Acoustic Doppler velocimeter (ADV) is a highly specialized device designed to record instantaneous velocity components at a single-point with a relatively high frequency. It achieves this by measuring the velocity of particles within a remote sampling volume based on the Doppler shift effect. The system includes a transmitter, which emits a high-frequency signal into the sample volume, and a receiver(s), which detects the shift in the frequency of the returning signal.

The probe head typically consists of one transmitter and two to four receivers. The sampling volume, located 5 or 10 cm from the tip of the transmitter, is a cylinder of water with a diameter of 6 mm and a height of 9 mm, although newer laboratory ADVs may have smaller sampling volume (e.g. Sontek→ microADV, Nortek→ Vectrino+). The sampling volume size is determined by the sampling conditions and manual setup.

In a standard configuration, an ADV system with N receivers records simultaneously 4.N values with each sample, including a velocity component, a signal strength value, a signal-to-noise (SNR) value, and a correlation value. The signal strength value represents the acoustic backscatter intensity and may relate to the suspended sediment concentration with proper calibration. The velocity component represents the velocity of the particles moving through the sampling volume and is measured along the line connecting the volume to the receiver.

The velocity component must be transformed into a Cartesian system of coordinates, and the trigonometric transformation may cause some velocity resolution errors. Although ADV has become a popular technique in both laboratory and field applications, some researchers have pointed out that the ADV signal outputs can include the combined effects of turbulent velocity fluctuations, Doppler noise, signal aliasing, turbulent shear, and other disturbances. These can all impact the quality and accuracy of the velocity data, leading to high levels of noise and spikes in all velocity components.

In turbulent flows, the ADV velocity outputs are a combination of Doppler noise, signal aliasing, velocity fluctuations, installation vibrations, and other disturbances. The signal may also be affected adversely by velocity shear across the sampling volume and proximity to boundaries. Lemmin and Lhermitte, Chanson et al., and Blanckaert and Lemmin discussed the inherent Doppler noise of an ADV system. Spikes may be caused by aliasing of the Doppler signal, and McLelland and Nicholas explained the physical processes that can cause them.