JOURNAL OF THE BALKAN GEOPHYSICAL SOCIETY, Vol. 2, No 3, August 1999, p. 63-74, 7 figs.

Properties of the magnetotelluric

frequency-normalised impedance

over a layered medium

Ahmet T. Basokur

Ankara Universitesi, Fen Fakultesi, Jeofizik Muh. B., Tandogan, 06100 Ankara, Turkey.

E-mail: basokur@science.ankara.edu.tr

( Received 21 January 1999; accepted 9 April 1999 )

*Abstract:* *The frequency-normalised impedance (FNI) function is introduced to give a physical significance to the magnetotelluric data. The FNI function is separated into its real and imaginary parts that have the same denominator to estimate their values in the high- and low-frequency limits. The behaviour of the apparent resistivity definitions and the phase is explained theoretically by considering the real and the imaginary parts of the FNI function. The oscillations on the apparent resistivity curves are explained by using ascending and descending types two-layer curves of the FNI function. *

The concept of the reciprocal geoelectric section is developed by using the properties of the FNI function. It has been shown that the apparent resistivity curves of a geoelectric section and those of corresponding to the reciprocal geoelectric section are symmetric. The axis of symmetry is the horizontal axis that intersects the vertical axis at unity.

The FNI function assists the interpreter by giving a clear idea about the number of layers, the resistivity ratio between consecutive layers and it serves a good approximation of the true resistivity values of the subsurface layers.

* Key Words: Frequency-normalized Impedance, Magnetotellurics, Apparent Resistivity Definitions*.