Acoustic Doppler Velocimeters (ADVs)
The ADV is a single-point, high resolution current meter, capable of recording three-dimensional velocity vectors. The ADV measures the velocity of the water by analysing the Doppler effect (frequency shift) of sound emitted by the sensor. The ADV uses a separate receiver and transmitter, both of which are constructed to generate very narrow beam patterns - the transmitter generates sound with most of the energy focussed in a narrow cone and the receiver is sensitive to sound reflected from a narrow angular range. The transducers are mounted so that their beams intersect at a volume of water located at some distance away from the sensor. Where the beams intersect is the volume of water in which the measurements are made - known as the sampling volume.
The transmitter generates a short pulse of sound at a known frequency, which travels through the water along the axis of the transmitter. As the sound beam passes through the sampling volume the its energy is scattered in all directions by suspended particulate matter (e.g. sediment, micro-organisms, bubbles). An ADV typically requires a concentration of >~10 mg/l scattering material for its operation. A small portion of the sound energy is scattered back along the receiver's axis and is sampled by the ADV, then processed to measure the change in frequency. The Doppler shift measured by one receiver is proportional to the velocity of the scattering particles along the axis of the transmitter and the receiver.
The typical noise level of the ADV is ~1% of the maximum recorded value during a deployment. For example, if the maximum recorded velocity during a deployment was 1 m/s, the noise level would be around 0.01 m/s.
