Noise in Ice-Tethered Profiler and McLane Moored Profiler velocity measurements

In order to measure current profiles, and most recently, turbulent fluxes, moored profiling instrument have been equipped with acoustic travel-time current sensors. Noise in the measured currents has exceeded expectations. A customized Falmouth Scientific acoustic current sensor on a McLane Moored Profiler (MMP) has a standard deviation of measured velocity that is 4.4% of the profiler velocity in still water and a modified Modular Acoustic Velocity Sensor (MAVS) on an MMP and an Ice-Tethered Profiler (ITP) has a standard deviation of 4.6% of profiler velocity. Both of these sensors measure velocity along four acoustic paths and the water velocities were computed neglecting their downstream paths. The noise in measured velocity profiles from the ITP is more obvious because the relative ocean currents sampled by the ITP profiles were relatively weak. The ITP with the custom MAVS velocity sensor was investigated for a Strouhal resonance of the sensor structure, motor commutator noise, vibration from the profiler rolling up and down a mooring cable, and the Strouhal wake of the sensor itself. The noise was found to be dominated by the latter. Acoustic travel-time current sensors do a good job of measuring a line integral of water velocity along their acoustic paths, but the presence of the acoustic transducers and their supporting structures may change the flow being measured. Some of the design tradeoffs of the sensor design are discussed. Two solutions to reduce velocity noise are described: over sampling and averaging and design of a sensor optimized for profiling. Over-sampling and averaging is shown to significantly reduce noise deriving from the smaller eddies shed by the sensor structure while retaining signals due to eddies larger than the sampling volume. Over-sampling and averaging can be utilized by the existing profilers with a program change while the second option requires a new sensor head. In a tow tank test through still water, over-sampling and averaging from 16 Hz down to 2 Hz reduced the standard deviation of velocity of paths normal to the flow from 5.2% to 2.2% of the profiler velocity.