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Electromagnetic Methods in Geophysics

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.

This slide shows an example of the resulting 2D inversion of our AMT data where cool colors show resistivity highs and warm colors show lows.  We computed this 2D inverse model from the electric field perpendicular mode data using the conjugate gradient, finite-difference method of Rodi (2001) and Mackie and a 100 ohm-m half-space starting model with topography. Various starting models were used to test the sensitivity of the model. 

  Example of an electrical resistivity profile
  Click on images for a larger view.

With the help of other geophysical and geologic data, we can interpret these resistivity models to help us understand the geometry of the subsurface structure better. This diagram shows how electrical resistivity data can be used in collaboration with gravity geophysical data and surficial mapping to provide greater insight into subsurface rocks and structures.

  Electrical combined with magnetic and gravity geophsyical data

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.
 

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