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" Ground Penetrating Radar * How It&amp;#039;s Done: GPR uses pulses of radio-frequency energy to see beneath the surface. In commercial use since the 1970s, it is today&amp;#039;s standard for detecting voids such as caves and tunnels. * Who&amp;#039;s Working on It: GPR is widely used in quality-testing roads, and to find unmarked graves, locate utility lines, trace subsurface geology, sweep for mines and search archaeological sites. * What the Limits Are: This method does not work well in moist mediums like clay and rarely penetrates deeper than 40 ft. Officials say that false alarms even at shallow depths waste time and money. Seismic Waves * How It&amp;#039;s Done: The way that vibrations just under the surface change as they pass through rock and dirt provides details about what&amp;#039;s below, and can show the presence of a tunnel. * Who&amp;#039;s Working on It: Researchers from the U.S. Army and Kansas Geological Survey in 2006 analyzed tunnel detection methods using seismic sensors in Kansas and along the U.S.-Mexico border in California. * What the Limits Are: To create real-time detectors, more powerful imaging software must be developed to filter out the waves&amp;#039; reactions to natural and man-made noises, such as wind and highway traffic. Electrical Resistivity * How It&amp;#039;s Done: Electrical currents can&amp;#039;t leap across empty space at low voltages. Metal electrodes staked in the ground could form a remotely monitored system that would tell solid rock from a void. * Who&amp;#039;s Working on It: While refusing to disclose its customers, Advanced Geosciences staff say the company has been approached by governments, including the U.S., to learn more. * What the Limits Are: A wide-ranging network would be expensive and would have to be hidden or disguised to avoid tampering. Microgravity * How It&amp;#039;s Done: When underground soil is removed, it causes very subtle changes in the Earth&amp;#039;s gravitational field. Lower gravity readings can indicate a tunnel.  * Who&amp;#039;s Working on It: In 2006 Western Kentucky University researchers field-tested robots in Texas studded with microgravity sensors to promote the concept of a mobile, deployable system. * What the Limits Are: Very high precision is required. The gravity differential for smuggling tunnels can be as slight as 10 microgals--measured against the Earth&amp;#039;s field of 100 million microgals. Cosmic Rays * How It&amp;#039;s Done: Muons are subatomic particles created by cosmic rays hitting the Earth&amp;#039;s atmosphere. The number of them detected underground varies with the mass above: If there&amp;#039;s a tunnel, more muons are found. * Who&amp;#039;s Working on It: Aside from a 1990 study by the U.S. Army, little research has been focused on this method. However, in 2006 a 12-year-old in San Diego converted two Geiger counters into a weak tunnel detector. * What the Limits Are: A large number of costly detectors, buried beneath the probable paths of illicit tunnels, would be needed. (Small detectors find few muons and therefore have low resolutions.)  RELATED STORIES * TECH WATCH: Cosmos CAT Scan Won&amp;#039;t Solve Border Security  * PLUS: 7 Updates From Homeland Chief for 2008   (Photograph by Getty Images) "^^<http://www.w3.org/2001/XMLSchema#string>
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