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

Adam Drozdowski Bedford Institute of Oceanography, Science Branch, Maritimes Region, Fisheries and Oceans Canada, Dartmouth, Nova Scotia, Canada

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Blair J. W. Greenan Bedford Institute of Oceanography, Science Branch, Maritimes Region, Fisheries and Oceans Canada, Dartmouth, Nova Scotia, Canada

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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.

Corresponding author address: Mr. Adam Drozdowski, Bedford Institute of Oceanography, P.O. Box 1006, Dartmouth NS B2Y 4A2, Canada. E-mail: adam.drozdowski@dfo-mpo.gc.ca

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.

Corresponding author address: Mr. Adam Drozdowski, Bedford Institute of Oceanography, P.O. Box 1006, Dartmouth NS B2Y 4A2, Canada. E-mail: adam.drozdowski@dfo-mpo.gc.ca
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