techniques

Oceans form two-thirds of the surface area of our planet. Our understanding of this environment even after several decades of research is best described as rudimentary. The watery abyss is home to the world's largest and most diverse biological ecosystems. Monitoring of these ecosystems and the study of the physical and biological evolution of the oceans is of paramount importance to our increased comprehension and understanding of the oceans. Efficient monitoring relies on the collection of data over long periods of time and in diverse sea conditions. Stationary or moving sonar systems have already proven invaluable for this task. However, most of these systems are fixed in their mode of operation, possessing little or no flexibility. Thus, when the environment changes data retrieval often becomes difficult or impossible. Changing the characteristics of the monitoring device to match the local sea conditions is a possible solution to this problem. In this paper the hardware and software implementation of a system that realises this function is presented together with some preliminary transmission tests in the Mediterranean Sea. The system, VERTLINK is designed to collect, store and transmit physical and bio-acoustical data to an end-user. The system, a half-duplex underwater communication link, com- prises two units-a sea-bed unit and a surface unit. The sea-bed unit, under the control of the surface unit, records oceanographic data and then transmits it to the sea-surface unit. The latter is linked to a computer station on-board a survey vessel via a physical cable or to shore station by means of a radio link. Using this system the end-user is able to change the way the sea-floor unit acquires and transmits data. The modulation rate/technique can be changed from 2-DPSK to 8-DPSK, allowing data retrieval to be changed according to sea noise conditions. The power level can be varied from 20% to 100%, to achieve more efficient use of power resources and the sea-floor unit can be told to release.

A mobile video/acoustic system (MVA), previously developed (Dudzinski et al. 1995), was modified with new hydrophones and a new hydrophone mount to increase the recorded frequency bandwidth and to decrease low frequency noise due to wobble in the physical mount, respectively. The MVA permits real-time synchronous recording of vocal and behavioural activities of individual dolphins. The system is swimmer-propelled and facilitates localisation of dolphin sound sources by associating video data of animal distributions with audio data from two hydrophones spaced relative to the human interaural distance as scaled to sound speed in water. A recent addition to the original design was a small second housing containing a Sony TCD D8 digital recorder together with a pre-processor circuit which detects the highly directional part of a dolphin's echolocation 'clicks' and makes these audible and recordable. The pre-processor comprises a preamplifier with band pass filtering which selects only signals between 90 kHz and 130 kHz. The filter output is then rectified and the low frequency envelope extracted by a further filter. This technique produces a low frequency pulse accurately representing the original signal amplitude which can be recorded on the R- DAT sound track. As the directionality of the original signal, the inter-pulse intervals and the pulse amplitudes are retained this band-limited data carries significant information and the loss of the high frequency spectrum is acceptable in this application. This 'click detector' data is recorded onto one track of the recorder while unprocessed low frequency signals from the hydrophone are captured on the second. The R DAT stores real-time information (date and time) in the sub-code of recorded signals so that video image data, with in-picture timecode, can be correlated accurately with the echolocation behaviour. Data is captured when dolphins swim directly towards the MVA and click detector unit. The click detector housing mounts to the dorsal surface of the MVA housing, and externally measures 37cm by 9cm by 5cm. The modified MVA system (including click detector) has been used to document the behaviour and vocalisations of interacting Atlantic spotted dolphins Stenella frontalis in the Bahamas and bottlenose dolphins Tursiops truncatus in Japan. Example data sets will provide evidence of this system's utility for examine the social lives and signal exchange among interacting, free-ranging dolphins.

