Magnetotelluric deep electrical sounding and resistivity

Abstract

Since the previous report to the IUGG in this field [Hermance, 1971], geoelectromagnetic induction studies have steadily increased in importance as a means of delineating tectonic structure in the upper few hundred kilometers of the earth. Such an advance is due especially to (1) the development of various computational techniques of modeling, (2) the accumulation of further data in tectonically interesting regions, (3) the increase in our knowledge of the relationships between the electrical conductivity of crustal and upper mantle materials and other parameters such as temperature, physical state, free water content, and seismic velocity, and (4) the ability to interpret many studies in terms of terrestrial conductivity structures which not only provide useful tectonic information not obtainable by other means but also are generally consistent with other relevant geological and geophysical data. Further stimulus has been provided by the availability of good electromagnetic sounding data for the moon obtained by the Apollo series of lunar landings.An outstanding problem remains the limited resolution of these studies as compared to seismology, for example. In consequence, average conductivity‐depth profiles of the upper few hundred kilometers still remain quite poorly determined for most of the earth, and a corresponding level of nonuniqueness is passed on to the models of two or three‐dimensional terrestrial conductivity structure.