Active source electromagnetic technique for conductivity profiling below the seafloor

The authors describe a new approach to the problem of active source electromagnetic sounding at sea and give initial results from its first use, across a mid-ocean ridge crest-the East Pacific Rise at 13°10´N. The electromagnetic energy is generated by a 100 m horizontal electric dipole of moment 6000 Am, which is towed close to the sea-bed. Square-waves with frequencies from 1/4 Hz and 8 Hz are transmitted and these propagate through the, generally, more resistive lithospheric half of the ocean-seafloor interface. The receivers are remotely operating sea bottom instruments, each equipped with an orthogonal pair of 12 m horizontal electric antennae. Electric fields of the order of 10^-10 V/m are measured. Initial results show porosities of 30% or more at 30 m below the seafloor decreasing to less than 5% at a depth of 1 km. No intrinsic difference is observed between the upper crustal conductivity structure of the rise axis and that of 100000 year old crust