8. The Magnetotelluric Method

SUMMARY In the magnetotelluric (MT) method, natural electromagnetic fields are used to investigate the electrical conductivity structure of the earth. Natural sources of MT fields above about 1 Hz are thunderstorms worldwide, from which lightning radiates fields which propagate to great distances. At frequencies below 1 Hz, the bulk of the signal is due to current systems in the magnetosphere set up by solar activity. In both cases the electromagnetic (EM) fields at the surface of the earth behave almost like plane waves, with most of their energy reflected but with a small amount propagating vertically downward into the earth. The amplitude, phase, and directional relationships between electric (E) and magnetic (H or B) fields on the surface depend on the distribution of electrical conductivity in the subsurface. By use of computed models, field measurement programs can be designed to study regions of interest within the earth from depths of a few tens of meters to the upper mantle. Equipment to carry out the measurements consists of magnetometers for the frequency range of interest; pairs of electrodes separated by suitable spacings to sense the electric field variations; plus amplifiers, filters, and suitable digital recording and processing systems to permit the signals to be captured and analyzed. The magnetometers in particular must have very low noise and great stability because those signals are so weak.