(B)

“At a time when a bomb smuggled by terrorists is as big a concern as one from a foreign power, delivered by missile or airplane, an attack at a port is definitely a more likely scenario,” says Thomas Cartledge, a nuclear engineer with the U.S. Defense Threat Reduction Agency (DTRA) in Fort Belvoir, Virginia. But forensic experts, who rely largely on nuclear test data collected years ago in Western deserts, lack a clear picture of how energy from a detonation would propagate in the highly saturated geology of many U.S. port cities. To remedy that, DTRA last October quietly staged Humming Terrapin: a 2-week test series at the Aberdeen Proving Ground in Mainland that detonated nearly 2 metric tons of conventional explosives to simulate nuclear blast effects in shallow water.

Since the 9/11 attacks, the U.S. government has mounted a major effort lo prevent a nuclear bomb from being smuggled into a port. It has outfitted points of entry with radiation detectors, and it is working with foreign ports toward a goal of having all U.S.-bound cargo scanned for nuclear materials before departure. But it’s well nigh impossible to track the myriad small craft flitting in and out of the 361 U.S. ports and 153,000 kilometers of open shoreline.“There are a zillion fishing boats that leave U.S. ports and nobody inspects them when they come home,” says Matthew Bunn, a specialist on nuclear terrorism at Harvard University’s Belfer Center for Science and International Affairs. “If there is highly enriched uranium metal that’s shielded and below the water line, it’s going to be really tough to detect at long range.”

In case the unthinkable happens, a sensor array called Discreet Oculus that is being installed in major U.S. cities would capture key forensic information. The array, which DTRA is still developing, would record radiation and seismic waves emanating from the blast “Discreet Oculus is up and running in several U.S. cities now,” Cartledge says. A sister system— a portable array that runs on battery or solar power called Minikin Echo—will be deployed at major events such as the Olympics or the Super Bowl. Data from Cold War-era nuclear testing and simulations are being used to calibrate the sensors.

Yet past U.S. testing is a poor proxy for detonations at a port, says Tamara VanHoose, a U.S. Army major and nuclear engineer at DTRA. A closer analog is a little-known campaign in 1963-64 in which the U.S. Air Force conducted a series of detonations of as much as 10 tons of chemical explosives at the bottom of Lake Superior. The tests offered a wealth of data on how seismic waves traverse the land-water interface, but they “were not instrumented to meet our needs,”VanHoose says.

Humming Terrapin aims to fill that gap. VanHoose and colleagues set up Discreet Oculus and two Minikin Echo arrays at Aberdeen, adding hydrophones, which are not currently included in either array. Another set of sensors probed how seismic signals ripple through East Coast rock layers.“These are wel-type geologies versus the granite geologies that we see out at the typical desert sites where we’ve done historic testing,” VanHoose says.