Underwater acoustic tracking techniques mostly rely on an active acoustic source triggering replies from an array of transponders. If the tracking equipment must remain passive, the transponders are replaced by hydro- phones. This research is to develop systems which can passively locate and track echo-locating cetaceans, with a view to reducing the by-catch of cetaceans in fishing nets, particularly in pelagic trawls. A 'flat' array of four hydrophones in two parallel streamers can be used to produce these tracks. The streamers are attached to the top of the trawl and 'fly' in a reasonably stable manner as the trawl is towed through the water. A fifth hydrophone is placed on the trawl headline, slightly above the plane of the two streamers, so as to obviate any ambiguities in the position-fixing computation. The bandwidth limitations of the coaxial cable transmission link to the towing vessel led to the use of a look-up table to determine positions. This paper describes the technology of the system and the position-fixing procedure associated with it.

Inter Pulse lnterval (IPI) measurements on clicks recorded from diving sperm whales have been confirmed to be useful in assessing the whale's body size (Gordon 1991, Goold 1996a, 1996b). Goold (1996a, 1996b) developed a cepstrum-based method to accurately measure the IPIs and to assess the animal size by taking into consideration the acoustic transmission properties if the spermaceti oi1 under different temperature and pressure conditions. To extensively apply IPI measurements, we developed a program, based on our custom real-time Digital Signal Processing Workstation (Pavan et al. 1997), to show in real-time both the spectrogram (spectrum vs time) and the cepstrogram (cepstrum vs time), optimised for this special purpose, of the click sequences received by means of a towed array. The cepstrogram based method resulted very sensitive and capable of displaying low level clicks otherwise difficult to see on the spectrogram and the real-time approach proved very helpful in browsing long recordings as well as in discriminating and counting different whales clicking at the same time. We extended the analysis on sighted and tracked whales to consecutive dives. Our results were consistent throughout whole dives and never showed the scattering of IP1 values previously by other authors, even though we confirmed IPIS slightly varied according to the variation of spermaceti properties with the progression of the dive. Therefore, spot measurements on short sequences of clicks should be considered reliable. Even though the method proved very effective, some problems still remain unresolved, in particular those concerned with the influence of long range sound propagation paths and of the relative position and orientation of the clicking whale in regard to the hydrophone. This research was carried out within a project granted by the Central Inspectorate for Sea Protection of the Italian Ministry of the Environment (1989-1996).

Classification of dolphin whistles has generally been a manual process requiring a trained observer, and calling for subjective and qualitative decisions. As a result it has been difficult to determine the differences and similarities between dolphin whistles. Software developed in the Underwater Acoustics Group at Loughborough University automates many of the aspects of classification of dolphin whistles, and provided quantitative probabilities for class memberships. Using the software, whistles may be detected on recordings, enhanced and background noise removed, their frequency-time contour traced and then encoded into a more compact data structure. Pattern recognition techniques can then more easily be applied to such encoded whistles. Examples of the use of the extraction, encoding, and classification methods are shown using the software, demonstrating the current levels of automation. Quantitative class probabilities for the signals are calculated which can be used to determine identities of different groups of dolphins in the field.

A statistical pattern recognition technique for classification by supervised learning was developed and applied to automated recognition of marine mammal sounds [J. Acoust. Soc. Am., 101(3), March 1997]. Training data identified by human experts are characterised by occupancy statistics associated with a multiple-resolution, binary partition of the unreduced observation space. Classification of a new sample is performed by Bayesian inference applied to these occupancy statistics. The classification algorithm is implemented in a simple, highly efficient computer program. The present work describes efficient encoding of the training data distributions for large numbers of training samples and classes, and efficient evaluation of a posteriori probabilities of class membership for classification of new samples. Data storage requirements and computational efficiency of the classification algorithm are compared with theoretical bounds.

