Basic Concepts
Electromagneticl geophysical methods are used to map the subsurface resistivity structure. The resistivity of geologic units ranges over many orders of magnitude and depends on their fluid content, porosity, degree of fracturing, temperature, and conductive mineral content. Unlike other geophysical methods (like gravity and magnetism), EM methods include a wide variety of techniques. Each technique, however, involves an EM source (natural or artificial) and measures one or more electric or magnetic field component.
One commonly used EM geophysical method is the magnetotelluric (MT) method. In this method, incoming magnetic fields are used to induce electric currents in the earth which contain information about the resistivity structure of the subsurface. We obtain information about these induced electric fields, or telluric currents, by measuring the magnetic and electric fields at the surface. GGSC scientists use an audiomagnetotelluric (AMT) system, which is similar to a MT system but measures a higher and narrower frequency range, therefore investigates the resistivity structure of the earth down to ~1 km depth.
One particular Audiomagnetotelluric system that we use is the Geometrics Stratagem system, a 4-channel, natural and controlled source tensor system recording in the range of 10 Hz-92 KHz. To augment the low signal in the natural field, we used an unpolarized transmitter comprised of 2 horizontal-magnetic dipoles in the range of 1-70 khz. Data are recorded with the electric field parallel and perpendicular to the regional fault strike and physiographic grain. |
Selected references:
McPhee, D.K., Chuchel, B.A., and Pellerin, L, 2006, Audiomagnetotelluric data from Spring, Cave, and Coyote Spring Valleys, Nevada: U.S. Geological Survey Open-File Report 2006-1164, 41 p.
Rodi, W., and Mackie, R. L., 2001, Nonlinear conjugate gradients algorithm for 2-D magnetotelluric inversion, Geophysics, 66, 174-187.
Vozoff, K., 1991, The magnetotelluric method, in Electromagnetic methods in applied geophysics: M.N. Nabighian, Ed., Society of Exploration Geophysicists, Tulsa, Oklahoma, vol. 2, part B, p. 641-711. |