Side-scan sonar observations of upper ocean processes

This thesis describes a study of near-surface ocean processes based upon observations made with a novel acoustic remote sensing streamlined instrument, ARIES II. The observations have been carried out on August 1995 as part of the LOIS-SES programme at a position on the Malin Shelf. ARIES II has been deployed on a ‘U-mooring’ at a mean transducer depth of 34.6 m, at about 4 km East from the 200 m isobath and into 146 m water depth. High-resolution observations in a deep ocean environment reveal a wide range of acoustic signatures: the tide, non-linear internal waves of depression, rain squalls, Langmuir circulation, wind-sea in the spectral band (0.1882-0.3106) Hz, swell [(0.0938-0.1016) Hz], wave groups (0.0078 Hz), breaking surface waves, bubble plumes and biological activity. The topics addressed include the performance of the streamlined instrument, the interpretation of acoustic imagery, the derivation of formulations to obtain quantitative information and the concurrent upper ocean processes captured in the images. ARIES II was operating continuously and unattended in a zone remote from shore, and it has proven to be a reasonably stable working framework that remains tilted 5.1° up to fore and 1.0° up to starboard. Variations of ±10° along the fore-aft axis of the frame are caused by surface wave groups during high wind conditions. Non-linear internal waves have induced a maximal tilt of 10° down to starboard and a maximal vertical excursion of 1.7 m. An important gap in knowledge filled by ARIES II regards its capability to provide detailed measurements of the internal wave straining. The current investigation has revealed that the main factors affecting the appearance of the acoustic features observed in sonographs are the instrument heading and the orbital motions of surface waves.