Several other tunnel detection technologies exist, either in practice or in theory. One technology uses seismic waves. Vibrations are generated and transmitted into the ground. Measurements of reflections and travel time are calculated, providing details about what’s below, including the presence, or absence, of a tunnel. One limitation is that natural and man-made noises, such as wind and highway traffic, tend to disturb the vibrations.
Electrical resistivity is also being developed to detect underground voids. Electrical currents are unable to leap across empty spaces at low voltages, so metal electrodes staked in the ground could form a remotely monitored system that would distinguish solid rock from a tunnel. A border-scale implementation of this technology would likely be too expensive and too difficult to conceal or maintain.
The Israelis use a fiber-optic technique, but it only detects digging, not existing tunnels.
No idea seems too far-out to investigate as a way to find tunnels, even cosmic rays have been considered as a potential way to detect tunnels. This pie-in-the-sky method would measure subatomic particles called muons, that are created when cosmic rays penetrate the atmosphere. The number of muons detected underground decreases as the mass above the sensor increases. Ergo, if there’s a tunnel, more muons are measurable.
In the same improbable vein, a microgravity technology developed at Western Kentucky University to explore highway roadbeds for sinkhole collapses was once also investigated as a way to find tunnels. The idea is, when soil is removed from under ground it causes minute changes in the Earth’s gravitational field. If a sensor with enough sensitivity could be placed in the right location, lower gravity readings might indicate a tunnel.
Probably, a more reliable tunnel detection method is gravity itself, as the Border Patrol has already discovered. Garcia says the Border Patrol routinely drives water trucks along portions of the border fence to soak the soil. The enormous weight of the trucks has been known to collapse shallow tunnels beneath the sandy earth. In fact, task force members joke that water trucks are their most reliable tunnel detection technology.
Truth is, all the tunnels discovered under the international border have either been discovered by luck, anonymous tips or good police work—none by tunnel detection technologies. That may change. Technology that’s better at detecting tunnels (rather than driving into one) may be at hand.
A government engineer at the Department of Energy’s Idaho National Laboratory (INL) has designed and built a handheld device that can detect tunnels through solid ground simply by holding the machine to the earth. Early tests with the prototype detector demonstrated positive detection of a buried trench through as much as 100 feet of hillside topsoil at INL’s barren research site on the Snake River Plain, 40 miles west of Idaho Falls.
The device is able to do this in less than 10 seconds. A tool with such capabilities could help border authorities find the underground routes smugglers use to bring drugs, weapons, and people into the country illegally. Military personnel might likewise use it to identify threats of intrusion or weapons caches. The Park Service and others could use it to find abandoned mine shafts. It’s conceivable the technology might also be used one day in caving or in mine and earthquake rescue efforts.
“This device can identify underground voids in most any soil conditions, including moist or rocky soil,” INL engineer Phillip West, the developer, says.
West calls the new unit the look-ahead sensor, or LAS. He says the device is called a look-ahead sensor rather than a look-around sensor because the earth-borne acoustic energy and subsequent measurements are made only in the forward direction.
West says the LAS finds bunkers and tunnels by measuring how dirt and rocks react in response to sound waves the machine transmits into the ground. A steering wheel-sized prototype beams sound waves into the earth for about 8 seconds, then uses special software to graph the returning signal. On the graph, solid earth shows as a rapidly rising line, but an underground tunnel produces a signal drop, represented on the graph as a dip.
“LAS provides an agile package ideally suited for use in the field to detect illicit storage areas and pathways that could otherwise threaten national security,” West says.