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A Mooring Design for Measurement of Deep Water Ocean Waves

A recent data acquisition program obtained ocean wave measurements in high-energy wave conditions offshore of southern Australia in 1400 meters water depth. A taut-wire subsurface mooring was employed to position acoustic Doppler current profilers (ADCPs) near the surface to resolve wave height and direction across a majority of the wave spectrum. The trick to this technique was quantifying and removing buoy/sensor motion contamination from the resulting observations. Motion corrections were achieved by using tandem ADCPs configured to sample the surface wave field as well as the attenuated wave field in deeper layers, using these deeper velocities as an indicator of sensor velocity. Subsequent data analysis showed remarkable agreement between wave spectra derived from (independent) velocity, surface track, and pressure records, providing first-order confidence in the fidelity of the results. This paper provides a description of the mooring design used to obtain deepwater wave measurements and offers analysis of the buoy motion in real ocean wave conditions. Buoy motion information (resolved to 1 Hz), provided by the ADCP heading, pitch, and roll sensors combined with the downward-looking ADCP velocity data in deeper layers, was used to calculate buoy excursions and accelerations. Our analysis of the buoy motion includes both time-and frequency-domain presentation of the system response. Empirical transfer functions between the wave forcing and resultant buoy response indicates that the buoy oscillates horizontally in phase with the wave orbital motion, but that the vertical motion is constrained by the mooring line. Comparison of tilt and buoy accelerations show that the instrument tilts were small (of order 1 degree in the worst case conditions)