Oceanic electric fields originate from (1) induction by magnetic pulsations in the ionosphere, (2) water motions across the earth's field and (3) electrochemical processes associated with sea floor materials and with tissues of marine organisms. Precise recording of these signals is difficult because of (1) their small size, and (2) a large and unavoidable noise occurs at the contact points between sea water and measuring devices.
The spectrum P of average natural electric activity at the sea floor in the band 10^-3 to 10⁺¹ cph roughly follows the trend P=kf^-1, f=frequency, k centered at 10^-2 μV² m^-2. The noise spectrum of typical Ag-AgCl electrode pairs P′ on the sea floor environment approximates k′= 10^-1 μV² with tenfold variations in both directions. Thus, achieving a 10¹ signal to noise ratio requires an electrode separation of 10 meters. Even so, the unavoidable electrode voltage mismatch, typically 10² to 10³ μV, fatally eradicates the signal datum unless considerably longer lines area used.
Rejection of electrode bias and noise to allow short electrode separations can be done by physically inverting the electrodes positions, which is most practically achieved with rotating instruments, or (2) by switching back and forth the sea water connections between electrodes and the salt bridge pipes which perform electrical contact with the ocean. A method to achieve this “water chopping” is described.
Examples of ionospheric and barotropic velocity signals recorded on the sea floor with 5m span instruments and presented.