Calculations of voltages for magnets-telluric modelling of a region with near-surface inhomogeneities
Date
1987
Authors
Poll, Helena Eva
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Abstract
In the magneto-telluric method, the electric field measurements are obtained by recording the voltages between grounded electrodes and dividing by their separation distances. Such a procedure gives the true value of the electric field only if it is uniform between the electrodes. In regions of near surface inhomogeneities this condition is not fulfilled and in extreme cases each electrode may be in contact with surface material of different resistivity. In this thesis a method is developed to modify an existing two-dimensional finite difference program so that voltages rather than electric fields are calculated at each node of the numerical grid in the B-polarization model. The numerical results are checked against the exact analytical expressions for voltage obtained for the control model of Weaver, Le Quang, and Fischer (1985) Both the analytical and numerical expressions for voltage are presented in a form suitable for programming and a method for dealing with an electrode profile that is not perpendicular to the strike is developed. (A listing of the Fortran programs for the analytic calculations and the Fortran subroutines for the numerical calculations are included in the appendix.) Finally real data obtained over the Gloucester fault in Canada are compared with results given by finite difference modelling based both on voltage calculations and on the more traditional electric field calculations. Although neither numerical method is in remarkably close agreement with the real data the voltage result is a better approximation to the observed behavior of the measured electric field. Some errors inherent in the magneto-telluric studies of such regions are discussed notably that of a spurious electric field parallel to the fault which arises in voltage calculations when the electrode profile is not perpendicular to the fault.