GEOLOGICAL SURVEY INVESTIGATIONS OF THE ELECTRICAL PROPERTIES OF THE CRUST AND UPPER MANTLE

The electrical properties of rocks deep in the crust and in the upper part of the mantle may be studied with various electrical methods. The U. S. Geological Survey has conducted such studies using three methods: conventional galvanic resistivity surveys, inductive resistivity surveys using the magnetotelluric field, and the measurement of electrical properties of rock samples at high temperatures. By combined sample measurements, galvanic resistivity measurements and magnetotelluric studies, the resistivity profile through the crust and upper mantle may be defined more closely than has been possible previously. We have recognized the existence of a heterogeneous surface layer consisting of sedimentary rocks and weathered igneous and metamorphic rocks. Within the crust, a marked decrease in resistivity is sometimes observed at depths of about 10 kilometers, perhaps corresponding to the Conrad discontinuity sometimes recognized by seismologists. It has not been possible to recognize an electrical boundary at depths corresponding to those at which the Mohorovičić discontinuity is noted by seismologists. However, a marked decrease in electrical resistivity occurs at somewhat greater depths, between 60 and 100 km.