# Dictionary:Magnetotelluric (MT) method

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(mag nēd’ ō t∂ loo, rik) A method in which orthogonal components of the horizontal and magnetic fields induced by natural primary sources are measured simultaneously as functions of frequency. Apparent resistivity ${\displaystyle \rho _{a}}$as a function of frequency ${\displaystyle f}$ is calculated:

${\displaystyle \rho _{a}={\frac {1}{\mu \omega }}{\Big (}{\frac {E_{i}}{H_{j}}}{\Big )}^{2},\omega =2\pi f}$

Also, ${\displaystyle \rho _{a}=0.2{\frac {Z^{2}}{f}}}$, where Z is the Cagniard impedance or tensor impedance (q.v.); see Vozoff (1972)[1].

Resistivity as a function of depth can be calculated for a layered Earth. For a nonlayered Earth, two apparent resistivity curves result from rotating the MT tensor impedance and interpretation is more involved (see tensor magnetotelluric method). The predominant sources of energy for magnetotelluric measurements are micropulsations having frequencies of less than 1 Hz. Sometimes magnetotelluric measurements are made at audio frequencies using energy from sferics; the method is then referred to as the audiomagnetotelluric method (AMT). See telluric current method.

## References

1. Vozoff, Keeva (1972). "The magnetotelluric method in the exploration of sedimentary basins". GEOPHYSICS 37 (1): 98–141. doi:10.1190/1.1440255.PDF version