Marine controlled-source electromagnetic sounding .2. The PEGASUS experiment

The marine controlled-source electromagnetic sounding method developed over the past 15 years at Scripps Institution of Oceanography employs a towed seafloor electric dipole transmitter of moment 4 x 10(4) Am and multiple free-vehicle seafloor electric field recorders. A survey of 40 Ma normal oceanic lithosphere in the northeast Pacific using frequencies of 0.25 to 24 Hz and synchronous stacking of 0.25- to 12-hour-duration detected signals at transmitter-receiver ranges between 5 and 95 km. One-dimensional electrical conductivity structure is recovered from the data using the Occam process of nonlinear regularized inversion. Repeated inversion of a model terminated with an essentially infinite conductor or resistor demonstrates that the maximum depth of inference for this experiment is about 30 km, well into the upper mantle, with bounds placed on conductivity to depths of 60 km. Structure shallower than about 1 km is comparable to that obtained by a similar experiment on the East Pacific Rise and by borehole logging, with a sharp increase in resistivity at depths of 600-800 m, although strictly our experiment is sensitive only to integrated square root of conductivity, or total attenuation, in the surface layers. The lower crust and upper mantle has a resistivity between 2 and 7 x 10(4) Omega m and a transverse resistance of at least 10(9) Omega m(2), suggesting at most 0.3% volume fraction of free water in the lower crust and some form of conductivity enhancement over mineral conductivity in the uppermost mantle. Although resolution is weak, below 30 km our data are compatible with a dry olvine model of mantle conductivity-temperature.