Earthquakes send energy rippling through the planet, but so does something decidedly human caused: an underground nuclear explosion. With the goal of monitoring the proliferation of nuclear weapons, scientists and engineers have been tasked with differentiating between these two types of energetic events. By collecting geophysical data from controlled detonations in the Nevada desert, researchers aim to do just that.
Rob Abbott, a seismologist at Sandia National Laboratories in Albuquerque, N.M., sums up the motivation for the project, known as the Source Physics Experiment: “Sometimes an explosion can look very earthquake-like, and sometimes an earthquake can look very explosion-like.”
Going Boom in the Desert
The Source Physics Experiment, which began in 2010, has conducted 10 controlled underground explosions at the Nevada National Security Site, a Rhode Island–sized facility roughly 105 kilometers northwest of Las Vegas. The detonations mimicked underground nuclear explosions, but the researchers used chemical explosives such as nitromethane rather than fission- or fusion-based bombs.
“We obviously can’t and don’t want to set off a nuclear explosion,” said Abbott, who is the science lead at Sandia National Laboratories for the Source Physics Experiment.
The team set off different-sized explosions ranging from roughly 100 to 50,000 kilogram equivalents of 2,4,6-trinitrotoluene, an explosive commonly known as TNT. The largest bundle of explosives filled a canister roughly 12 meters long and 2 meters wide. “It’s huge,” said Abbott. “It comes on a flatbed truck.”
All of the explosions occurred in boreholes dug into either porous alluvial soil or granite. They were set off at depths ranging from 30 to 385 meters. That’s far shallower than most earthquakes, but drilling boreholes is very expensive, and the Source Physics Experiment used two, the larger of which was 2.4 meters in diameter.
Sensors, Sensors, Everywhere
Each explosion was carefully orchestrated. “Like anything, practice makes perfect,” said Abbott. “We do multiple dry runs.”
Abbott and his colleagues monitored the explosions from trailers located roughly 2 kilometers away after placing sensors like accelerometers into nearby instrument boreholes. In addition to the belowground sensors, instruments such as geophones and high-speed video cameras and surface mapping techniques (lidar, photogrammetry, and synthetic aperture radar) were used to measure how the detonations dynamically deformed, shocked, and otherwise affected their surroundings. The Source Physics Experiment team obtained measurements at a range of distances from the explosion: Some instruments were as close as 10 meters, and others were several hundred kilometers away.
Regional seismic networks run by such institutions as the University of Nevada, the University of Utah, and the California Institute of Technology detected the detonations as well. Metaphorically, no rock was left unturned, said Abbott. “The geologists came out on their hands and knees to look for microcracks.”
Danny Bowman, a geophysicist and atmospheric scientist at Sandia National Laboratories involved in the Source Physics Experiment, launched balloons containing pressure sensors above the explosion sites. Airborne experiments are important because most of the sound associated with an explosion goes straight up, said Bowman. “A ground-based sensor actually catches a small fraction of the energy.”
Bowman and his colleagues measured infrasound, low-frequency pressure fluctuations that can’t be heard by humans. By analyzing ripples in pressure and knowing the properties of the atmosphere, Bowman and his colleagues can reconstruct the physics of the explosion. “It’s kind of like seismology of the atmosphere,” said Bowman.
Hundreds of people at Sandia National Laboratories, Lawrence Livermore National Laboratory, Los Alamos National Lab, and other institutions have been involved in the Source Physics Experiment, which detonated its final explosion earlier this year. Data analysis is now ongoing, and it’ll take a while, said Abbott. “We’ve taken a ton of data.”
P’s and S’s
Comparing the Source Physics Experiment measurements with data from earthquakes will reveal the geophysical differences between nuclear explosions and seismic activity.
Scientists already have a few clues for telling these two types of events apart, said Abbott. Seismic activity tends to produce a higher fraction of shear (S) waves than explosions. That’s not surprising, Abbott said, because an explosion displaces material radially around it, producing compression (P) waves. But explosions also make S waves, and Source Physics Experiment researchers want to know why. “A major goal is to figure where those shear waves are coming from in a theoretically purely compressive, isotropic source,” said Abbott.
The Source Physics Experiment team is also studying how parameters such as burial depth and sediment type affect the signals recorded from an explosion.
Researchers around the world are looking forward to analyzing Source Physics Experiment data, which will be made freely available. Open science accelerates progress in research, said Timo Tiira, a seismologist at the University of Helsinki not involved in the research. “This experiment creates an important ground truth database.”
—Katherine Kornei (@katherinekornei), Freelance Science Journalist