Water currents of large water bodies are continuously monitored and recorded in order to foresee any major changes in the characteristics of waves. This is done by using an instrument known as Acoustic Doppler Current Profiler, which uses sound energy in order to generate a "current profile"
An Acoustic Doppler Current Profiler or ADCP is an instrument used to measure water currents with the help of sound propogation technique. It measures , records and monitors the velocity of currents in order to generate a wave profile. It can be configured in several ways according to the requirement. Side-listening is done in rivers and canals in order to monitor the velocities of different layers. Downward listening is done by mounting ADCP on a boat or any floating structure to measure the various underlying water currents. This method is generally used for long term studies or research purposes.
ADCP is used for both - monitoring natural phenomena and also for commercial purposes. It is being widely used in oceonography for measuring the velocity of water flow in estuary, rivers, canals and large streams. It has been extremely useful in locating underwater oceanic tornadoes that have a history of many major disasters. But the foremost help it provides is for weather forecasting. By studing the changes in the velocity and movement of water currents in oceans, changes in weather condition can be easily predicted.
ADCP also has a diversified commercial usage. It is used in locating the deep sea currents that might damage the drilling processes, in measuring the flow of liquid through the underwater pipelines and also for monitoring the flow of freshwater melting off an iceberg. They are also widely used at ports and harbors to predict "high and low tides" in order to manage the ships traffic.
ADCP works on the same principle as that of a sonar. It uses ceramic transducers in order to transmit and receive sound signals. The transducers are positioned in such a way that sound waves travel in different, but known directions. The sound waves leave and arrive at the transducer face with a shift in their frequency. The entire working of the system is based on the Doppler Effect.
Batteries are generally used to supply power. All the ADCPs are self-contained and work remotely to provide the required data. Optical Fiber cables or other subsea methods are used in order to transfer data to the shore. Many ADCPs provide live monitoring to the shore. They are also fixed on ROVs and deep sea submarines to provide navigational help in areas where GPS system becomes inefficient for use.
Doppler Effect and ADCP
As ADCPs work on the principle of Doppler effect, we will first see as to what is Doppler effect?
The Doppler effect explains the changes in the wave frequency as received by an observer moving relative to the source of the wave. This can be easily understood by taking an example of a vehicle sounding a siren and moving towards the observer. As the vehicle approaches the observer, the frequency received increases. When the vehicle reaches the observer, the frequency emitted by the vehicle is equal to that received by the observer. And when it moves away from the observer, the frequency received reduces.Thus the total Doppler effect is an outcome of all the three - the motion of observer, the motion of source and the motion of medium.In case of ADCP, the frequencies of sound waves as experienced by the source and observer will be relative to the medium in which the sound waves are transmitted. i.e water.
Thus in ADCP, the relative velocity of water currents shifts the frequency of the sound waves that are transmitted and received by the transducer. If there is an obstacle in the water, the frequency received by the tranducer will be further shifted because of the relative velocity between the obstacle and water. The final number of the received frequency passes through a series of complex mathematical calculations and critical assumptions in order to derive the exact "profile" of water currents over a varied range of depths.
Alec Eden The search for Christian Doppler, Springer-Verlag, Wien 1992.