Acoustic Doppler Techniques

Abstract

Doppler backscatter current meters have been used for the last 35 years to make relative velocity measurements in the ocean. While the initial widespread application of the principle was for ship speedometers (Harvard University Acoustic Research Laboratory, 1945), Doppler flowmeters have more recently been used by the oceanographic community. In general, these devices transmit high frequency sound in a narrow beam. The sound scatters off plankton and other drifting (and swimming) objects in the sea. The Doppler shift of the returning sound gives information about the component of scatterer velocity along the beam. Acoustic frequencies from several kilohertz to ^~10 mHz have been used for Doppler sensors. The Doppler shift associated with a given relative velocity increases with increasing frequency. The higher the frequency, however, the more rapid is the acoustic attenuation with range.