MOORING VIBRATION AS A SOURCE OF CURRENT-METER ERROR AND ITS CORRECTION

Data from instrumented moorings are used to show that rate underreadin g of Aanderaa paddle-wheel rotor current meters is a result of mooring vibration induced by vortex shedding from spherical in-line buoyancy. Direct evidence of this mooring motion is provided by accelerometer r ecords that reveal high-frequency (0.2-0.4 Hz) displacements normal to the absolute flow direction that are as much as 0.5 m in amplitude. C urrent meter measurements are up to 40% lower than concurrent rates me asured by a ship-mounted acoustic Doppler current profiler (ADCP). The result supports the hypothesis of Loder and Hamilton that rate underr eading occurs when cross-how motion of the current meters causes a rot or shielding effect arising from meter misalignment with the fluctuati ng relative water velocity. Measurements from an instrumented mooring in which spherical in-line buoyancy is replaced with streamlined flota tion are also presented. Here the variance of the measured cross-flow acceleration is typically reduced by two orders of magnitude, and curr ent meter and ADCP records are in excellent agreement. Besides greatly improving mooring stability and current meter data quality, use of th e streamlined packages also reduces the overall drag on the mooring th ereby significantly reducing instrument excursions and inclinations.