Acoustic Doppler Current Profiler
An Acoustic Doppler Current Profiler (ADCP or ADP) is a hydroacoustic current meter similar to a sonar, attempting to measure water current velocities over a depth range using the Doppler effect of sound waves scattered back from particles within the water column. The term ADCP is used synonymously for all kind of acoustic current meters although the abbreviation originates from the name of an instrument series by Teledyne. ADCPs have been commercially available since the late 1980s. The working frequencies range from 38 kHz to several megahertz.
ADCPs contain piezoelectric oscillators to transmit and receive sound signals. The traveling time of sound waves gives an estimate of the distance, the red or blue shift can be converted to a velocity. In order to measure 3D velocities, at least three vector components have to be estimated, this is why the instrument typically has four of them.
Further components of an ADCP are an electronic amplifier, a receiver, a mixer, a clock to measure the traveling time, a temperature sensor, a compass to know the relative rotation, and a pitch/roll sensor to know the horizontal. An analog-to-digital converter and a digital signal processor are required to sample the returning signal in order to determine the Doppler shift. A micro processor evaluates the sound velocity at the instrument position using the seawater equation of state, and uses this to estimate the velocities. This procedure assumes that the same density in the water column nearby is mainly determined by temperature, i.e. that the salinity has a pre-configured constant value. Finally, the results are saved on a memory card.
Depending on the mounting, one can distinguish between side-listening, downward- and upward looking ADCPs.
Mounted on moorings within the water column or directly at the seabed, water current and wave studies may be performed. They can stay underwater for years at a time, the limiting factor is the lifetime of the battery pack. Depending on the nature of the deployment the instrument usually has the ability to be powered from shore, using the same umbilical cable for data communication. Deployment duration can be extended by a factor of three by substituting lithium battery packs for the standard alkaline packs.
Some harbor managers also use ADCPs to nowcast tides and local currents which helps them to optimize the ship traffic in a busy port. Another application is the measurement of water flow through sewer pipes.
By adjusting the signal processing, ADCPs can be employed as acoustic Doppler Velocity Log, this feature is usually called bottom-track. Sound waves bouncing off the sea floor can be used to determine the velocity vector of the device. Combing this with a position fix, compass heading, and data from the acceleration sensors (typically by use of a Kalman Filter), the position of the vehicle with the ADCP can be determined. This may help to navigate submarines, autonomous and remotely operatored underwater vehicles.
Advantages and disadvantages
The major advantage of ADCPs is no moving parts are used which may be subject to biofouling. Besides, a single instrument is enough to cover up to 1000 m of water column, this allows more accurate estimations of flow patterns.
However, the cost per device of roughly tens of thousands of euros is relatively high. Considering the assumptions made – constant temperature and salinity – the choice of a mechanical instrument might be more appropriate where one of the variables is non-constant, i.e. within the thermocline (depth range with prominent temperature gradient) or halocline (salinity changing with depth). NOTE: that when significant stratification is present, the horizontal velocities that are measured by the ADCP are correct, but the vertical velocites and the ranges to the cells could be in error by up to 3% if there is a 35psu difference across the thermocline.
As any acoustical instrument, it contributes to noise pollution in the ocean which may interfere with cetacean navigation and ecolocation. The effect depends on the frequency and the power of the instrument but most ships already have one or more echo sounders which also contribute to noise pollution.
- William J. Emery, Richard E. Thomson (2001). Data analysis methods in physical oceanography. Gulf Professional Publishing. p. 83. ISBN 978-0-444-50757-0. Retrieved 2011-02-06.
- Hogan, C Michael (10 2011). "Icon Encyclopedia of Earth Topics". Washington, D.C.: Environmental Information Coalition, National Council for Science and the Environment. Retrieved 2012-09-13.