water acoustic measurement system

The US Naval Ocean Research and Development Activity has developed a high-frequency acoustic measurement system for use in shallow water. The heart of this system is a pair of submersible towers supporting acoustic transmitting and receiving instrumentation. These towers are transported to an experimental staging area, assembled, and acoustic instrumentation installed. They are towed to a preselected measurement site, then the chambers on each tower are flooded, thereby settling slowly to the ocean bottom. Stability and dynamic response analyses were used to determine the towing and deployment stability envelopes for the towers. The acoustic transmitting system uses a pair of narrow-beam parametric acoustic sources operating at secondary frequencies ranging from 20 to 180 kHz. The acoustic receiving systems consist of a pair of 16-hydrophone, two-dimensional arrays with broadband capabilities up to 250 kHz. These systems have been used to make high-resolution bottom scattering measurements in shallow water off the coast, south of Panama City, Florida     

Digital waveform codings for ocean acoustic telemetry

M-sequence waveform coding with a single long codeword has been considered as the basis for long-range underwater acoustic telemetry for one user. (An m-sequence is a periodic, binary, linear-law maximal-length sequence. If the span of the law is n, the maximal length L-2ⁿ=1). For a given law, a single m-sequence transmits a maximum of log₂ (L) bits of source information per channel word. To increase the number of bits per word, families of m-sequences and Gold codes are considered and compared to a single m-sequence. A hypothetical idealized multipath channel with added white Gaussian noise is assumed. Coding using families of m-sequences is recommended because it requires a smaller bit-energy-to-noise ratio than other waveform codes to achieve an equivalent codeword error probability     

Spatial diversity processing for underwater acoustic telemetry

A large increase in the reliability of shipboard or stationary underwater acoustic telemetry systems is achievable by using spatially distributed receivers with aperture sizes from 0.35 to 20 m. Output from each receiver is assigned a quality measure based on the estimated error rate, and the data, weighted by the quality measure, are combined and decoded. The quality measure is derived from a Viterbi error-correction decoder operating on each receiver and is shown to perform reliability in a variety of non-Gaussian noise and jamming environments and reduce to the traditional optimal diversity system in a Gaussian environment. The dynamics of the quality estimator allow operation in the presence of high-power impulsive interference by exploiting the signal and noise differential travel times to individual sensors. The spatial coherence structure of the shallow water acoustic channel shows relatively low signal coherence at separations as short as 0.35 m. Increasing receiver spacing beyond 5 m offers additional benefits in the presence of impulsive noise and larger-scale inhomogeneities in the acoustic field. A number of data transmission experiments were carried out to demonstrate system performance in realistic underwater environments     

Adaptive equalization techniques for acoustic telemetry channels

A tutorial review of adaptive equalization techniques for combating intersymbol interference in high-speed digital communications over time-dispersive channels is given. Various equalizer structures and the associated adaptive algorithms, including both fractionally spaced and symbol-spaced equalizers, are presented. Also considered is the application of adaptive equalization techniques to underwater acoustic telemetry channels     

Marine sediment acoustic measurement system

This paper describes a system that has been developed to measure compressional wave speed in cored marine sediments onboard ship. The structure enables one to secure an extruded core sample to its base and to move acoustic probes to a desired location, implant them to a specified depth into the sample and perform the measurement. The acoustic measurement system is a pulse-time delay system measuring time difference over a fixed path length and the temperature of the sediment. The time difference and temperature measurement systems are comprised of task oriented components and are housed in a single portable box. The system is adaptable to the various sample sizes obtained with the coring apparatus presently in use. Initial field tests indicate that ship motion has no effect on the system. Data collected from cores has been classified according to sediment type and displays good agreement with data presented by Hamilton (1970). The difference in compressional wave velocity, based on sediment type, for the two studies is 5 m per sec.     

