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.
Once the signals have been recorded they must be processed and analyzed. Processing is normally done in the frequency domain because the theory is simpler than in time domain. Hence processing begins with Fourier transformation, from which earth impedances to the incident waves as functions of frequency, direction, and position are computed. Processing in many systems is now done in real time, while the signals are being acquired.
These computed impedances are next interpreted in terms of electrical conductivity versus position and depth. Numerical models for one-dimensional, twodimensional, and three-dimensional structures are used for this last step. Interpretation is the most difficult part of the method because information is seldom complete and the models are never complex enough to represent a real earth. For that reason, and to use the MT data to best advantage, other available external data such as well logs, seismic, and any other electrical data are commonly utilized to help with interpretation.
The main shortcoming of the method is the difficulty of obtaining data in electrically noisy areas or where the surface is unstable.
The strengths of the method are its unique capability for exploration from very shallow depths to very great depths without artificial power sources and with little or no environmental impact. At high frequencies, audio frequency MT (AMT) has been used to map groundwater and major base metal deposits at depths from 50–100 m to several kilometers. However, the major application of MT is to petroleum exploration in areas where reflection seismology is very expensive or ineffective, such as in extreme terrain and beneath volcanics. Another major successful application has been to geothermal exploration.
Major research problems at the moment concern interpretation in areas of complex three-dimensional structure, and improvements in production rate.
Figures & Tables
Electromagnetic Methods in Applied Geophysics, Volume I, Theory presented the mathematical and physical foundations common to all EM methods. The purpose of Volume I was to help facilitate the understanding of the theory involved and to provide a limited amount of interpretational aids. Volume II, Applications is devoted to a method-by-method treatment of the principal EM techniques in common use.