Instrumentation for the High Resolution Measurement of Ocean Surface Waves and Currents over km Square Areas
Final rept. 1 Apr 2001-31 Dec 2002
SCRIPPS INSTITUTION OF OCEANOGRAPHY LA JOLLA CA MARINE PHYSICAL LAB
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The objective of this project, funded by the Office of Naval Research, was to develop imaging sonar with increasingly higher resolution. With modern advancements in digital electronics, it is now feasible to digitize incoming sonar data at the carrier frequency, in this case, approximately 3 x 50 kHz. Subsequent homodying and data compression can be done using software. The sonar technology was tested on two occasions on the research platform RV FLIP. On the first occasion it was mounted on FLIP at a depth of 20 meters and operated over a 20-day period with FLIP moored in 200 meters of deep water. In these summer experiments the mixed layer depth was very thin. The sonar was positioned in the upper thermocline, where sound is strongly downward refracted. Thus, the sonar scattered primarily from the sea surface for the first 800-meter range, and subsequently recorded a mix of surface and bottom echoes to ranges greater than 2.5 km. A second data collection opportunity occurred in Sep-Oct 2002, when FLIP was moored off the Hawaiian Island of Oahu, observing large amplitude internal waves generated by tidal flow over the Keana Ridge. Here the depth of the mixed layer was 25 meters and breaking waves, which provide subsurface bubbles as scattering targets, were common. The sonar was operated continuously for about 20 days, achieving ranges of 1.5 km from pure surface scattering. The dominant signal seen was the surface wave field, which was quite energetic during trade wind conditions. When these signals were low-pass filtered in time, images of underlying Langmuir cells emerged. Further filtering has begun to reveal the internal wave signature. The next developmental task is to create a real-time analysis and display capability to match the speed of this sonar, which digitally recorded at a rate of 100 Gigabytes per day.
- Physical and Dynamic Oceanography
- Acoustic Detection and Detectors