LAUSR

An analysis is made of Earth‐surface geoelectric fields and voltages on electricity transmission power‐grids induced by a late‐phase E3 nuclear electromagnetic pulse (EMP). A hypothetical scenario is considered of an explosion of several hundred kilotons set several hundred kilometers above the eastern‐midcontinental United States. Ground‐level E3 geoelectric fields are estimated by convolving a standard parameterization of E3 geomagnetic field variation with magnetotelluric Earth‐surface impedance tensors derived from wideband measurements acquired across the study region during a recent survey. These impedance tensors are a function of subsurface three‐dimensional electrical conductivity structure. Results, presented as a movie‐map, demonstrate that localized differences in surface impedance strongly distort the amplitude, polarization, and variational phase of induced E3 geoelectric fields. Locations with a high degree of E3 geoelectric polarization tend to have high geoelectric amplitude. Uniform half‐space models and one‐dimensional, depth‐dependent models of Earth‐surface impedance, such as those widely used in government and industry reports informing power‐grid vulnerability assessment projects, do not provide accurate estimates of the E3 geoelectric hazard in complex geological settings. In particular, for the Eastern‐Midcontinent, half‐space models can lead to (order‐one) overestimates/underestimates of EMP‐induced geovoltages on parts of the power grid by as much as ± 1,000 volts (a range of 2,000 volts)—comparable to the amplitudes of the geovoltages themselves.