An acoustic remote sensing study of an arctic submarine spring plume

Colbourne, Eugene B. (1987) An acoustic remote sensing study of an arctic submarine spring plume. Masters thesis, Memorial University of Newfoundland.

[English] PDF (Migrated (PDF/A Conversion) from original format: (application/pdf)) - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. Download (10MB) [English] PDF - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. (Original Version)

Abstract

A brackish water plume rising from a submarine spring was mapped using a 192 kHz acoustic sounder and a microwave positioning system. The submarine spring is located at 47 m depth at the head of Cambridge Fiord, Baffin Island. The results demonstrate the advantages of using acoustic remote sensing techniques to detect and characterize buoyant plumes in the ocean. The computer enhanced acoustic images yield critical information on maximum height of rise, plume shape, horizontal extent and vertical velocities in the rising plume. Additional observations included CTD measurements from a launch moored at different positions relative to the plume axis, and visual observations together with CTD and current measurements from a submersible. -- The initial geometry and water properties of the flow at the vent together with the ambient stratification were used as inputs to a standard numerical model of a buoyant plume. The maximum height of rise calculated by the model is within the range determined from the acoustic images. The experimental and numerical plume widths show the same general increase with height. The results indicate a freshwater discharge rate of 0.05 to 0.10 m³/sec. Vertical velocities calculated by the model are of the same order as those inferred from the acoustic images. However, discrepancies exist between calculated and observed variations of velocity with height. Numerical calculations of vertical velocities at 10 m height are about 35 cm/sec and decrease linearly to about 20 cm/sec at 20 m height. Vertical velocities calculated from the acoustic images have a similar magnitude but do not decrease in the 10 m to 20 m height range. -- It is shown in the thesis that the radial decay of the acoustic backscatter is too rapid to be explained by simple radial spreading of conservative point scatterers. In fact the radial decay corresponds better to a simple model of turbulence decay. Also experimental observations indicate that the scatterers are not bubbles, suspended sediments of biological organisms. The correlation between the acoustic backscatter intensities and the fine structure in the temperature and salinity field plus the radial decay of these quantities in the spreading plume point to the backscatter mechanism being acoustic refractive index fluctuations caused by turbulent mixing.

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