10.9 Eulerian Measurements of Currents
Eulerian techniques have used many different types of current meters attached to many types types of moorings or ships.
Moorings (Figure 10.18) areplaced on the seafloor by ships. The moorings may last for months to longer than a year. Because the mooring must be deployed and recovered by deep-sea research ships, the technique is expensive. Subsurface mooring shown on the right in the figure is preferred for several reasons: the surface float is not forced by high frequency, strong, surface currents; the mooring is out of sight and it does not attract the attention of fishermen; and the floatation is usually deep enough to avoid being caught by fishing nets. Measurements made by moorings have errors due to:
Acoustic-Doppler Current Meters and Profilers
Two types of acoustic current meters are widely used. The Acoustic-Doppler Current Profiler, called the ADCP, measures the Doppler shift of sound reflected from water at various distances from the instrument using sound beams projected into the water just as a radar measures radio scatter as a function of range using radio beams projected into the air. Data from the beams are combined to give profiles of current velocity as a function of distance from the instrument. On ships, the beams are pointed diagonally downward at 3–4 horizontal angles relative to the ship's bow. Bottom-mounted meters use beams pointed diagonally upward.
Ship-board instruments are widely used to profile currents within 200 to 300 m of the sea surface while the ship steams between hydrographic stations. Because a ship moves relative to the bottom, the ship's velocity and orientation must be accurately known. GPS data have provided this information since the early 1990s.
Acoustic-Doppler current meters are much simpler than the ADCP. They transmit continuous beams of sound to measure current velocity close to the meter, not as a function of distance from the meter. They are placed on moorings and sometimes on a CTD. Instruments on moorings record velocity as a function of time for many days or months. The Aanderaa current meter (figure 10.19) in the figure is an example of this type. Instruments on CTDs profile currents from the surface to the bottom at hydrographic stations.
|Department of Oceanography, Texas A&M University
Robert H. Stewart, email@example.com
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Updated on October 18, 2006