Deep ocean profiles from electromagnetic and acoustic Doppler measurements

An instrument is described for measuring profiles of horizontal velocity as a function of depth in the deep ocean. The method is a hybrid technique based on the principles of electromagnetic induction and acoustic Doppler and is mobile since not dependent on bottom-installed equipment. The EM method measures weak electric currents in the sea induced by the motion of the water through the earth\´s magnetic field. The resulting velocity profile reveals the velocity shears but is relative to an unknown, depth-independent reference velocity. The reference velocity is determined by acoustic Doppler measurements of the absolute velocity of the instrument as it nears the sea floor. The two methods are incorporated into a single freely-falling probe which measures and internally records the electric and acoustic signals and other variables such as temperature and vehicle orientation. The method yields velocity determinations every 5-10 m with an uncertainty of about cm/s. A round trip in 6000 m of water lasts about 3 hours. Data from this method have been used to study mid-ocean eddies, internal waves, and the Gulf Stream.