Magnetic distortion of the magnetotelluric tensor: a numerical study

Abstract

Decomposition of the magnetotelluric (MT) tensor when the electric field alone is subject to local galvanic distortion has received much attention in recent years. Recently some authors have extended such procedures to include the effects of the associated magnetic field distortion as well. With the aid of a three-dimensional modelling program the validity of the assumptions made in these analyses have been examined by considering the response over a range of periods of a small conductive cube at the surface of the earth and near a two-dimensional fault. The study indicates that the inclusion of magnetic distortion is necessary and important at short periods when induction occurs in the anomaly itself, but that the elements defining the magnetic distortion matrix become complex at such periods so that the assumptions underlying the theory are no longer valid. At longer periods the magnetic distortion matrix does become real and frequency-independent and therefore determinate, but its effect on the response becomes insignificant compared with that due to electric distortion. At these longer periods the phases of the regional impedances and the ratios of the electric distortion parameters are correctly recovered whether magnetic distortion is present or not. Calculations were repeated for a resistive cube, and also for a highly resistive region beneath the surface layer, with no significant enhancement of magnetic distortion. It is concluded that, at least for such models, inclusion of magnetic distortion in decompositions of the MT tensor does not, in general, offer any improvement over the conventional decompositions in which only distortion of the electric field is taken into account.