An instrument typically mounted to the bottom of a ship, or tied to a mooring, which uses sound waves to detect the motion of particles in the water. Sound waves are emitted into the water column at a frequency of around 150 KHz and the echo of the sound bouncing off small particles in the water can be used to determine the motion of the particles. This is based on the principle of Doppler Shift.

Assuming the particles are moving with the motion of the water, the speed and direction of water movement can be determined and continuously measured throughout a research cruise, ultimately providing a ‘map’ of the currents within the area the ship has traveled. The strength of the echo will also provide a measure of the size and density of the particle, giving an indication of what types of particles are present.