An acoustic doppler current profiler (ADCP) is a
hydroacoustic current meter
A current meter is an oceanographic device for flow measurement by mechanical, tilt, acoustical or electrical means.
Different reference frames
In physics, one distinguishes different reference frames depending on where the observer is locat ...
similar to a
sonar
Sonar (sound navigation and ranging or sonic navigation and ranging) is a technique that uses sound propagation (usually underwater, as in submarine navigation) to navigate, measure distances ( ranging), communicate with or detect objects o ...
, used to measure
water current velocities over a depth range using the
Doppler effect
The Doppler effect (also Doppler shift) is the change in the frequency of a wave in relation to an observer who is moving relative to the source of the wave. The ''Doppler effect'' is named after the physicist Christian Doppler, who described ...
of
sound waves
In physics, sound is a vibration that propagates as an acoustic wave through a transmission medium such as a gas, liquid or solid.
In human physiology and psychology, sound is the ''reception'' of such waves and their ''perception'' by the br ...
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
The hertz (symbol: Hz) is the unit of frequency in the International System of Units (SI), often described as being equivalent to one event (or cycle) per second. The hertz is an SI derived unit whose formal expression in terms of SI base uni ...
to several
megahertz
The hertz (symbol: Hz) is the unit of frequency in the International System of Units (SI), often described as being equivalent to one event (or cycle) per second. The hertz is an SI derived unit whose formal expression in terms of SI base ...
.
A similar device is a ''
SODAR
Sodar, an acronym of sonic detection and ranging, is a meteorological instrument used as a wind profiler based on the Wave scattering, scattering of sound waves by atmospheric turbulence. Sodar equipment is used to measure wind speed at various ...
'', which works in the air and uses the same principles for wind speed profiling.
Working principle

ADCPs contain
piezoelectric
Piezoelectricity (, ) is the electric charge that accumulates in certain solid materials—such as crystals, certain ceramics, and biological matter such as bone, DNA, and various proteins—in response to applied stress (mechanics), mechanical s ...
transducers to transmit and receive sound signals. The traveling time of sound waves gives an estimate of the distance. The frequency shift of the echo is proportional to the water velocity along the acoustic path. To measure 3D velocities, at least three beams are required. In rivers, only the 2D velocity is relevant and ADCPs typically have two beams. In recent years, more functionality has been added to ADCPs (notably wave and turbulence measurements) and systems can be found with 2,3,4,5 or even 9 beams.
Further components of an ADCP are an
electronic amplifier
An amplifier, electronic amplifier or (informally) amp is an electronic device that can increase the magnitude of a Signal (information theory), signal (a time-varying voltage or Electric current, current). It is a two-port network, two-port ...
, a
receiver, a clock to measure the traveling time, a
temperature sensor, a compass to know the heading, and a pitch/roll sensor to know the orientation. An
analog-to-digital converter
In electronics, an analog-to-digital converter (ADC, A/D, or A-to-D) is a system that converts an analog signal, such as a sound picked up by a microphone or light entering a digital camera, into a Digital signal (signal processing), digi ...
and a
digital signal processor
A digital signal processor (DSP) is a specialized microprocessor chip, with its architecture optimized for the operational needs of digital signal processing. DSPs are fabricated on metal–oxide–semiconductor (MOS) integrated circuit chips. ...
are required to sample the returning signal in order to determine the
Doppler shift
The Doppler effect (also Doppler shift) is the change in the frequency of a wave in relation to an observer who is moving relative to the source of the wave. The ''Doppler effect'' is named after the physicist Christian Doppler, who described t ...
. A
temperature
Temperature is a physical quantity that quantitatively expresses the attribute of hotness or coldness. Temperature is measurement, measured with a thermometer. It reflects the average kinetic energy of the vibrating and colliding atoms making ...
sensor is used to estimate the
sound velocity at the instrument position using the
seawater equation of state, and uses this to estimate scale the frequency shift to water velocities. This procedure assumes that the
salinity
Salinity () is the saltiness or amount of salt (chemistry), salt dissolved in a body of water, called saline water (see also soil salinity). It is usually measured in g/L or g/kg (grams of salt per liter/kilogram of water; the latter is dimensio ...
has a preconfigured constant value. Finally, the results are saved to internal memory or output online to an external display software.
