A velocity profiler based on the principles of geomagnetic induction

Abstract

Abstract 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 Earth's 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.