Near-surface Doppler Sonar measurements in the Indian Ocean

Abstract

A ship-mounted Doppler Sonar has been used to estimate near-surface current profiles in the Somali Current region of the Indian Ocean. The system was deployed on R.R.S. Discovery in May and June 1979. Four 75-kHz, narrow beam transducers were mounted on the ship in the so-called ‘Janus configuration’. Acoustic pulses of 50 or 100 ms duration were transmitted, and echoes due to volume scattering were converted using a Fast Fourier Transform processor to a series of power spectra corresponding to successive range gates. The spectra were averaged over typical sets of 256 pulses, and the Doppler shift observed for each beam and at each depth was used to calculate the velocity of the scatterers with respect to the ship. Two sets of sample data are presented with derived current profiles, which are compared to those produced with a conventional profiling current meter. The problems with the data are discussed, in particular that of poor vertical resolution, and the effects of short-period (e.g., wave-induced)_motion of the ship on such measurements.