A QUANTITATIVE METHODOLOGY TO EXTRACT REGIONAL MAGNETOTELLURIC IMPEDANCES AND DETERMINE THE DIMENSION OF THE CONDUCTIVITY STRUCTURE

A methodology to determine quantitatively the dimensionality of the do minant conducting structures and the resolution of the structural para meters in magnetotelluric data is presented. In addition, the method r ecovers the regional impedance responses when the regional structure c an be characterized, at least approximately, as 1- or 2-D. The methodo logy is based upon three general models of the MT tensor, each of whic h has a distinct parameterization and physical interpretation. A weigh ted statistical residual describes quantitatively the fit of the model response to the data within the scatter of the measured data and henc e permits: (1) tests of dimensionality, (2) determination of the appro priate strike angle, and (3) recovery of the regional responses. The m ethod has been tested extensively with synthetic data and proven to be successful. These synthetic studies give insight into the different p hysical parameterizations and the stability of the parameters determin ed. We describe and illustrate some of these synthetic studies. With f ield data, the methodology is not always as straightforward, but its a pplication to a great many sites has proven valuable. Data from two cl osely spaced sites, which are both affected by strong but very differe nt 3-D effects, are analysed to illustrate the geological significance of the results. The analyses reveal and recover regional responses wi thin the data which indicate the presence of electrical anisotropy loc ated deep in the crust and upper mantle. Analyses of the entire data s et, of which these two form a part, confirm this finding.