Abstract

Nine deep resistivity probes were made in southeastern Nebraska, southwestern Iowa, central Pennsylvania, southwestern and northeastern New York, Vermont, New Hampshire, and Maine. The surveys were designed to determine the requirements for crustal-scale resistivity surveys in areas where there is no thick overburden of conductive sedimentary rocks. The Schlumberger array was used for spacings up to one km, the dipole method for wider spacings. A source dipole moment in the range 10 4 to 10 5 amp-m was used with the dipole method. Electric field measurements were made at distances up to 60 km from the source dipole. These separations were adequate to determine the presence of resistant basement rock, but not adequate to penetrate the crust.Penetration of the crust might have been obtained if the source dipole had been stronger by an order of magnitude, but experience gained in surveys made in northeastern New York and in central Vermont suggests that lateral changes in surface resistivities may present a difficult problem at large dipole spacings. In order to penetrate the highly resistive screening portion of the crust, galvanic resistivity surveys should be supplemented by inductive surveys which would not require such large separations of source and detection coils for the measurements.

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