Mooring Vibration as a Source of Current Meter Error and Its Correction

Data from instrumented moorings are used to show that rate underreading 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 records 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. Current meter measurements are up to 40% lower than concurrent rates measured by a ship-mounted acoustic Doppler current profiler (ADCP). The result supports the hypothesis of Loder and Hamilton that rate underreading occurs when cross-flow motion of the current meters causes a rotor shielding effect arising from meter misalignment with the fluctuating relative water velocity. Measurements from an instrumented mooring in which spherical in-line buoyancy is replaced with streamlined flotation are also presented. Here the variance of the measured cross-flow acceleration is typically reduced by two orders of magnitude, and current meter and ADCP records are in excellent agreement. Besides greatly improving mooring stability and current meter data quality, use of the streamlined packages also reduces the overall drag on the mooring thereby significantly reducing instrument excursions and inclinations.