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A geomagnetic variation study on mature oceanic lithosphere in the North Atlantic just south of Bermuda has revealed the presence of at least one layer of low electrical resistivity. The low resistivity layer of approximately 10 ohm-m has been found at three widely spaced seafloor sites with crustal ages of 85, 110 and 150 million years. There is also evidence that the resistivity increases to greater than 20 ohm-m below about 100 km. Apparent resistivity and phase versus period are calculated using the vertical gradient of the horizontal magnetic field variations to estimate the seafloor electric field. The vertical gradient method assumes that the seasurface magnetic variations can .be estimated from a nearby land station and that no local magnetic induction occurs at either reference or seafloor site. Both assumptions are critically evaluated during the analysis. Seafloor observations are modeled using the Monte Carlo technique. Estimates of the smoothed resistivity structure as well as the resolution and precision of the estimates are made using the Backus-Gilbert method. Models are shown to be severely data limited. Resolution is found to be poor in the upper 30-40 km of the lithosphere due to the lack of reliable data at periods shorter than 30 minutes. The uncertainty involved in estimating the magnetic field at the seasurface

Geomagnetic variations in the Northwest Atlantic : implications for the electrical resistivity of the oceanic lithosphere