Intercomparison of MAVS with VACM, VMCM, FSI 2-D, and Aanderaa RCM 11 current meters from a mooring

An intercomparison of four current meters at 2000-m depth in the Ultramoor experiment of Woods Hole Oceanographic Institution near Bermuda in summer 2000 provided an opportunity to examine characteristics of mechanical, Doppler, and acoustic travel-time current meters in a very low velocity environment. This mooring in 4000 m depth had subsurface buoyancy at 300 m. Typical tidal current of only 4 cm/s added to mean current of only 2 cm/s caused the mechanical VACM sensors to stall intermittently. The two acoustic travel-time current meters, MAVS and FSI 2-D, tracked tidal fluctuations well although there was some offset drift over two months of recording. The Aanderaa RCM 11 acoustic Doppler current meter response to tidal fluctuations fell between that of the acoustic travel-time current meters and the mechanical sensors. Energetic vertical oscillations for 24 hours in two events 50 days apart suggest mooring response to possible hurricane passage. Only MAVS sensed vertical velocity so there is no corroborating evidence that the subsurface buoy responded to internal wave oscillations excited by the hurricane passage. In addition to testing three sensing modalities, the intercomparison displayed benefits from alternative sampling schemes, one being the monitoring of mooring motion with a vertical axis of velocity sensing. An advantage of acoustic travel-time sensing in such low currents is the linearity of the technique through zero velocity but a disadvantage is the uncertainty in the exact zero point. At 2000-m depth, fouling is not an issue but availability of acoustic scatterers for Doppler sensors is a concern. Dropout of scattered signal from the fluid may lead to spurious lock-on to side lobe signals scattered from mooring hardware. This may contaminate estimates of velocity and mimic the effect of stalling from the mechanical sensor of the VACM. Short intervals in time show these effects during the several months of the deployment. Selection of sensors for deep current monitoring is important for observations of global transport of heat, water, trace gases, and dissolved chemicals. Tests of sensor characteristics in actual moored configurations are invaluable.