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A Palestinian man prepares to transport fuel through one of the tunnels between the Gaza Strip and Egypt.
One key to a cease-fire in the Gaza conflict is Israel's determination to stop the smuggling of weapons and explosives through tunnels from Egypt to Gaza.
There are thought to be hundreds of tunnels along Egypt's 9-mile border with Gaza, used to supply everything from black-market food and cigarettes to components for the rockets fired by Palestinian militants into southern Israel.
Israel has destroyed many of those tunnels in the current offensive, but it wants assurances that Hamas won't be able to re-arm itself by digging new passageways.
The Memorandum of Understanding signed by Secretary of State Condoleeza Rice and Israeli Foreign Minister Tzipi Livni on Friday says the U.S. will speed up its efforts to "provide logistical and technical assistance and to train and equip regional security forces in counter-smuggling tactics."
The regional security forces are those of Egypt, which has had limited success blocking tunnels from its end. The U.S. has already promised to provide Egypt with $33 million worth of equipment designed to detect tunnels, but no one involved is saying exactly what technologies might be employed.
Tunnel Hunters Use Science, Technology
Charles Oden, a geophysicist at the Colorado School of Mines, says there are a number of systems that might be used, ranging from ground-penetrating radar to remote sensing from satellites, that can detect tiny changes in the level of the ground. But, he says, all of them have problems.
"Ground-penetrating radar has perhaps the highest resolution of any standard geophysical technology," Oden says. "But its Achilles' heel is that there are a lot of types of soil that are opaque to radar." For instance, radar isn't much good at penetrating soils with a lot of clay content or soils that contain salt water, he says.
Archaeologists working in the Nile Valley have found that sand is "very transparent" to GPR, Oden says, so much so that it has been used successfully to find underground sites. The soil along the Egyptian border with Gaza is sandy, but some of the smuggling tunnels that have already been found are as much as 50 to 60 feet deep, and may be beyond the radar's range.
Lessons Learned Along The Border With Mexico
Ground-penetrating radar is used to search for smuggling tunnels along the U.S. border with Mexico. Mike Carney, of U.S. Immigration and Customs Enforcement, says it's good at mapping cavities under the ground. Carney, a deputy special agent in charge of ICE's San Diego office of investigations, notes though, that a highly sophisticated tunnel found in 2006 was 80 feet deep in some places.
"Something that deep," he says, "would be very hard to detect."
Another technique that might be used is seismic measurement. Oden says the technology can be used to detect the sound of digging or movement underground. Seismic sensors can also measure the normal, ambient sound caused by earth movement, traffic and other sources, and find anomalies — places where the sound is different. Oden cautions that it's easy finding anomalies, though, and harder to explain what's causing them.
Electro-magnetic induction — "essentially it's a fancy metal detector," Oden says — can be used to detect wiring underground, if the tunnels are provided with electric lighting systems. The problem is that it can't detect tunnelers using flashlights or miners' headlamps.
InSAR is a type of radar satellite system that can detect minor changes in the Earth's crust, in some cases, down to the centimeter. Oden says the system can observe places where the ground is sinking slightly because of excavations deep below.
Oden says that if he were advising the effort to find tunnels, he'd try to use several different techniques at once, starting with remote sensing.
"No geophysical technique works all the time, so you try using several and compare notes between them," he says.
Carney says Immigration and Customs Enforcement has been working closely with the military along the U.S.-Mexico border and with contractors who are working on tunnel-finding technology. He says that although a lot of the technology shows promise, "it still hasn't been developed to the point where we can take it over an area of the border and say with absolute certainty there's a tunnel."