Recent improvements in ocean current measurement from motional electric fields and currents

Abstract

Scientific studies of ocean dynamics and transports require rapid and accurate observations of velocities over large spatial scales. One opportunity to provide some of the needed data is available in the naturally occurring electric currents and magnetic field. These EM signals arise from the motion of sea water through the Earth's magnetic field and contain information about the local and regional velocity. The mixture of local and distant responses is often considered a liability, but this seeming problem can be turned into a great benefit. The local contribution is the water velocity, v , at the sensor location. The distant contribution is determined primarily by the average velocity over the vertical water column, denoted as \bar{v}^{\astr} . The apparent velocity sensed by an instrument moving with the local water velocity is equal to v - \bar{v}^{\astr} . A method to separate these contributions and, thereby, isolate \bar{v}^{\astr} is to obtain simultaneous EM and navigational information. The difference between an independent measurement of the local horizontal velocity and the EM-derived velocity yields \bar{v}^{\astr} . Because \bar{v}^{\astr} is usefully close to being \bar{v} , the vertically averaged velocity, the method holds significant potential for improving current measurement technology. Measurements of motionally induced electric fields and currents are being made from submarine cables, towed electrode systems and free-fall profilers. Significant new information about steady and variable ocean transport has been obtained from these methods. Examples of the recent progress in measurements, interpretations and instrumentation are discussed, and a new device to measure \bar{v}^{\astr} , called the Towed Transport Meter, is described.