Orest Diachok

Ambient noise vertical directionality in the northwest Atlantic

The vertical directionality of the ambient noise field was measured at a site between Cape Hatteras and Bermuda over the frequency range 45–100 Hz. The noise level was observed to decrease rapidly at angles beyond ±20 ° from the horizontal by as much as 20 dB. Slight peaks were observed near ±15 ° and at the horizontal. These peaks are hypothesized to be due to deep ocean shipping and slope enhanced signals from shipping located on the continental shelf edge, respectively. Theoretical calculations based on a parabolic equation propagation model and measured environmental information support these hypotheses.

Elements of a geoacoustic model of the upper crust

Elements of a geoacoustic model of the geologically young (< 10 million years) upper crust, the top few hundred meters of the basalt subbottom, are described. The model is based on analysis of low‐frequency reflectivity versus angle data at a 1 million‐year‐old sediment‐free site, as well as limited Deep Sea Drilling Project downhole logging results, and extrapolated seismic refraction measurements. The upper crust at this site, which is in close proximity to the crest of the East Pacific Rise, may be characterized by low interfacial velocities (Vp∼2800 m/s and Vs∼800 m/s), large gradients (3–5 s−1), and substantial rms roughness (∼5 m). The low‐inferred shear speed implies no shear critical angle. Hence, low‐frequency energy incident on the bottom at small grazing angles is in large part transmitted into the rock, refracted by the gradient, and reradiated into the water. Scattering loss occurs at both the initial incidence and the interaction of the refracted energy at the boundary. Low interfacial shear...

TOPOGRAPHIC RELIEF AND SEDIMENT THICKNESS : THEIR EFFECTS ON THE THERMAL EVOLUTION OF THE OCEANIC CRUST

We compiled a suite of 490 heat flow measurements from the Pacific Ocean in areas with less than 85 m of sediment. Heat flow patterns at different basement ages vary with topographic relief. Areas with reliefs 300 meters are conductively blanketed at 70–90 m.y. and convect out to ≥ 90–110 m.y.

Interpretation of the spectra of energy scattered by dispersed anchovies

The spectra of backscattered energy by dispersed anchovies, which were reported by Holliday (1972), reveal several peaks at frequencies that correspond to theoretically calculated resonance frequencies of year classes of anchovies. Theoretical calculations are based on concurrent measurements of distributions of swim bladder dimensions and a modified form of Minnaert’s (1933) equation. Differences between calculated and measured values of the mean lengths of the second-, third-, and fourth-year classes are within experimental uncertainties (±8%). The calculated mean lengths of juvenile anchovies are in good agreement with historical measurements of the bounds on this parameter (Butler, 1989). Matching of theoretical calculations and measurements of backscattered energy level versus frequency yields estimates of the total Q of the spectral line, QT, and the relative number density per year class. The resultant estimate of QT of adult anchovies is approximately 4.4. This value of QT is consistent with labor...

Concurrent inversion of geo- and bio-acoustic parameters from transmission loss measurements in the Yellow Sea.

This paper describes results of a simultaneous inversion of bio-acoustic parameters of fish (anchovies) and geo-acoustic parameters of the bottom from transmission loss (TL) measurements in the Yellow Sea, which were reported by Qiu et al. [J. Sound Vib. 220, 331-342 (1999)]. This data set was selected because the bio-absorptivity at their site was extremely large, 40 dB at 1.3 kHz at 5 km, and measurements were made between multiple source and receiver depths and ranges. Measurements were made at night when anchovies are generally dispersed. Replica fields were calculated with a normal mode model, which incorporates bio-absorption layers. The inversion was based on minimizing the rms difference, delta, between measured and calculated values of TL at all ranges and source and receiver depths, and involved a simultaneous search for bio-layer depth, bio-layer thickness, bio-alpha, geo-sound speed, and geo-alpha. The resultant small value of delta, +/- 1.7 dB, confirmed that the model, which was assumed in replica field calculations, was realistic, and that inverted parameters were meaningful. In particular, the inverted depth of the bio-absorption layer, 6.9 +/- 0.3 m, was consistent with theoretical calculations of the depth, 5.8 +/- 1 m, of 10-cm-long anchovies Engraulis japonicus, the dominant species in the Yellow Sea.

Bioacoustic resonance absorption spectroscopy

Absorption losses at the resonance frequencies of sardines up to about 18 dB at 1.3 kHz at night, 15 dB at 1.7 kHz during the day and 35 dB at 2.7 kHz at dawn were observed at a range of 12 km during Modal Lion, a multidisciplinary experiment designed to isolate absorptivity due to fish from other effects on long‐range propagation at a relatively shallow (83 m) site in the Gulf of Lion. 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. The depths and thickness of layers estimated from sound propagation measurements were in good agreement with echo sounder data. The measured resonance frequencies of dispersed fish at night were within 13% of theoretical computations, based on swim bladder dimensions which were derived from near coincident samples of adult (∼16 cm long) sardines. A smaller absorption line at 3.9 kHz...

