TABLE OF CONTENTS Magnetotellurics employ naturally occurring electromagnetic fields to investigate the earth's internal resistivity distribution. The interaction of electromagnetic fields with the earth's crust and mantle generates induction currents resulting in secondary magnetic fields used to determine subsurface resistivity ?~( E/H) 2. Combined with other geophysical methods, the distribution of rock types, sediments, and other geologic structures can be mapped out for petroleum, mineral, and groundwater exploration.
Naturally occurring electromagnetic field sources vary in frequency between roughly 10 -4 - 10 4 Hz giving rise to telluric currents in the earth's crust. Electromagnetic fields on the low end of this frequency range result from sun spot activity. Variations in the earth's magnetic field can occur with periods of about 100 s and longer. The interaction of the solar wind with earth's ionosphere gives rise to Schumann resonances ~0.05 Hz in the earth-ionosphere spherical resonant cavity. The North American 60 Hz power line grid actually excites the 9th Schumann harmonic resonance. Higher frequency magnetotelluric sources above the kHz range result from lightning in the atmosphere.
Because the electromagnetic penetration depth varies with frequency, penetration depths may vary between 10 2 10 5 m, also depending on the subsurface resistivity distribution. The electrical resistivity of igneous rocks can vary roughly between 10 3 10 6 ?m, sedimentary rocks between 10 10 3 ?m, and rocks containing partial melts between 10 -2 - 10 ?m.
