Estimating the Geoelectric Field and Transmission Line Voltages During a
Geomagnetic Storm in Alberta, Canada Using Empirical Magnetotelluric
Impedance Data: The Influence of Three-dimensional Electrical Structures
in the Lithosphere
Abstract
Estimating the effect of geomagnetic disturbances on infrastructure is
an important problem since they can induce damaging currents in electric
power transmission lines. In this study, an array of magnetotelluric
(MT) impedance measurements in Alberta and southeastern British Columbia
are used to estimate the geoelectric field resulting from a magnetic
storm on September 8, 2017. The resulting geoelectric field is compared
to the geoelectric field calculated using the more common method
involving a piecewise-continuous 1-D conductivity model. The 1-D model
assumes horizontal layers, which result in orthogonal induced electric
fields while the empirical MT impedance data account for fully 3-D
electromagnetic induction. The geoelectric field derived from empirical
MT impedance data demonstrates a preferential polarization in southern
Alberta, and the geoelectric field magnitude is largest in northeastern
Alberta where resistive Canadian Shield outcrops. The induced voltage in
the Alberta transmission network is estimated to be ~120
V larger in northeastern Alberta when using the empirical MT impedances
compared to the piecewise-continuous 1-D model. Transmission lines
oriented northwest-southeast in southern Alberta have voltages which are
10-20% larger when using the MT impedances due to the polarized
geoelectric field. As shown with forward modelling tests, the
polarization is due to the Southern Alberta British Columbia conductor
in the lower crust (20-30 km depth) that is associated with a
Proterozoic tectonic suture zone. This forms an important link between
ancient tectonic processes and modern-day geoelectric hazards that
cannot be modelled with a 1-D analysis.