Adaptive detection for shallow-water acoustic telemetry withcochannel interference

Adaptive decision feedback equalization (DFE) has recently been used to enable high-rate data transmission through shallow-water acoustic channels. This adaptive receiver successfully tracks and suppresses intersymbol interference due to a dispersive multipath channel. However, acoustic modems which are used for network applications must also contend with interference due to cochannel signals from proximal modems. In this work, we propose and evaluate a multiuser receiver with cochannel interference suppression. The advantages of this multiuser receiver in the presence of strong cochannel interference are shown by a performance comparison to a bank of the single-user DFEs described above. Conclusions are supported in part by the demodulation of experimental data for two simultaneous cochannel signals and by a steady-state performance analysis     

Performance limitations in underwater acoustic telemetry

Performance limitations in digital acoustic telemetry are addressed. Increases in computational capabilities have led to a number of complex but practical solutions aimed at increasing the reliability of acoustic data links. These solutions range from ocean-basin scale data telemetry to video-image transmission at a few hundred yards' distance. The opportunity to implement highly complex tasks in real time on modest hardware is a common factor. The data rates range from 1 to 500 kb/s and are much slower than satellite channels, while acceptable system complexity is higher than virtually any other channel with comparable data throughput. The basic performance bounds are the channel phase stability, available bandwidth, and the channel impulse response fluctuation rate. Phase stability is of particular concern for long-range telemetry, channel fluctuation characteristics drive equalizer, and synchronizer design; the bandwidth limitation is a direct constraint on data rate for a given signaling method     

Design and calibration of an acoustic telemetry system subject to upwelling events

The experiments described were designed to calibrate a hexagonal array of VEMCO VR2 receivers and transmitters (model V9P-6L-S256) in isothermal and stratified water columns off the south coast of South Africa. The array, configured with 500 m between receivers, was designed to study the influence of water temperature and turbidity on the spawning behaviour of chokka squid Loligo reynaudii. Range tests comprised fixing a single VR2 receiver 2 m from the seabed and placing a V9P transmitter at distances of 0 m, 75 m, 150 m, 225 m, 300 m, 375 m, 450 m and 500 m from the receiver for periods of 10 minutes at each position under isothermal conditions and in the presence of a thermocline. The data indicated a range of 300 m for the former and 75 m for the latter conditions. The field performance of the V9P transmitter in a non-stratified water column compared well with the theoretical range of 352 m calculated using software to calculate range. System saturation was investigated by repeating the range test using four, eight and 14 transmitters simultaneously. Field data indicated a significant decrease in signal detections due to signal collisions when more than eight transmitters were active simultaneously. It was demonstrated that the hexagonal configuration of VR2s is optimal during isothermal conditions but inadequate during stratified conditions when acoustic dead zones of 350 m between VR2 receivers can occur.     

A synthetic bandwidth measurement system for acoustic remotesensing of geoacoustic properties of bottom sediments

A series of narrowband acoustic transmissions may be combined into an equivalent broadband signal - with a corresponding increase in resolution in the time domain. This procedure is called synthetic bandwidth processing. If ray methods are used in combination with inverse theory to determine the otacoustic properties of bottom sediments, the increased time resolution aids in the identification of arrivals which are closely spaced in time. This, in turn, permits improved estimates of the spatial distribution of properties and more accurate otacoustic models. Signal processing techniques and guidelines for synthesizing multioctave broadband pulse-like signals are presented. Also described is the instrumentation used to implement the technique in real ocean environments     

Acoustic telemetry system for underwater control

Underwater acoustic communication in the multipath environment encountered in shallow water is restricted mostly by signal fading. It degrades the signal detection and time synchronization required for reliable acoustic communication. An approach to time synchronization and to the frequency diversity method is presented. A communication algorithm for obtaining a reliable acoustic underwater link, and offering an easy-to-implement decoding scheme is introduced, and system realization is described     

The state of the art in underwater acoustic telemetry

Progress in underwater acoustic telemetry since 1982 is reviewed within a framework of six current research areas: (1) underwater channel physics, channel simulations, and measurements; (2) receiver structures; (3) diversity exploitation; (4) error control coding; (5) networked systems; and (6) alternative modulation strategies. Advances in each of these areas as well as perspectives on the future challenges facing them are presented. A primary thesis of this paper is that increased integration of high-fidelity channel models into ongoing underwater telemetry research is needed if the performance envelope (defined in terms of range, rate, and channel complexity) of underwater modems is to expand     

The measurement of marine geothermal heat flow by a multipenetration probe with digital acoustic telemetry and insitu thermal conductivity

