Microscopic scale conductivity as explanation of magnetotelluric results from the Alps of Western Switzerland

Abstract

Recent MT soundings carried out in the Penninic Alps of Valais have shown the presence of a very good, outcropping conductor. Extremely high conductivity was attributed to the presence of graphite. To verify this assumption, the electrical properties of borehole black shales were measured under simulated physical conditions (electrical frequency, hydrostatic confining pressure, internal fluid pressure, temperature). These measurements showed that under all physical conditions (electrical frequency, in the 0.005-200 Hz interval; hydrostatic confining pressure up to 39 MPa; internal fluid pressure up to 23 MPa; temperature up to 180 degreesC) one of the samples studied was very conductive (resistivity less than 2 Omegam under all physical conditions). Interestingly, despite similar macroscopic aspect, other samples from a nearby borehole were found to be only slightly conductive. Chemical, mineralogical and petrographic investigations revealed that the enhanced electrical conductivity is mostly due to textural characteristics (such as grain size and carbon film distribution at the grain boundaries) rather than to chemical differences.