Atlantic Lithosphere Sounding

Abstract

We have measured electromagnetic fluctuations at a number of stations on islands and on the sea floor of the Atlantic. Magnetic fields H were measured on the Bahamas, on Bermuda and at two sea floor stations. Horizontal components E of the electric field were measured on Bermuda and at three deep sea stations. The latter form a triangular array with 100km legs on the Hatteras Plain. The age of the underlying lithosphere is estimated at 130 million years.E at the sea floor is dominated by the Lorentz force U×B below 0.1cpd. These fields have a typical horizontal scale less than 100km and are unsuitable for deep electromagnetic soundings but provide limits to shallow conductivity. At higher frequencies, up to the oceanic cut off of several cycles per hour, the fields are of ionospheric origin.The impedance tensor has been estimated at Bermuda and two sea floor stations for the ionospheric fields. On Bermuda the local influence of the island has been removed by Larsen's method for comparison with the sea floor impedances. At both sea floor stations where impedances were measured, only the component generated by eastward E and northward H is well determined. Models of lithospheric conductivity which are consistent with all measurements show diminishing conductivity below a crustal layer, then sharp rises at 70 and 250km.