U.S. Geological Survey Open-File Report 2007-1194
Figure 13. An Aquadopp acoustic profiler mounted on a grate prior to deployment. The Aquadopp emits pulses of energy that are reflected from scatterers suspended in the water. The return pulse has a change in frequency called a “Doppler shift.” Because Doppler shift is proportional to the component of water flow along the beam, trigonometry is used to convert the returned acoustic signal into eastward, northward, and vertical components of water flow.