This paper suggests a new approach based on narrow-band, low-frequency data using air-deployed shots recorded on widely distributed large aperture vertical arrays. This approach uses fast, cheap, and high S/N data. Results to date with a simulated three-dimensional (3-D) eddy environment show that efficient characterization of the environment plus careful selection of the source/array geometry can lead to highly accurate estimates of the 3-D sound-speed profiles, e.g., maximum errors less than 0.2 m/s.
An environmental/acoustic model of sound propagation in the Arctic Ocean, which accounts for reflection losses from ridged sea ice, has been developed. In this model sea-ice ridges are represented as infinitely long, randomly distributed, elliptical half-cylinders. Under-ice reflection losses for acoustic wavelengths either large oF small compared to ridge dimensions are computed from theoretical equations as a function of average keel depth and width, number of ridges/km, and grazing angle. Numerical values of under-ice reflection loss as a function of grazing angle are then incorporated into ray-theoretical computations of transmission loss assuming a single sound-speed profile which is characteristic of the central Arctic Ocean. The validity of the concepts and approximations, the limitations of the model, and the accuracy of coincident measurements of environmental and acoustic parameters required to validate the model are discussed. To illustrate the predictions and accuracy of the model under diverse ice conditions, several comparisons of theoretical and experimental determinations of under-ice transinission loss in the central Arctic Ocean are presented. Midwater absorption loss, a complicating feature of transmission loss measurements at high frequencies, is also considered. Subject Classification: [ 43 ] 30.20, [ 43 ] 30.30.
Absorption losses at the resonance frequencies of sardines, 1.3 kHz (18 dB) at night, 1.7 kHz (15 dB) during the day and 2.7 kHz (35 dB) at dawn, were observed at a range of 12 km at a shallow water site in the Gulf of Lion in September of 1995. These observations were made during Modal Lion, a multidisciplinary experiment, which was designed to isolate absorptivity due to fish from other effects on long range propagation. Systematic changes in the resonance frequency of dispersed sardines were consistent with concurrent echo sounder observations of the vertical migration of sardines at twilight. Comparison of transmission loss measurements with a numerical sound propagation model that incorporates absorption layers in the water column permitted estimation of the average absorption coefficient, depth and thickness of absorption layers. Depths and thickness of layers estimated from sound propagation measurements were in good agreement with echo sounder data. Measured resonance frequencies, f0, of dispersed fish were in good agreement with theoretical computations based on measured swim bladder dimensions. The measured resonance frequency of sardines in schools, which were at a depth of about 65 m, was about 0.6 times the resonance frequency of dispersed sardines. The observed frequency shift is consistent with an analytical equation of the fundamental resonance frequency of a “cloud” of bubbles, an average separation between fish in schools of about 0.8 fish lengths and 5×103 fish per school. These estimates are consistent with previously published estimates of these parameters. The resonance frequencies of other absorption lines are consistent with the hypothesis that absorptivity due to sardines generally manifests itself in pairs of absorption lines, which correspond to the resonance frequencies of an ensemble of individual sardines, and the resonance frequencies of an ensemble of schools. Matching of theoretical calculations with inferences of the absorption coefficient permitted estimation of the average number of dispersed sardines per unit volume, which also consistent with previously published estimates. The results presented here suggest the possibility of long term tomographic mapping of fish parameters over large areas using broadband transmission loss measurements.
Absorption losses of up to about 10 dB at night and 25 dB at twilight at the resonance frequency of pelagic fish, at a range of 12 km were observed during MODAL LION, a multidisciplinary experiment designed to test the feasibility of isolating biological scattering from other effects on long-range propagation at a relatively shallow (83 m) site in the Gulf of Lion. Comparison of measurements with a theoretical model that incorporates absorption layers in the water column permitted estimation of the average fish length (or bladder size), the number of fish/m3, and the depth of the layers. Good agreement between estimated parameters and echosounder and direct sampling results suggests the possibility of long-term tomographic mapping of fish parameters over large areas.
Quasisynoptic measurements were made to determine the spatial variability of ambient noise at the Arctic ice-water boundary over a frequency range from 100 to 1000 Hz. The results of this investigation show that the ice-water boundary acts as a spatially well-defined source of ambient noise with measured noise levels near a compact edge about 12 dB higher than open water levels and about 20 dB higher than levels far in the ice field. Measured noise levels near a diffuse ice edge were about 4 dB higher than open water levels, and about 10 dB higher than levels far in the ice field. The relatively high noise levels at the ice edge are probably generated by wave and swell interactions with individual ice floes.
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