## Publication 33

##
Calculations of voltages for magnetotelluric modelling of a region with near-surface inhomogeneities

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H. Poll, J.T. Weaver, and A.G. Jones

### Abstract

In the magnetotelluric method, measurements of the components of the horizontal electric field are obtained by
recording the voltage between grounded electrodes and dividing by their separation distances. This procedure gives the
true value of the electric field only if it is uniform between the electrodes. In regions of near-surface inhomogeneity this
condition is not fulfilled, and in extreme cases each electrode may be in contact with surface material of different
resistivity. It is therefore suggested that voltages, rather than electric fields, should be computed in the two-dimensional
modelling of such regions in the B-polarization mode, and that magnetotellunc impedance calculations for comparison
with real data should be based on voltages. A method for modifying an existing finite difference program is described,
and sample calculations of voltage differences in the control model of Weaver, LeQuang and Fischer are checked
against the exact analytic results that can be obtained. 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
conventional electric field calculations.

### Source

Physics of the Earth and Planetary Interiors, **53**, 287-297, 1989.

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