A SIMPLE TECHNIQUE FOR ANALYZING AND PARTLY REMOVING GALVANIC DISTORTION FROM THE MAGNETOTELLURIC IMPEDANCE TENSOR - APPLICATION TO ABITIBI AND KAPUSKASING DATA (CANADA)

Galvanic distortion of the regional electrical field by surficial local heterogeneities disturbs the regional inductive response. We present a simple method for (a) detecting the presence of galvanic distortion in the measured impedance tensor, (b) defining the regional dimensionality of the background (1-D or 2-D; 3-D is excluded from the analysis) and (c) recognizing whether or not measurements were taken in the true regional axes (if the regional tensor is 2-D). In the case of distortion of a 1-D regional tensor, a tensor decomposition does not bring more information than that given by the effective impedance (Berdichevsky & Dmitriev 1976). In the case of a 2-D regional tensor, if the measurement axes do not coincide with the regional principal axes, both apparent resistivities and phases will be a effected. Whether or not measurement and principal axes coincide will define the form of distortion model to use in the removal technique. A simple inversion method is used for this purpose. The approach, which is a cross between that presented by Bahr (1988) and Groom & Bailey (1989) and does not remove static shift, is tested on synthetic and real data. The results of the application to real data suggest that this type of analysis should be carried out prior to any interpretation of magnetotelluric data collected in regions of complex geology.