In July, Chinese researchers urged their government to increase the country's readiness for defending against a high-altitude electromagnetic pulse (EMP) attack. Just over a year ago, a group of American researchers released a report warning that China possessed the capability to conduct an EMP attack against the United States. Military and non-proliferation experts are worried about the growing temptation by nuclear-armed countries to engage in a first-strike EMP attack using nuclear weapons that, while avoiding direct casualties, could prove devastating to electric grids and electronic devices from smartwatches to supercomputers.
The enormous potential of an electromagnetic pulse released by the high-altitude detonation of a nuclear weapon has been recognized for some time. In 1962, the U.S. conducted an atmospheric test of a 1.45 megaton thermonuclear weapon, code-named Starfish Prime, 250 miles above Johnston Island in the Pacific Ocean. Over 1,000 miles away, the blast knocked out electricity supply in parts of Hawaii and disrupted telephone service for a period of time. In addition, radiation from the test damaged several satellites in low-Earth orbit, taking them out of service. Decades later, the Commission to Assess the Threat to the United States from Electromagnetic Pulse (EMP) Attack determined as early as 2008 that the U.S. would face catastrophic consequences from an EMP attack given its growing dependence on electronics of all forms and complete reliance upon the electrical grid.
And yet, until now, government and industry risk assessments about EMP attacks and their effects on the power grid have been based on oversimplified models of the solid Earth that assume zero variation in depth or composition. But, as it turns out, the actual effects on the power grid of an electromagnetic pulse in outer space are strongly determined by the three-dimensional distribution of rocks beneath our feet.
Landmark research, the product of collaboration between the U.S. Geological Survey (USGS) and the University of Colorado, illuminates the role of the solid Earth in determining the magnitude of any EMP hazard. In an interview, USGS geophysicist Jeffrey J. Love, the lead author on the new report, explains that a high-altitude EMP produces three sequential waveforms with different impacts on electrical systems: E1, E2, and E3.
The high-frequency E1 pulse disrupts consumer electronics and tends to get the most attention; E2 behaves more-or-less like lightning, and, fortunately, our electrical systems are (largely) hardened against its effects. The E3 waveform is the lowest-amplitude part of the EMP signal, but because it is the longest-lasting part, covering periods from about 0.1 seconds to several hundred seconds, it has the potential to cause catastrophic damage to the electrical grid through its interactions with the solid Earth. (See the illustration, below.)
A nuclear explosion in the upper atmosphere or in space (off screen) would push a current of ions and electrons through the atmosphere (B), producing a magnetic field (A) that would in turn induce currents and electric fields in the earth beneath it. Regions of high conductivity (D) would more readily carry that current, while regions of low conductivity (F), would not. Instead, the larger electric field in these regions (E) could help coax current out of the ground and through the wires of high-capacity power lines (C)—yielding a potentially grid-crippling power surge induced by the explosion's EMP. John MacNeill
Love notes that three factors conspire to form a geoelectric hazard for power grids: "The level of EMP magnetic disturbance, the conductivity of the surrounding Earth, and the specific parameters of the grid itself." The new study used existing survey data—originally collected for geological exploration—in a small region of the eastern-midcontinental U.S., covering portions of Arkansas, Missouri, Illinois, Mississippi, Kentucky, and Tennessee. Researchers from the USGS then secured permission from property owners to place sensors on the ground for measuring the natural variation in the Earth's magnetic field over several weeks. They also used voltmeters to measure the time-varying electric field at the same locations. These two measurement types provide estimates of surface impedance—an electromagnetic property that depends on rock conductivity.
Love and his co-authors used these survey data to assess the impact of an E3 EMP waveform generated by a high-altitude detonation of a nuclear weapon with a yield of several hundred kilotons—the benchmark description of an EMP event in the literature. They also included USGS research on natural, magnetic-storm disturbances across the continental U.S.—in regions such as the electrically resistive metamorphic and igneous rocks of northern Minnesota or east of the Appalachian Mountains as well as electrically conductive sedimentary rocks of Michigan or Illinois.
All in all, they found that EMP hazards had not been accurately mapped in complex geological settings. They call for USGS to analyze surface impedance across regions like the eastern mid-continent. They add that USGS should pay special attention to the eastern U.S, where magnetic-storm hazards are already known to be high. Of course this is also where many of the nation's largest cities are located.
"Better coordination across scientific disciplines is necessary," Love says. "By bringing together weapons engineers, space scientists, and geophysicists, we can achieve a holistic approach to EMP threat assessment and, with that, prioritize improvements."