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

Abstract

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 and the large error estimates combine to give low overall precision. The diurnal results do not agree with the continuum results if the continuum is corrected for latitudinal variations of the source field between the reference station and seafloor sites. Data at periods as short as 10 minutes are required to resolve structures in the upper 30 km of the mantle. Artificial source fields may be necessary to obtain periods short enough to resolve crustal features. Periods longer than diurnal will be required to study sub-lithospheric resistivity variations.