Processing methods
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
Narrowband signals are signals that occupy a narrow range of frequencies or that have a small fractional bandwidth. In the audio spectrum, ''narrowband sounds'' are sounds that occupy a narrow range of frequencies. In telephony, narrowband is ...
". 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" or "broadband". This method improves the space-time resolution by a factor of 5 (typical). Commercially, this method was protected by US patent 5615173 until 2011. The pulse-to-pulse coherent method 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.
Applications
Depending on the mounting, one can distinguish between side-looking, downward- and upward-looking ADCPs. A bottom-mounted ADCP can measure the speed and direction of currents at equal intervals all the way to the surface. Mounted sideways on a wall or bridge piling in rivers or canals, it can measure the current profile from bank to bank. In very deep water they can be lowered on cables from the surface.
The primary usage is for
oceanography
Oceanography (), also known as oceanology, sea science, ocean science, and marine science, is the scientific study of the ocean, including its physics, chemistry, biology, and geology.
It is an Earth science, which covers a wide range of to ...
.
The instruments can also be used in
river
A river is a natural stream of fresh water that flows on land or inside Subterranean river, caves towards another body of water at a lower elevation, such as an ocean, lake, or another river. A river may run dry before reaching the end of ...
s and
canal
Canals or artificial waterways are waterways or engineered channels built for drainage management (e.g. flood control and irrigation) or for conveyancing water transport vehicles (e.g. water taxi). They carry free, calm surface ...
s to continuously measure the
discharge.
Mounted on
mooring
A mooring is any permanent structure to which a seaborne vessel (such as a boat, ship, or amphibious aircraft) may be secured. Examples include quays, wharfs, jetties, piers, anchor buoys, and mooring buoys. A ship is secured to a mooring to ...
s 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
An umbilical cable or umbilical is a cable and/or hose that supplies required consumables to an apparatus, like a rocket, or to a person, such as a diver or astronaut. It is named by analogy with an umbilical cord. An umbilical can, for example, ...
for data communication. Deployment duration can be extended by a factor of three by substituting
lithium battery packs for the standard alkaline packs.
Bottom tracking
By adjusting the window where the Doppler shift is calculated, it is possible to measure the relative velocity between the instrument and the bottom. This feature is referred to as bottom-track. The process has two parts; first identify the position of the bottom from the acoustic echo, then calculating the velocity from a window centered around the bottom position. When an ADCP is mounted on a moving ship, the bottom track velocity may be subtracted from the measured water velocity. The result is the net current profile. Bottom track provides the foundation for surveys of the water currents in coastal areas. In deep water where the acoustic signals cannot reach the bottom, the ship velocity is estimated from a more complex combination of velocity and heading information from
GPS,
gyro
Gyro may refer to:
Science and technology
* GYRO, a computer program for tokamak plasma simulation
* Gyro Motor Company, an American aircraft engine manufacturer
* '' Gyrodactylus salaris'', a parasite in salmon
* Gyroscope, an orientation-sta ...
, etc.
Discharge measurements
In rivers, the ADCP is used to measure the total water transport. The method requires a vessel with an ADCP mounted over the side to cross from one bank to another while measuring continuously. Using the bottom track feature, the track of the boat as well as the cross sectional area is estimated after adjustment for left and right bank areas. The discharge can then be calculated as the
dot product
In mathematics, the dot product or scalar productThe term ''scalar product'' means literally "product with a Scalar (mathematics), scalar as a result". It is also used for other symmetric bilinear forms, for example in a pseudo-Euclidean space. N ...
between the vector track and the current velocity. The method is in use by hydrographic survey organisations across the world and forms an important component in the stage-discharge curves used in many places to continuously monitor river discharge.
Doppler velocity log (DVL)
For underwater vehicles, the bottom tracking feature can be used as an important component in the navigation systems. In this case the velocity of the vehicle is combined with an initial
position fix, compass or gyro
heading, and data from the
acceleration sensor
An accelerometer is a device that measures the proper acceleration of an object. Proper acceleration is the acceleration (the rate of change of velocity) of the object relative to an observer who is in free fall (that is, relative to an inertia ...