The Atlantic bottlenose dolphin Tursiops truncatus is the best studied example of successful adaptation of biosonar to the reverberant and variable VSW/SW environment. With evolved and learned adaptations, it can successfully search for, detect and classify objects in conditions that usually defeat the best artificial sonars. The adaptations of the transmitting, receiving and analysing systems that reduce reverberation and noise are reviewed as background for recent studies of the dynamics of biosonar performance in open-fields; work that suggests still further means of enhancing the S/N ratio. The open-field work uses new experimental methods that control for additional extraneous variables as well as instruments that continuously record the animal's location, head-attitude, head-azimuth, interpulse intervals and emission spectra. The instruments include a high- speed data acquisition package that is carried by the animal and a guide boat that is equipped with a high-accuracy DGPS, an interactive field-display for the driver and a computer that coordinates time, position and data acquisition. Systematic measurements with these methods allow for the analysis of the animal's distribution of biosonar energy in 3D space, time and frequency as a function of practice effects, object distance, grazing angle and environmental variables.

A subtidal-zone (<10 m) system for sound transduction, acquisition, and archival was developed to study and monitor harbour seals. In developing the transduction system - a multi-channel hydrophone array - several engineering challenges were solved: cable armour was applied and smooth low-noise hydrophone cages were designed to protect against animal bites. Helical anchors were used to attach the cable to the sandy bottom; they are adjusted as necessary when sand is moved by currents. The cable was weighted so that it sank into the sand to prevent cable strum from wave action. The acquisition and archival system had several engineering goals: simultaneous multi-channel sound acquisition; changeable sampling rate; high data rate (200 Kbyte/s); large storage volume to allow continuous long- term monitoring; configurable recording times. The system developed was a PC with a multi-channel data acquisition board (SignaLogic), custom software that allows automatic reconfiguration (channels recorded, sampling rate, etc.) at preset times, and a high-capacity (4 Gbyte uncompressed) data DAT drive. The tape drive software allows acquired data to be accessed directly as computer files, obviating a separate re-acquisition step. The acquisition system compares favourably to multi-track tape recorders in price and very favourably in recording capacity.

Real-time and continuous passive acoustic monitoring of the ocean is one goal of the Monterey Bay Aquarium Research lnstitutes Ocean Acoustic Observatory project. Continuous real-time monitoring makes it possible to observe episodic and non-predictable phenomena such as biological and geological events. Animal calls, turbidity flows, underwater landslides and volcanic eruptions are examples of such events. MBARI has developed a state- of-art real-time system for multi-channel acquisition, beamforming, processing and archiving of acoustic data. The system is capable of collecting data from up to 32 hydrophones at the maximum sampling rate of 200 kHz per channel and can form up to 68 simultaneous beams in real time. A four- processor application accelerator performs various signal processing functions on the raw or beamformed data. Spectral analysis such as spectrogram computation, automatic detection, localisation, classification and display of sounds can be performed in real time. An event-based recording scheme can be selected to record only acoustic events of interest. The system is expandable to handle up to 512 channels of data from fixed or towed hydrophone arrays. This paper presents a detailed description of the acoustic observatory system, implemented algorithms and results from fixed or towed arrays from the Monterey Bay ocean region in Northern California.

The study of marine mammals is generally characterised by individuals or small teams working on a very tight budget. The use of advanced electronic technology to assist these studies has traditionally been severely limited by funding. However, the trend in microelectronics, driven by the demands of the personal computer and home entertainment markets is to continually reduce the cost of technology and to make more powerful systems available to the mass market. The marine mammal research area can now benefit from this advanced technology to provide a number of useful tools. At the lowest levels, the advances in basic analogue device performance now allows the building of hydrophone-preamplifier units with good noise performance and high dynamic ranges to achieve small, low-cost analogue channels for underwater acoustic data acquisition. The current generation of analogue-to-digital convertors allows the full bandwidth of these signals to be converted for subsequent digital processing. The demands of the personal computer market are making computer power more easily available to the researcher at a number of levels, ranging from the new RISC-based personal digital assistants which are ideal for data acquisition in the field to high performance desk-top machines based on Pentium or PowerPC processors capable of demanding spectrum analysis tasks. This paper explores the advances made in recent years in both analogue and digital devices to demonstrate the level of technology now available and conceptually design a number of systems that could be assembled to aid marine mammal research.