Magnetic Signals from an Ocean Eddy.

Abstract

The movement of sea water in the magnetic field of Earth causes a process of motional electromagnetic induction. As a result, electric currents flow both in the sea water, and in the material of the seafloor. There has been long-standing interest in observing the magnetic fields of such motional induction, especially at periods longer than a day; that is, longer than the main ocean tides. Conditions appear to have been favourable for such observations during an experiment which took place in 1983/84 on continental Australia and the floor of the adjacent Tasman Sea (the Tasman Project of Seafloor Magnetotelluric Exploration, or Tasman experiment). The particular circumstances were the passage of an active ocean eddy across a line of seafloor magnetometers and electrometers; the presence on the seafloor of a 1 km pile of porous (and thus electrically-conducting) sediments; and the simultaneous observation of magnetic fluctuations at sites remote from the eddy. Using remote-reference techniques, the magnetic signals due to the passage of the eddy are clearly identified, and are of some 30 nT in amplitude. These combine with the corresponding seafloor electric field signals, of some 30 μV.m-1 in amplitude, to give seafloor magnetic to electric signal ratios of 1.14 ± 0.19 nT·μV-1·m. Applying basic theory for quasi-steady rectilinear flow to this figure gives a seafloor conductance value of 910 ± 150 S, and so a “leakage value” of 0.06 ± 0.01. Both these latter figures agree with estimates for the same quantities calculated independently, from knowledge of the Tasman seafloor sedimentary column; an agreement which supports the application of quasi-steady theory to the present case. The result has an application to oceanography, in allowing an estimate of the integrated velocity, or barotropic flow, above a station where such seafloor electric and magnetic observations are made. The estimate of seafloor conductance, and the implication that conductivity must greatly decrease below the sediments, is a geological result of value, and suggests the similar application of such measurements in other situations. As a case history, there are lessons to be learned from the present instance regarding the operation of ocean-bottom geomagnetic observatories, especially in areas where there are active ocean movements and high seafloor conductances.