A magnetotelluric study of southern Vancouver Island.
Date
1973
Authors
Nienaber, Wilfred
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Abstract
Magnetotelluric studies involve a determination of the electrical subsurface structure of the earth from measurements of the naturally occuring electromagnetic variations at the surface of the earth. In the present work magnetotelluric measurements made at Victoria, B.C. are analyzed. The results are indicative of an anisotropy in the surface impedance with the electric field in approximately the NS direction enhanced by a factor of four or more. The enhancement is possibly due to the effects of discontinuities in the surface layer conductivity at a geological fault, the Leech River Fault, and a seawater channel, the Straits of Juan de Fuca and Georgia, at Victoria.
As is usually the case in anisotropic situations, calculations using the two different orthogonal sets of electric and magnetic field measurements yielded two different results. Often in magnetotelluric work a model of the electrical substructure is proposed on the basis of one set of measurements. In the present work, an interpretative technique employing both sets of measurements is used. The resulting model proposes a highly resistive surface layer followed by a highly conducting layer over a highly resistive earth. The resistivities proposed for the three layers are approximately 4000, 9, and 1000 ohm-m respectively where the first two layers are each 10 km thick.
The validity of the rotational transformation, normally applied to magnetotelluric data in anisotropic situations, is examined experimentally for telluric measurements. A comparison of polarization plots and of power spectra for telluric data transformed into a new coordinate system and for actual measurements in that system only partially verify the form of the transformation. The observed discrepancies may be due to a rather poor quality of signals at certain periods or the possibility that a scalar representation for the conductivity is inadequate.