Acoustic Doppler current profiler
An acoustic doppler current profiler (ADCP) is a hydroacoustic current meter similar to a sonar, used 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 a generic term for all acoustic current profilers, although the abbreviation originates from an instrument series introduced by RD Instruments in the 1980s. The working frequencies range of ADCPs range from 38 kHz to several megahertz.
A similar device is a SODAR, which works in the air and uses the same principles for wind speed profiling.
Processing methods[edit]
Three common methods are used to calculate the Doppler shift and thus the water velocity along the acoustic beams. The first method uses a monochromatic transmit pulse and is referred to as "incoherent" or "narrowband". The method is robust and provides good quality mean current profiles but has limited space-time resolution. When the transmit pulse consists of coded elements that are repeated, the method is referred to as "repeat sequence coding"[1] or "broadband". This method improves the space-time resolution by a factor of 5 (typical). Commercially, this method was protected by US patent[2] 5615173 until 2011. The pulse-to-pulse coherent method[3] relies on a sequence of transmit pulses where the echo from subsequent pulses are assumed not to interfere with each other. This method is only applicable for very short profiling ranges but the corresponding improvement in space time resolution is of order 1000.
Advantages and disadvantages[edit]
The two major advantages of ADCPs is the absence of moving parts that are subject to biofouling and the remote sensing aspect, where a single, stationary instrument can measure the current profile over ranges exceeding 1000 m. These features allow for long term measurements of the ocean currents over a significant portion of the water column. Since the start in the mid-1980s, many thousand ADCPs have been used in the world oceans and the instrument has played a significant role in our understanding of the world ocean circulation.
The main disadvantage of the ADCPs is the loss of data close to the boundary. This mechanism, often referred to as a sidelobe interference, covers 6–12% of the water column and, for instruments looking up toward the surface, the loss of velocity information close to the surface is a real disadvantage. Cost is also a concern but is normally dwarfed by the cost of the ship required to ensure a safe and professional deployment.
As any acoustical instrument, the ADCP contributes to noise pollution in the ocean which may interfere with cetacean navigation and echolocation.[6] The effect depends on the frequency and the power of the instrument but most ADCPs operate in a frequency range where noise pollution has not been identified to be a serious problem.