The IEEE Seventh Working Conference on Current Measurement Technology

Current and Wave Monitoring and Emerging Technologies

March 13-15 | Bahia Hotel | San Diego, CA, USA

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Deep ocean experience with acoustic current meters

Nelson Hogg and Dan Frye

MS 21
Woods Hole Oceanographic Inst.
Woods Hole , MA USA
02543

Phone: 508.289.2791
Email: nhogg@whoi.edu

Dan Frye
MS 18
WHOI
Woods Hole, MA 02543

As part of a program to develop a 5-year subsurface mooring capable of periodic telemetry ("Ultramoor") we have tested candidate current meters based on acoustic travel time (the Nobska MAVS and the FSI 3DACM) and Doppler (the Nortek Aquadopp and the Aanderaa RCM-11) principles. The advantages of these modern instruments over our longstanding workhorse, the VACM, are many, but the most important are that they have no moving parts and they have modern communications interfaces. However, in the first deployment of Ultramoor south of Bermuda we discovered consistent disagreements between the instruments with the RCM-11 registering lower speeds and the MAVS and 3DACM higher speeds than our reference VACM. Direction biases were also evident. The second Ultramoor deployment in Nov 2001 has confirmed these discrepancies and led us to deploy a third, short mooring offshore Bermuda expressly to compare the current meters. This mooring included a VMCM and an RCM-8, as well as 2 RCM-11s and a 3DACM, all positioned at about 4000 m depth in 4300 m of water. This comparison showed good agreement between the instruments, but left open the possibility that low scattering levels higher in the water column were causing the Doppler based instruments to underestimate the current.

As a "final" test of this possibility we deployed 3 instruments on a wire below a CTD configured to measure the (known) lowering rate of the CTD as they were lowered from the surface to the bottom-passing through relatively high and low scattering environments. The results and implications from all these tests will be presented and discussed.

Submitted on October 10, 2002

Sponsored by the Current Measurement Technology Committee (CMTC) of the IEEE Oceanic Engineering Society.

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