Magnetotelluric measurements across the Wisconsin Arch

Abstract

Magnetotelluric observations, consisting of measurements in the period range from 1000 sec were made at five stations across the Wisconsin arch. Scalar and tensor apparent resistivity values were computed from the power density estimates of the horizontal field components. Electrical resistivity models of the earth at each of the five sites were obtained by fitting the apparent resistivity values of one-dimensional models to the measured values. The preferred conductivity models show a layered crustal structure which agrees well with the crustal seismic profile at the four best sites. The most striking result is an order-of-magnitude increase in conductivity at about 12 km depth, which corresponds to an intracrustal velocity interface. Near the crust-mantle boundary an increase in resistivity, comparable to that of the upper crust, is observed. Indications of a conductivity increase at 70 km depth were obtained at two of the stations; one of the models tenuously suggests another steeper conductivity increase at a depth of 170 km.