An acoustic Doppler current profiler is commonly used to measure currents in water.
An ADCP works much like a sonar instrument. Piezoelectric transducers send sound pulses through a water column, and suspended particles scatter some of that sound back toward the instrument. The travel time estimates the distance to the scattering region, while the echo’s Doppler frequency shift reveals the component of water velocity along each acoustic beam.
By combining measurements from several angled beams, an ADCP can calculate current direction and three-dimensional velocity over many depth cells. Instruments may be mounted on the seabed, a ship, a buoy, a bridge piling, or a riverbank. Oceanographers use them for circulation and wave studies, while river scientists use them to profile flow and estimate discharge.
The instrument does not measure perfectly near every boundary. Sidelobe interference can create a zone of lost data near the surface or bottom, and the sound speed estimate depends on factors such as temperature and salinity. ADCP should also not be confused with SODAR: SODAR applies similar acoustic principles to wind measurement in air, whereas an ADCP is chiefly associated with water.