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 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 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.
A diver deploying a bottom-mounted acoustic doppler current profiler to investigate the hydrodynamics of coral reef systems. Click image for larger view.
Measuring currents is a fundamental practice of physical oceanographers. By determining how ocean waters move, scientists can determine how organisms, nutrients and other biological and chemical constituents are transported throughout the ocean. Ocean waters have varied temperatures and in places like the warm Gulf Stream the movement of water means the movement of heat. Heat transport in the ocean is a critical component of the global heat budget and, therefore, it contributes to global climate change. Because of its high-resolution and ability to sample deep within the ocean interior, the ADCP is an efficient tool for sampling a large section of the ocean in a limited amount of time.
On large research vessels the ADCP is permanently mounted on the bottom of the ship’s outer hull. A typical unit (75 kHz) is powerful enough to sample waters as deep as 700 m (2275 ft). During operation, the ADCP is sending out and receiving several acoustic pulses every second. An on-board computer processes the returned signal and a real-time display of the magnitude and direction of the current throughout the water column is produced on the computer monitor. This way, scientists can observe the changing ocean current structure nearly continuously while the ship is in motion. The data are stored on CDs so that scientists can conduct a thorough analysis when they return to their laboratory after the cruise. ADCP technology is very robust and the system requires little technical support or training to operate.
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