. The sensor suite is combined (typically by use of a
Kalman filter
In statistics and control theory, Kalman filtering (also known as linear quadratic estimation) is an algorithm that uses a series of measurements observed over time, including statistical noise and other inaccuracies, to produce estimates of unk ...
) to estimate the position of the vehicle. This may help to navigate submarines,
autonomous
In developmental psychology and moral, political, and bioethical philosophy, autonomy is the capacity to make an informed, uncoerced decision. Autonomous organizations or institutions are independent or self-governing. Autonomy can also be defi ...
, and
remotely operated underwater vehicle
A remotely operated underwater vehicle (ROUV) or remotely operated vehicle (ROV) is a free-swimming submersible craft used to perform underwater observation, inspection and physical tasks such as valve operations, hydraulic functions and other g ...
s.
Wave measurements
Some ADCPs can be configured to measure surface
wave height
In fluid dynamics, the wave height of a surface wave is the difference between the elevations of a crest and a neighboring trough. ''Wave height'' is a term used by mariners, as well as in coastal, ocean and naval engineering.
At sea, the ...
and direction. The wave height is estimated with a vertical beam that measures the distance to the surface using the echo from short pulses and simple peak estimation algorithms. The wave direction is found by cross correlating the along-beam velocity estimates and the wave height measurement from the vertical beam. Wave measurements are typically available for seafloor-mounted instruments but recent improvements permit the instrument to be mounted also on rotating subsurface buoys.
[
]
Turbulence
ADCPs with pulse-to-pulse coherent processing can estimate the velocity with the precision required to resolve small scale motion. As a consequence, it is possible to estimate
turbulent
In fluid dynamics, turbulence or turbulent flow is fluid motion characterized by chaotic changes in pressure and flow velocity. It is in contrast to laminar flow, which occurs when a fluid flows in parallel layers with no disruption between ...
parameters from properly configured ADCPs. A typical approach is to fit the along beam velocity to the
Kolmogorov structure configuration and thereby estimate the dissipation rate. The application of ADCPs to turbulence measurement is possible from stationary deployments but can also be done from moving underwater structures like gliders or from subsurface
buoy
A buoy (; ) is a buoyancy, floating device that can have many purposes. It can be anchored (stationary) or allowed to drift with ocean currents.
History
The ultimate origin of buoys is unknown, but by 1295 a seaman's manual referred to navig ...
s.
Advantages and disadvantages
The two major advantages of ADCPs is the absence of moving parts that are subject to
biofouling
Biofouling or biological fouling is the accumulation of microorganisms, plants, algae, or small animals where it is not wanted on surfaces such as ship and submarine hulls, devices such as water inlets, pipework, grates, ponds, and rivers that ...
and the
remote sensing
Remote sensing is the acquisition of information about an physical object, object or phenomenon without making physical contact with the object, in contrast to in situ or on-site observation. The term is applied especially to acquiring inform ...
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 current
An ocean current is a continuous, directed movement of seawater generated by a number of forces acting upon the water, including wind, the Coriolis effect, breaking waves, cabbeling, and temperature and salinity differences. Depth contours, sh ...
s 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
An ocean current is a continuous, directed movement of seawater generated by a number of forces acting upon the water, including wind, the Coriolis effect, breaking waves, cabbeling, and temperature and salinity differences. Depth contours, ...
.
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
Noise pollution, or sound pollution, is the propagation of noise or sound with potential harmful effects on humans and animals. The source of outdoor noise worldwide is mainly caused by machines, transport and propagation systems.Senate Publi ...
in the ocean which may interfere with
cetacean
Cetacea (; , ) is an infraorder of aquatic mammals belonging to the order Artiodactyla that includes whales, dolphins and porpoises. Key characteristics are their fully aquatic lifestyle, streamlined body shape, often large size and exclusively c ...
navigation and echolocation.
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.
References
{{Authority control
Sonar
Physical oceanography
Oceanographic instrumentation
Ocean currents
Watercraft components