Ocean acoustic tomography: a scheme for large scale monitoring

Temporal coherence of acoustic signals propagating through fluctuating oceans has received much attention in the literature as the rate of signal coherence loss impacts the sonar performance. This paper presents the measurement results of temporal coherence of acoustic signals propagating through shallow water using data from three experiments in three different parts of the world. The data are broadband signals covering three frequency bands: low frequencies below 1.2 kHz, mid frequencies between 2 and 5 kHz, and high frequencies between 18 and 22 kHz. The source‐receiver range covers 3, 5, 10, and 42 km. Measurement issues and environmental impact on the temporal coherence are discussed. The signal coherence time, defined by the time that the signal becomes decorrelated, is deduced from the data. The signal coherence time varies with the source‐receiver range and the signal frequency and is significantly influenced by the medium fluctuations. The data can be fit with a simple power law dependence on the range, frequency, and sound speed standard deviations. [This work is supported by the U.S. Office of Naval Research.]

Measurements of temporal coherence of sound transmissions through shallow water

Coral reef fish communities often include hundreds of sympatric species which are of great inter- est to reef conservation and fisheries managers. Long- term acoustic monitoring of fish sounds can be used to infer periodic reproductive activity and changes in pop- ulation abundance. However, limited records of sound production by coral reef species have precluded the ap- plication of acoustic monitoring at the population or community levels. We used rebreather and digital acoustic/ video techniques to produce a sound library for fishes on coral reefs of west Hawai'i Island, HI, USA. We documented 85 sounds produced by 45 (47%) of the 96 resident species that were associated with agonistic interactions and resource defense, reproduction, nest defense, feeding, and vigilance behaviors. Most non- feeding sounds consisted of single or trains of pulse events <100 ms long that were distributed across a spectrum of <100 to 1000 Hz with the majority of peak frequencies between 100 and 300 Hz. Agonistic sounds created during competitive interactions over food, space, or nest brood resources were identified for dam- selfishes, surgeonfishes, butterflyfishes, and trigger- fishes, among others. Reproductive sounds associated with courtship, spawning, or nest defense were pro- duced by damselfishes, goatfishes, butterflyfishes, par- rotfishes, and surgeonfishes, as well as wrasses and Moorish idols. The distinct adventitious feeding sounds recorded for some parrotfishes and triggerfishes oc- curred in a higher frequency band (2�6 kHz) and may be useful indicators of feeding activity and rates of reef bioerosion. This is the first study to characterize the spe- cies-specific behavior soundscape that can be applied to acoustic monitoring of a coral reef fish community.

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Acoustic behaviors in Hawaiian coral reef fish communities

Publisher Summary The mechanisms of sound production by many fishes and the sounds themselves have formed an active area of research. Although sound production may be restricted to some as unknown fraction of all fish species, it is apparent that all fishes must be capable of receiving acoustic stimuli. Sound detection in an aquatic medium presents certain problems not normally encountered by terrestrial organisms, for example, the separation of pressure from displacement detection seems to be characteristic of aquatic animals. Any study of sound production or sound detection in fishes necessitates an understanding of the acoustic properties of water as a medium. Many species, such as certain deep-sea forms, appear to possess the means for sound production, but have not yet been recorded. Three general types of sonic mechanisms are present in fishes: stridulatory, hydrodynamic, and swim bladder. Stridulatory sounds are produced by friction of teeth, fin spines or bones. Hydrodynamic sounds result from swimming movements, especially during rapid changes of direction or velocity. The swim bladder acts as a sound projector when it is vibrated by contiguous or attached muscles.

6 Sound Production and Detection

Passive acoustic and digital video record- ings were used to investigate sonic activity and behav- ior of red grouper Epinephelus morio on the West Florida Shelf. Red grouper were found to produce a unique series of low-frequency (180 Hz peak) pulses consisting of 1 to 4 brief (0.15 s) broadband pulses and a 0.5 to 2 s growl (short call); occasionally these were followed by a rapid series of 10 to 50 broadband pulses (pulse train). Sound production was ob served through- out the day and night, but most sounds occurred between sunrise and sunset, with a noticeable increase during late afternoon. Behaviors asso ciated with sound production included solitary male activity and court - ship interactions, indicating that sound production is likely related to spawning activity. Thus, passive acoustics could be an effective tool in monitoring red grouper reproduction and defining critical habitat of a keystone species.