BIOACOUSTIC ABSORPTION SPECTROSCOPY: ESTIMATION OF THE BIOMASS OF FISH WITH SWIMBLADDERS

Lavery, A.C., Stanton, T.K., Chu, D. & McGehee, D.M. (accepted). Three-dimensional modelling of acoustic backscattering from fluid-like zooplankton. Accepted by the J. Acoust. Soc. Am. Stanton, T.K. (1989). Simple approximate formulas for backscattering of sound by spherical and elongated bodies. J. Acoust. Soc. Am. 86, 1499-1510. Stanton, T.K. (1990). Sound scattering by spherical and elongated shelled bodies. J. Acoust. Soc. Am. 88, 1619-1633. Stanton, T.K., Wiebe, P.H., Chu, D., Benfield, M.C., Scanlon L., Martin, L. & Eastwood, R.L. (1994). On acoustic estimates of zooplankton biomass. ICES J. Mar. Sci. 51, 505512. Stanton, T.K., Chu, D. & Wiebe, P.H. (1998a). Sound scattering by several zooplankton groups. I. Experimental determination of dominant scattering mechanisms. J. Acoust. Soc. Am. 103, 236-253. Stanton, T.K., Chu, D. & Wiebe, P.H. (1998b). Sound scattering by several zooplankton groups. II. Scattering models. J. Acoust. Soc. Am. 103, 236-253. Stanton, T.K., Chu, D., Wiebe, P.H., Eastwood, R.L. & Warren, J.D. (2000). Acoustic scattering by benthic and planktonic shelled animals. J. Acoust. Soc. Am. 108, 535550. Stanton. T.K. & Chu, D. (2000). Review and recommendations for the modelling of acoustic scattering by fluid-like elongated zooplankton: euphausiids and copepods. ICES J. Mar. Sci. 57, 793-807.

Very-low-frequency under-ice reflectivity

This paper describes a direct method to model under‐ice reflection loss from analysis of ice draft data taken from a region of the Arctic near the FRAM IV experiment site. The water–ice boundary is modeled as a random distribution of infinite elliptical half‐cylinders fixed to a free surface. Burke and Twersky’s theory [J. Acoust. Soc. Am. 40, 883–895 (1966)] of scattering from a single cylindrical protuberance is used to calculate reflectivity from a distribution of scatterers. Individual ice ridge keels are identified from ice draft data resulting in a ridge keel depth distribution function spatially coincident with FRAM IV acoustic propagation paths. The scattering amplitude is weighted by the keel depth distribution function providing an effective ridge keel depth which is used to calculate the under‐ice reflectivity. The predicted reflection loss is in good agreement with those inferred from normal‐mode methods applied to FRAM IV acoustic field data received on a larger aperture vertical array.

Experimental demonstration of sound‐speed inversion with matched‐field processing

Matched‐field processing is shown to be effective for estimating sound speed in a deep, range‐independent ocean environment. The amplitude and phase of signals from a distant source measured on a large aperture vertical array are sensitive to changes in sound speed. This sensitivity is exploited to infer the environment’s sound‐speed profile by matching predicted and measured amplitude and phase. Simulations and an initial experimental demonstration of the power of the technique are based on a modified version of Munk’s canonical sound field model. This model is used in a simulated environment with a 15‐Hz source where source and receiver locations are known. The simulation demonstrates that changes in the depth and strength of the modeled sound channel axis directly result in trackable errors in range and depth as well as in reduction of the received power level. The same model is used to determine sound‐speed profile using a 15‐Hz signal from a 244‐m explosive source detected on a 675‐m vertical array a...

Effects of Upper Crustal Geoacoustic Parameters on Low Frequency Sound

A seismo-acoustic paradigm of the upper crust is proposed, based on analysis of low-frequency reflectivity versus angle data at a 0.5 million-year old sediment-free site, and on spatially well sampled seismic refraction measurements at sediment-free and thinly sedimented sites in the Pacific Ocean. Crustal velocity measurements at thick sediment covered sites are evidently substantially higher, and are excluded from the analysis presented here. Interfacial compressional and shear speeds of thinly sedimented, geologically young upper crust (between 0.5 and 5 million years) at this site are estimated to be about 2800 m/s and 800 m/s respectively. At such sites sub-basement gradients are approximately 4 s-1. The rms roughness of the basement, which is Fresnel zone size dependent, has significant effects at frequencies as low as 10 Hz. Low shear speeds imply no shear critical angle. Hence, at low shear speed sites, low-frequency energy incident on the bottom at small grazing angles is in large part transmitted into the rock, refracted by the gradient, and reradiated into the water. Scattering loss occurs at both the initial incidence and the interaction of the refracted energy at the boundary. Low interfacial shear speeds lead to large grazing angles at the boundary for the transmitted shear waves, large wavenumbers and, hence large boundary scattering losses; small changes in interfacial shear speed produce large changes in subsurface boundary-scattering loss, and hence in the reflection coefficient. Sediment-free/thinly sediment-covered crustal shear speeds are projected to increase with age, eventually (at an unknown age) becoming faster than the speed of sound in water, resulting in critical angle reflection.