The Theory, Measurement, and Applications of Very-Low-Frequency Magnetotelluric Variations

Past and present observations of magnetic and telluric field variations are reviewed. Particular attention is given to measurement of the related E and H fields for very-low-frequency (0.001-1.0 cps) vertically incident waves and the characteristics and probable origins of these waves are discussed. Current techniques for measuring these waves include orthogonal sets of earth current probes, mutually perpendicular air-core loops, high-mu cores wound with many turns, and modem optical pumping magnetometers. Simultaneous observations of the E and H fields of these waves may be employed in geophysics for vertical profiling, resistivity determinations, the location of faults, axes of inhomogeneity, and ore bodies. In addition such observations are of great value in observing the complex interplay of the solar corpuscular stream ("solar wind") and the earth's magnetic field, and may also be employed as indicators of high altitude nuclear events.