Computing the energy yield of an explosion just below, at, or above the ground poses difficult challenges, while deep-underground blasts are more readily evaluated. The new capability could help investigators figure out what types of explosives were used in such near-surface detonations and how much damage was done to structures, said Michael Pasyanos of Lawrence Livermore National Laboratory in Livermore, California, leader of the team that came up with the technique. It could also help emergency planners anticipate which areas are likely to be most affected if an explosion were to occur, he added.
Starting from seismic signals produced by explosions, the new tool can estimate the energy released by military ordnance, mining events, industrial accidents, plane crashes, terrorist attacks, and other close-to-the-Earth explosions, according to a new study accepted for publication in Geophysical Research Letters, a journal of the American Geophysical Union.
Pasyanos and his colleague Sean Ford calibrated the new tool using a series of controlled surface explosions at the White Sands Missile Range in New Mexico in 2012. Then, Pasyanos saw footage of a May 2014 Syrian explosion. "I saw the video of this and I said, 'wow'! And the next thing was, 'Can I see that? And if I can see that, I think I can model it.' "
Syrian rebels claimed to have detonated 60 tons of explosives in a tunnel under a military base in northwest Syria, which sent debris hundreds of feet into the air and destroyed the western entrance of the base. Pasyanos and his colleagues used data from seismic stations in Turkey that recorded the explosion, along with information about the geology of the area and news reports, to estimate the yield of the event.
Unlike deep-underground explosions, those near or on the surface release energy in many different forms, making it hard to piece together the total energy released by these events. There is currently no standard method that scientists can use to calculate their yield. Blast energy gets released as heat, noise, seismic waves, and in other forms. The seismic and acoustic signals produced by the explosion can provide some information about its magnitude, but they only represent a fraction of the total energy released in a blast.
The new method uses the relationships between these various forms of energy and the signals from the blast to calculate the total energy output. It also takes into account how the energy and the signals produced vary if the same amount of explosive is detonated at different depths relative to the surface or even slightly above it. For instance, an explosion hundreds of feet underground will generate different forms of energy and different signals than an explosion on the surface. Having established these relationships using known explosions, scientists can process a seismic signal with the new method to calculate the total energy of an unfamiliar explosion.
The new research builds on a technique Pasyanos and his colleagues developed for determining the energy released by underground explosions, like nuclear tests. The researchers modified this technique to take into account how proximity to the Earth's surface affects the different types of energy and signals produced by an explosion.
"It essentially makes this method applicable to a totally new class of events than what we had traditionally been looking at," Pasyanos said.
When tested on the Syrian explosion, the new method found that the energy released was likely between 20 and 40 tons, depending on how deep the explosives were buried. The calculated yield could be less than the 60 tons the rebels claimed for many reasons: the rebels could have exaggerated or over-reported the yield, the energy density of the explosives they used could be less than the energy density of TNT that is used as a reference explosive in the new method, or not all of the explosive material might have detonated.
Given these and other uncertainties about the Syrian explosion, Pasyanos said his team's estimate of the yield is reasonable. The next step is to test the method with other near-surface explosions, he said.
As in the case of the Syrian blast, the new tool can be used over large distances, Pasyanos explained, allowing scientists to calculate yields of explosions that occur hundreds or thousands of kilometers away from a seismic station.
"We think that this could be applicable to a large class of problems," he said.
Keith Koper, an associate professor of geology and geophysics at the University of Utah in Salt Lake City, said the new study shows the tool works well but needs additional testing. If the new technique passes additional tests, it could be a very important tool for investigators, law enforcement officials, and others who need to understand the energy released by different near-surface explosions, said Koper, who was not involved with the new study. Seismometers around the world are continuously capturing signals from these types of events, he noted.
"I think it is something that really could be used in the future when there's different kinds of accidents or whether there's a terrorist attack or whatever it is that causes a big boom," Koper said. "I think this could be one of the tools that is going to be really valuable for people who are doing a forensic investigation of whatever incident it is."
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