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Measurements of low frequency acoustic backscatter from the sea surface

University of British Columbia

The overall objective of this thesis was to predict, model and measure low frequency acoustic backscatter from the sea surface zone (SSZ). In particular, the objectives were fourfold: to relate the acoustic backscatter Doppler spectrum to the directional waveheight spectrum (DWS) through a perturbation analysis; to develop instrumentation suitable for measuring the properties of acoustic backscatter from the SSZ; to design and implement signal processing hardware and software to process raw data from the instrument; and to deploy the instrument and make measurements to test the validity of the predictions of the theoretical development. A theoretical framework was developed to enable a test of the acoustic analogue of the Coastal Ocean Dynamics Applications Radar (CODAR) technique, using beamforming techniques to simulate the CODAR antennas. Expressions relating the CODAR antenna outputs to the output of an array of omnidirectional acoustic point sensors were developed, and mathematical algorithms and techniques were derived to extract information about the DWS of surface gravity waves from acoustic Doppler backscatter measurements with the array. Models were developed and implemented, showing the expected form of the power spectral density of the acoustic Doppler backscatter seen by single omnidirectional receivers, and the expected form of data products of the beamformed array. An acoustic instrument — the Upward-Looking Sonar Array System (ULSAS) — was developed for stand-alone, remotely controlled operation in both bottom-situated and deep-water, surface-tethered configurations. This device can collect and store large quantities of acoustic data from a multi-element array, under the control of a distant operator over a radio link. The bottom-situated version was deployed in the coastal waters of British Columbia, and the deep water version was deployed in the recent Surface Wave Processes (SWAPP) experiment. A preliminary test of the acoustic CODAR technique was made, yielding information consistent with the known wind and wave field. The form of the non-directional part of the extracted DWS followed approximately the expected k⁻⁴ shape for k values above saturation. Beamforming results using frequency-domain data show that the Doppler-shifted acoustic backscatter is directional in nature. These are the first results of this kind to be reported. The deep-water version of ULSAS was tested for the first time during the SWAPP cruise. In spite of a problem limiting the power output of the projector, estimates of the surface scattering strength parameter over angles of incidence less than 45° were made, showing some surprising departures from the Chapman-Harris empirical formula for S₅ , and interesting angular structure. Measurements of the ambient noise field were also made under calm conditions and during 14 kt winds.

Text

2011-01-31

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http://hdl.handle.net/2429/31022

Oceanography

University of British Columbia

Graduate