Investigation of Large-Scale Inhomogeneities in the Earth by the Magnetotelluric Method

In the magnetotelluric analysis proposed by Cagniard, a resistivity profile of a homogeneous stratified earth model is determined from the ratio of the tangential components of the natural electric and magnetic fields at the surface of the earth. This type of analysis is often confused by the fact that apparent resistivity estimates obtained from data recorded at one site on different occasions may vary considerably. Also the apparent resistivities computed from one component of the E field and its orthogonal H field component may be significantly different from those computed from another pair of orthogonal field components. Cantwell points out that these variations may, at least in part, be due to two-dimensional inhomogeneities and/or anisotropies together with randomly polarized magnetotelluric fields. Accordingly he has suggested testing the wave impedance of the earth as a rank two tensor. This paper describes a measurement and analysis procedure developed using a model based on a two-dimensional wave impedance tensor. It was found that a rotation of the measuring axis to minimize the main diagonal terms in the impedance tensor yields a reasonably consistent estimate of the direction and magnitude of a major inhomogeneity in the apparent resistivity profile.