Title

An Experimental Study Of The Coherent Under-Ice Reflectivity Of Sound In The Greenland Sea Marginal Ice Zone

Date of Award

1987

Availability

Article

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Applied Marine Physics/Ocean Engineering

Abstract

The coherent component of acoustic under-ice reflectivity was investigated as a function of frequency and grazing angle in the Marginal Ice Zone (MIZ). Measured reflectivity was compared with predictions by scattering theories to test the hypothesis of ridge scattering dominance in the MIZ. Explosive source acoustic signals were received on a vertical array, during the Marginal Ice Zone Experiment (MIZEX 84), and deconvolved to separate the direct and surface-reflected arrivals. Data was available from multiple days and ranges, resulting in an ensemble of ice-scattered measurements. The surface arrivals were aligned in time and coherently averaged over grazing angle for frequency bins from 64 to 256 Hz. Coherent reflectivity decreased with grazing angle from 12$\sp\circ$ to 35$\sp\circ$ and with frequency from 64 to 256 Hz. Remote sensing data, taken during the experiment, showed that the ice cover was primarily composed of small floes ($<$200 m in diameter). Laser ice-surface height measurements were analyzed to infer a pressure ridge sail height distribution or surface rms roughness for input to corresponding scattering theories. The measured reflectivity was compared with reflectivity from smooth elastic plate theory, perturbation theory for a rough pressure release surface, perturbation theory for a rough elastic surface, and Burke-Twersky theory for hard and soft, infinite, semi-elliptical cylinders on a soft plane. Perturbation theories were limited to the low frequencies ($<$96 Hz) and low grazing angles ($<$30$\sp\circ$) due to the small waveheight criterion. The Burke-Twersky soft boss theory gave predictions of the coherent reflectivity at grazing angles below 30$\sp\circ$ and frequencies from 64 to 256 Hz, within 1 dB of the measured mean reflectivity when a ridge keel-to-sail ratio of 6.5 was assumed. The data-theory comparison suggested that under-ice scattering in the MIZ, at angles less than 30$\sp\circ$, and frequencies less than 256 Hz, is dominated by ridge-like scattering which depends on the geometric rather than the physical properties of the ice.

Keywords

Physics, Acoustics

Link to Full Text

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