Electromagnetic induction in the oceans and inferences on the constitution of the earth

This review concentrates on the uncertainties surrounding interpretation of sea floor impedance measurements. Oceanic motionally induced signals prove to be noise generators which limit the low frequency range of usable signals. At high frequencies the screening by a thick ocean and by the sediments and rocks of layer two present insuperable barriers to detection of poorly conducting rocks in the depth range 2 to 30 km below the sea bottom by usual methods. The conductivity of this layer is important for the interpretation of all ocean impedance measurements because it determines the width of a boundary zone at the continental margins of the ocean. If the conductivity is as low as 10^–5 S/m the bounding zone begins to fill the whole ocean. It is suggested that use of an active, manmade EM source can provide signals at the sea bottom capable of resolving the uncertainty.