An Intercomparison of Acoustic Current Meter Measurements in Low to Moderate Flow Regions

Abstract

Abstract Instrumented, subsurface moorings deployed in the Scotian shelf and slope regions of the North Atlantic provide data in low to moderate flows for a current meter intercomparison. The primary instruments being evaluated are two acoustic Doppler single-point current meters, the Aanderaa Seaguard (SG) and the Teledyne RD Instruments (RDI) Doppler volume sampler (DVS), which are compared against older-generation single-point current meters and acoustic Doppler current profilers. Analysis showed that the root-mean-square (RMS) of the speed difference between concurrent instrument combinations was in the range of 1.0–1.6 cm s−1, which is about 3%–6% of the upper limit of speeds observed at these sites. Best agreement was between the DVS and the nearby Seaguard (RMS speed difference of 1.2 cm s−1), during the shelf deployment, and between the Aanderaa recording current meter 11 (RCM11) and the nearby Seaguard (1.0 cm s−1), during the slope deployment. Speed differences larger than 4 cm s−1 were uncommon, occurring less than 1.5% of the time. Slight overspeeding of one of the Seaguards is traced to an intentional alteration in the instruments' sampling strategy. The DVS compass had a slight meandering tendency that caused it to routinely disagree with other instruments by as much as 15° for hours at a time. The disagreement was random in direction and had no impact on most of the comparisons, but it did produce a 15% smaller magnitude of mean current. Subsequent to this field test, Teledyne RDI redesigned the DVS and replaced the compass with a new sensor.