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Sound production of red grouper Epinephelus morio on the West Florida Shelf

In July 1972, as part of the cooperative studies between the Miami and Michigan Project MIMI research groups, continuous fixed‐system underwater sound propagation tests at 406 Hz were commenced between a sound source off Eleuthera, Bahamas, and receivers at Bermuda, a distance of 1250 km, and at positions intermediate in range with respect to the 1250‐km path. Preliminary studies of long‐period CW transmission fluctuations at Bermuda and at one intermediate position have been completed. The long‐period phase and amplitude fluctuations are qualitatively quite similar to those observed in the Straits of Florida and in other relatively shallow water locations. Phase fluctuations associated with the ocean tides are a marked feature of the data. With averages taken over two‐day time spans, significant variations in the statistical properties of transmission loss have been observed over a five‐month time period. Shorter‐period fluctuations in acoustic phase (time scale of a few minutes to a few hours) have simi...

Long-period fluctuations of CW signals in deep and shallow water

Sound waves of 420 Hz continuously transmitted across the Florida Current from a fixed source to fixed hydrophones are used to investigate relationships between temporal fluctuations of the waves and changes of Florida Current transport and water‐column temperatures. The amplitude and phase of the received waves show variations with periods in the range from seconds to weeks and longer‐term trends. Characteristically, variations of acoustic phase correspond to changes of transport and temperature, but fluctuations of acoustic amplitude are noiselike and not readily identifiable. Long period changes of transport and temperature result in slowly changing phases of the multipath arrivals and varying patterns of interference and amplitude fading. Spectra of the amplitude fluctuations show components over a wide frequency range which mask components corresponding to transport and temperature changes. The above characteristics also appear in calculations with ray‐propagation models in which sound speed versus depth was varied with time in accordance with measured variations. Since the models have plane parallel boundaries, characteristic features of the variations appear to be independent of surface and bottom irregularities. This raises questions of modeling amplitude fluctuations from transport and temperature changes obtained from acoustic phase measurements. The paper summarizes relevant observations and analyses.

Fixed‐System Studies of Underwater Acoustic Propagation

A hydrophone assembly with terminals the Lerner Marine Laboratory, Bimini, Bahamas, has been installed on the east bank of the Straits of Florida in order to study the sounds produced by marine animals in the natural environment. Sounds from two bottom‐mounted hydrophones, one in 100 ft of water and the other in 1200 it, were recorded on dual track, magnetic tape on a 24‐h basis. The tapes were played back at eight times recording speed and monitored for sounds possibly of marine animal origin. More than 25 categories of sounds were observed during the initial period from November 1960 to July 1961. A number of the categories showed repetitive, diurnal patterns and possible seasonal patterns of soniferous activity. In general, sound categories the shallow and deep sites were different. Usually, only a single category was heard from a hydrophone at one time. Some tentative identifications have been made.

Hydrophone Installation for the Study of Soniferous Marine Animals

Results are presented of measurements of the performance of Savonius rotor type instrumentation being used for high resolution current profile studies in the Straits of Florida. The instrumentation housing is designed to hold the rotor horizontally during a profile measurement of the water column, but internal and external forces act to displace the profilers from their designed horizontal axis and introduce errors into their measurement of current velocity. The necessity for determining the occurrence and magnitude of these errors prompted this investigation into the effects of tilt on a Savonius rotor exposed to a turbulent flow regime, plus the development of an instrument to measure rotor inclination. Although the rotor tested was not a true Savonius rotor and actual velocity profiles are not uniform, the results are believed to be applicable to similar instrumentation. Significant differences in tilt effects between the measurements in this study and previous results were observed. Recommendations for reexamination of housing designs and careful calibration under controlled conditions are included.

The Effects of Tilt on a Savonius Rotor Exposed to a Turbulent Flow Regime

A 24-foot compliant-tube parabolic reflector, with a flextensional transducer at its focus, has yielded acoustic source levels up to 122 dB?baryd1 at 420 Hz in 85 feet of water off Fowey Rocks near Miami, Fla. The projector is part of a research program to study underwater sound propagation and related environmental parameters. Modulation signals of continuous wave pulses and pseudorandom sequences are transmitted by wire from the Institute of Marine Sciences Laboratory, Miami, to the transducer, a distance of 12 miles. Bottommounted hydrophones are employed at various distances, with the principal sensors on the island of Bimini, which lies 45 miles eastward across the Florida Straits. The reflector provides a main lobe of 28° at the half-power points; the back lobes are down more than 25 dB and the directivity index is 15 dB. The effective mechanical Q of the transducer-reflector combination is 4. At the present depth, the transducer is cavitation limited to an acoustic output of 4 kW.

Morton Kronengold

Papers

Long-period fluctuations of CW signals in deep and shallow water

Fixed‐System Studies of Underwater Acoustic Propagation

Hydrophone Installation for the Study of Soniferous Marine Animals

The Effects of Tilt on a Savonius Rotor Exposed to a Turbulent Flow Regime

Directional 420-Hz Sound Source