The ADCP, or Acoustic Doppler Current Profiler, allows us to measure currents throughout the water column and is a considerable improvement on the old fashioned current meters which could only measure one depth at once.
We use two main types, ship mounted, which is mounted on the bottom of the FRS Algoa and allows us to measure the currents wherever we go, and moored ADCPs which we leave in place to monitor what happens to the currents over longer periods of time. Shallow ADCP moorings are deployed by divers, whilst deeper ones are deployed from the ship (the pictures on this page are of deep water moorings). The floats serve to keep the mooring upright, and when we return to the site, we use a device called an "acoustic release" which listens for a special sound signal to release the mooring from the bottom, and the floats carry it to the surface where we pick it up again. We usually use acoustic releases in pairs, as the equipment and data are very valuable, and losing them due to a failed acoustic release is very frustrating!
The ADCP system basically works by using sound waves ("Acoustic" means sound) to calculate how minute particles in the water are moving, and using this, based on the resonable assumption the small particles don't move themselves very well, works out in which direction and how fast the water is moving.
The system sends out "pings" of known (generally very high) frequency, and then measures how long these take to bounce back off these minute objects. Because of effect motion on sound waves, called the doppler effect, we know that if the reflected signals come back with a higher frequency, the particles are moving towards the sensor, if they come back with a lower frequency, the particles are moving away, and if the frequency stays the same, the particles aren't moving. To work out where the particle is in the water column, we simply time how long the ping takes to come back; divide that number by two and multiply by the speed of sound in water of that density, and you have the distance of the particle away from the sensor.
If you want to hear the doppler effect in action, next time an emergency vehicle's siren passes you or someone honks their car horn as they go past or even a train goes past, listen to how the pitch ("note") of the sound seems to go up as if comes towards you and then go down as it passes you.
The ADCP unit is the cylinder in the middle of the floats; the round windows at the top are where the sound waves are tranmitted and received.
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