Magnetic distortion of the magnetotelluric tensor: a numerical study

Decomposition of the magnetotelluric (MT) tensor when the electric field al one is subject to local galvanic distortion has received much attention in recent years. Recently some authors have extended such procedures to includ e the effects of the associated magnetic field distortion as well. With the aid of a three-dimensional modelling program the validity of the assumptio ns made in these analyses have been examined by considering the response ov er a range of periods of a small conductive cube at the surface of the eart h and near a two-dimensional fault. The study indicates that the inclusion of magnetic distortion is necessary and important at short periods when ind uction occurs in the anomaly itself, but that the elements defining the mag netic distortion matrix become complex at such periods so that the assumpti ons underlying the theory are no longer valid. At longer periods the magnet ic distortion matrix does become real and frequency-independent and therefo re determinate, bur its effect on the response becomes insignificant compar ed with that due to electric distortion. At these longer periods the phases of the regional impedances and the ratios of the electric distortion param eters are correctly recovered whether magnetic distortion is present or not . Calculations were repeated for a resistive cube, and also for a highly re sistive region beneath the surface layer, with no significant enhancement o f magnetic distortion. It is concluded that, at least for such models, incl usion of magnetic distortion in decompositions of the MT tensor does not, i n general, offer any improvement over the conventional decompositions in wh ich only distortion of the electric field is taken into account.