Directional Wave Measurements from a Subsurface Buoy with an Acoustic Wave and Current Profiler (AWAC)

Abstract

Directional wave measurements in deep water locations are intrinsically difficult to measure without the use of a surface wave buoy. Traditional acoustic Doppler current profilers do not have the appropriate data collection and processing technique to be mounted on a subsurface buoy. Nortek developed the SUV wave data collection and processing technique for measuring ocean waves from a subsurface buoy using a Nortek acoustic wave and current profiler (AWAC). In 2006 Nortek initiated a collaborative experiment to validate the SUV method and explore mooring performance by deploying two Nortek AWACs on different shape subsurface buoys offshore of Lunenburg Bay, Nova Scotia, Canada. A surface wave buoy was located nearby as an independent reference. The AWACs were deployed from September to November 2006 and measured waves over 4 m in significant wave height during three storms. The results indicate that the acoustic surface tracking (AST), used to measure non-directional wave properties, was a robust technique and worked very well with the AWACs deployed on a subsurface buoy. Greater than 99% of all AST measurements passed the quality control checks (comparable to results from a bottom mounted AWAC) and measurements of wave height and period were in excellent agreement with the surface wave buoy. The wave directional estimates were in good agreement with the surface wave buoy, but indicated clear frequency bands with increased directional uncertainty. An analysis of buoy motion suggests that the frequencies of poor directional estimates are coincident with the natural frequency of the mooring system. Guidance is offered to design a subsurface buoy which has a natural frequency outside of the wave band such that this technique may be used widely for offshore directional wave measurements.