The Effect of Magnetotelluric Spatial Data Sampling in Geoelectric Hazard Estimation

Abstract:

Many analyses of geoelectric fields induced by geomagnetic disturbances utilize data from sparse, national-scale magnetotelluric (MT) surveys. For example, in the United States, geoelectric hazard analyses often leverage EarthScope MT data that were collected with a nominal station spacing of 70 km. Although such data are a valuable source of information about the role of solid-Earth electrical conductivity in giving rise to geomagnetically induced currents (GICs), their adequacy for resolving the spatial heterogeneity of the surface geoelectric field has not previously been critically examined. Because the Earth’s bulk conductivity structure can vary by orders of magnitude on length scales as short as hundreds of meters, comparable to the inductive length scale at short periods, the effect of spatial data sampling must be considered. To address this problem, we evaluate the degree to which integrated geoelectric field estimates along electrical transmission lines differ between using only national-scale magnetotelluric data (specifically EarthScope data) and using denser regional-scale magnetotelluric data. We find that EarthScope-style data sampling provides a useful first-order estimate of geoelectric hazard patterns. However, we also find that using higher-density data can significantly change line-averaged electric field estimates at the level of individual transmission lines by up to an order of magnitude. Although EarthScope-style MT data sampling is valuable in assessing geoelectric hazards at a national scale, in some locations further data acquisition is necessary to fully characterize geoelectric hazards. We conclude that the underlying regional geology, specifically the combination of lithosphere-scale lithotectonic structure and geologic history, is a critical factor in determining the need for denser MT data sampling.