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Current Metering

Acoustic Doppler Current Profiler (ADCP)

The Acoustic Doppler Current Profiler (ADCP) measures the speed and direction of ocean currents using the principle of “Doppler shift”. Anyone who has ever heard a train whistle is familiar with the Doppler effect. When the train is traveling towards you, the whistle’s pitch is higher. When it is moving away from you, the pitch is lower. The change in pitch is proportional to the speed of the train. The ADCP exploits the Doppler effect by emitting a sequence of high frequency pulses of sound (sonar pings) that scatter off of moving particles in the water. Depending on whether the particles are moving toward or away from the sound source, the frequency, or pitch, of the return signal bounced back to the ADCP is either higher or lower.
Particles moving away from the instrument produce a lower frequency return and vice versa. Since the particles move at the same speed as the water that carries them, the frequency shift is proportional to the speed of the water, or current. The ADCP has 4 acoustic transducers that emit and receive acoustical pulses from 4 different directions. Current direction is computed by using trigonometric relations to convert the return signal from the 4 transducers to ‘earth’ coordinates (north-south, east-west and up-down). Because the emitted sound extends from the drilling rig or ship down to the bottom of the ocean, the ADCP measures the current at many different depths simultaneously. This way, it is possible to determine the speed and direction of the current from the surface of the ocean to the bottom.

ADCP technology is very robust and the system requires little technical support or training to operate.
 
Head of an ADCP with the four transducers
Head of an ADCP with the four transducers. Click for larger image.
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