Thomas B. Sanford

A velocity profiler based on the principles of geomagnetic induction

A technique to determine the variations of horizontal velocity between the sea surface and bottom, based on the measurement of electric currents generated by the motion of the seawater through the Earths magnetic field, has been developed. A freely-falling Electro-Magnetic Velocity Profiler, EMVP, senses the currents as a function of depth. Velocity is inferred as the equivalent velocities required to generate the observed electric currents in the local geomagnetic field. The inferred velocity profile is relative to an unknown, but depth-independent, velocity contribution. Hence, the profiles are of the depth-variable velocity components. The device is released from the surface, falls to a preset depth or to the sea floor and returns to the surface. Both descent and ascent take about 90 min in water 6000 m deep. The r.m.s. velocity errors are about 1 to 2 cm s−1 at the vertical resolution of independent methods, 50 m and larger. Simultaneous profiles by two identical EMVPs have r.m.s. velocity differences of less than 0.7 cm s−1 at a vertical resolution of 10 m. A second electric measurement on the instrument yields a velocity profile within 0.3 cm s−1 r.m.s. of the first electric measurement every 10 m. A performance level of ±1 cm s−1 at a vertical resolution of 10 m is indicated. This level is expected in the absence of strong magnetotelluric currents, which can produce errors as large as 10 cm s−1 during infrequent (10 to 20 times per year) periods of strong temporal fluctuations of the geomagnetic field. Temperature, electrical conductivity, pressure, and other variables are recorded twice each second. The digitally recorded data are processed aboard ship to obtain velocity and density profiles.

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 earths 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 \pm1 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.

Performance of an absolute velocity profiler based on acoustic doppler and electromagnetic principles

Prepared for the National Science Foundation, Office of the International Decade of Ocean Exploration under Grant OCE76-24605.