Abstract
A smooth two-dimensional least-squares inversion was applied to the COPROD2 magnetotelluric (MT) data provided by the Geological Survey of Canada. Selection of the optimum smoothness at each iteration was based on a statistical criterion ABIC (Akaike's Bayesian Information Criterion), which was derived from the Bayes statistics and the entropy-maximization theorem. Six frequencies, below 2 Hz, were picked for the inversion. Both TM and TE mode data at 35 sites were used for the weighted inversion in which the weights were calculated from the observation errors listed in the data file. The MT responses and their partial derivatives with respect to model parameters were calculated with a finite-element method. The inverted model basically shows a three-layer earth: conductive, resistive, and conductive from the surface. Two major conductive anomalies are recognized in the resistive layer of 100 km thickness. One is interpreted as the North American Central Plains (NACP) anomaly, and the other corresponds to the Thompson Belt (TOBE) anomaly.
