Current profilers are sensors which enable scientists to estimate the speed and direction of marine currents. The systems deployed aboard the vessels of the French Oceanographic Fleet are Acoustic Doppler Current Profilers (ADCP) which can assess currents in the water column to a depth of 1,000 metres. These sensors are fitted permanently to the ships, either on the keel or in special fairings fastened under the keel.
Acoustic Doppler current profilers or ADCPs use piezoelectric transducers to transmit and receive acoustic signals, as well as an electronic/software component for the generation and acquisition of signals. The acoustic signal transmitted by the sensor at a given frequency propagates through the water and is back-scattered by particles and plankton (krill, copepods, pteropods, etc.) present in the water. The backscatter is received by the sensor and processed.
Data analysis is founded on the following hypothesis: particles which cause the acoustic signals to backscatter do not have any speed of their own, but move instead with the mass of water in which they are suspended. By determining the speed of these particles, scientists can, therefore, measure the speed of the marine currents.
The travelling time between any given transmission to its reception enables the scientist to determine the distance from the transducer. The shift in the frequency of the echoes received compared to the signals transmitted is proportional to the relative speed of the particles compared to the ship (Doppler effect). In order to reconstitute the 3D components of this speed, at least three beams are required. ADCPs generally comprise four beams positioned at 30° from the vertical (see figure below), two in the bow of the ship (one starboard, the other port) and two in the stern (one starboard, the other port).
Next, the speed, rolling, pitching and heading of the ship must be determined (GPS position and attitude control unit) in order to estimate the speed of the marine currents within a georeferenced zone.
Three frequencies are available for this sensor: 38, 75 and 150 kHz. Performance in terms of range and vertical resolution depends on the sensor’s frequency. Performance is shown in the table below. Range also depends on the conditions of acquisition (ship noise, quantity of particles suspended in the water, etc.)