Part III: Multidimensional Magnetotellurics
9.1 IMPEDANCE OF A HORIZONTALLY INHOMOGENEOUS MEDIUM
Up to this point we have considered the Tikhonov-Cagniard model, which is the fundamental model of magnetotellurics. The model is very simple. A plane monochromatic wave E?, H? excites the plane earth consisting of homogeneous isotropic layers with horizontal boundaries.
10.1 QUESTIONS AND STAGES IN MAGNETOTELLURIC INTERPRETATION
The impedance tensor, [Z], and the Wiese-Parkinson matrix (the tipper), [W], are determined as functions of the period, T. Interpretation of these functions leads to the construction of a geoelectric model for the region under study. This procedure consists of three basic steps.
11.1 MULTIDIMENSIONAL DEFINITION OF THE INVERSE MAGNETOTELLURIC PROBLEM
We will revert to the plane model of the inhomogeneous earth that we considered in Section 9.1, and assume that the size of the area essential for MTS does not exceed several hundred kilometers and that a primary field within this area is horizontally unifor.
Figures & Tables
Magnetotellurics in the Context of the Theory of Ill-Posed Problems
In 1950, A. N. Tikhonov published a paper, “On determination of electric characteristics of deep layers of the earth’s crust” in the proceedings of the Academy of Sciences of the USSR (Doklady, Akademia Nauk SSSR). In this paper, Tikhonov examined the relations between the horizontal components Ex, Hy of the magnetotelluric field (the natural time-varying electromagnetic field of cosmic origin), and introduced the impedance Z = Ex/Hy as a quantity characterizing the electric conductivity of the earth’s interior. A one-dimensional model disregarding the lateral effects was used for impedance interpretation. In this way, the feasibility of sounding the earth through the magnetotelluric observations at a single point on the earth’s surface was demonstrated, and new information about conductivity in the mantle was obtained.
This simple idea gave impetus to the development of a new geophysical method called magnetotelluric sounding, or MT sounding, or simply MTS. This method is a variation of frequency sounding. With all its strengths and weaknesses, it has found wide utility in commercial electric exploration and deep geoelectric investigations. A new branch of geophysics, given the name magnetotellurics, has come into being.