The design and use of a marine heat probe with capability for measuring thermal conductivity insitu with high accuracy, and providing digital acoustic transmission of data to the ship, is described. The instrument employs the violin bow strength member and parallel sensor string configuration suggested by C. R. B. Lister. Several hundred measurements have been made in the deep ocean on multipenetration or pogostick profiles using a 3 m probe and in deep inlets of western Canada using a 7 m probe. The insitu thermal conductivity technique using a calibrated heat pulse has been studied in detail through laboratory calibration of the probe in materials of known conductivity, through numerical models, and through comparison of insitu measurements with needle probe measurements on sediment cores taken from the same sites. The insitu technique permits a conductivity accuracy of better than ±5% with a recording time of 7 minutes following 7 minutes in the bottom to establish the geothermal gradient. The pulse heating is also more energy efficient than the conventional continuous heating technique.Contribution from the Earth Physics Branch No. 807.     

High-frequency acoustic backscattering characteristics for acoustic detection of the red tide species Akashiwo sanguinea and Alexandrium affine

Harmful algal blooms (HABs), caused by the overgrowth of certain phytoplankton species, have negative effects on marine environments and coastal fisheries. In addition to cell-counting methods using phytoplankton nets, a hydroacoustic technique based on acoustic backscattering has been proposed for the detection of phytoplankton blooms. However, little is known of the acoustic properties of HAB species. In this study, as essential data to support this technique, we measured the acoustic properties of two HAB species, Akashiwo sanguinea and Alexandrium affine, which occur in the South Sea off the coast of Korea. Due to the small size of the target, we used ultrasound for the measurements. Experiments were conducted under laboratory and field conditions. In the laboratory experiment, the acoustic signal received from each species was directly proportional to the cell abundance. We derived a relationship between the cell abundance and acoustic signal received for each species. The measured signals were compared to predictions of a fluid sphere scattering model. When A. sanguinea blooms appeared at an abundance greater than 3 500 cells/mL, the acoustic signals varied with cell abundance, showing a good correlation. These results confirm that acoustic measurements can be used to detect HAB species.     

A low-cost acoustic positioning system for small manned submarines

A low cost range-range acoustic positioning system using modified "off-the-shelf" components was developed for use with small manned submarines. Range information is displayed for the submarine pilot and relayed to a microcomputer aboard the surface support vessel. Software developed for the surface-vessel positioning also plots, displays, and records the submarine position. This system was successfully used to navigate the submarine "Porpoise" during search operations off the Rhode Island coast.     

海底沉积物声衰减同轴差距衰减测量法与其他测量方法的对比研究

以精度较高的现场测量Hamilton的原位测量所得数据为标准,对比研究了钱正明的直立同轴衰减测量法、刘强的垂直轴差距衰减测量法和我们课题组的同轴差距衰减测量法等3种研究海底沉积物声衰减的实验室测量方法。通过3种实验测量方法所得数据跟Hamilton原位测量所得数据进行对比,分析了以上实验室方法的优缺点,并分析了声衰减与样品长度和测量频率的关系。同轴差距衰减测量法与直立同轴衰减测量法相比,前者在消除测量仪器和耦合衰减误差方面,以及在实验样品长度方面更合理。同轴差距衰减测量法与垂直轴差距测量法相比,同轴差距衰减测量法在原理上保证了声波传播能量接收的同轴性和同指向性,其实验数据比利用相同实验仪器的垂直轴差距测量法的数据更准确。     

Drag-type in-situ acoustic measurement system: Design,operation, and experimental results

The Drag-type In-Situ Acoustic Measurement System (DISAMS) is a new instrument designed for rapid measurement of seabed sediment acoustic parameters, including the sediment sound velocity and acoustic attenuation coefficient. The DISAMS consists of six independent acoustic probes arranged symmetrically such that each side has one transmitting probe and two receiving probes. The entire operation is controlled and monitored in real time by a deck control unit on board, and the center frequency of the DISAMS is 30?kHz. The DISAMS can record full waveforms to determine the sound velocity and attenuation coefficient in the seabed sediment. In addition to site measurements, the DISAMS can also carry out towing measurements, resulting in improved efficiency compared with existing in-situ acoustic measurement systems. This article presents the configuration, electronics, and tests of the DISAMS in detail. Laboratory tests were conducted in a sediment pool, sea trials were carried out in the Zhoushan Islands, and sound velocity and attenuation coefficient data were obtained. The test results demonstrated that the DISAMS performed well and was able to measure the sound velocity and attenuation coefficient rapidly and accurately in both site measurements and towing measurements.