Scripps Institution of Oceanography

An active source EM method has been used on the deep seafloor near the East Pacific Rise. The source consisted of an 800 meter long horizontal antenna grounded at the ends and supplied with 100 ampere excitation in a range of frequencies extending from 1/4 to 3 Hz. Power for the source was supplied from a surface ship. Receivers were autonomous units each detecting the horizontal components of the electric field by a crossed pair of 9 m antennas. The r.m.s. noise voltage in the receivers was several times greater than the thermal agitation noise in the electrode-ta-sea water resistance of 4 ohms. Electromagnetic induction from turbulence induced by water flow past the electrodes may be a contribution to the excess noise. A signal/noise ratio of more than 20 dB was evident at a distance of 18.9 km from the source in a bandwidth of 2 X 10 -3 Hz.