Underground Radar Systems: Fighting Tunnels with Tunnels

September 2010
Topics: Radar, Border Security, Sensor Technology
To aid the U.S. Border Patrol in the time-consuming and difficult task of locating smuggling tunnels, MITRE has designed a prototype system to boost the capability of ground-penetrating radar.
Underground radar systems play a critical role in homeland security.

The Arizona city of Nogales sits snug up against the Mexican town of the same name. Separating the sister cities is a border fence erected to impede the flow of illegal immigrants and drugs from Mexico into the United States and the flow of cash and weapons in the opposite direction. While U.S. Border Patrol agents keep a sharp eye to make sure nothing or no one sneaks over the fence, they have found they must also plant a keen ear to the ground to make sure no smugglers are tunneling under the fence.

Last summer, while pursuing a tip from an alert citizen, the border patrol in Nogales surprised a smuggler sawing a hole from beneath the floor of an empty building. Under the building the agents found a tunnel that stretched 83 feet to its origin in Mexico. The tunnel, a sophisticated construction complete with ventilation and lighting, was the largest of the more than 60 tunnels the border patrol had uncovered in Nogales. Along the entire Mexican border, agents have uncovered more than 100 smuggling tunnels.

But finding each of these tunnels is time-consuming and difficult. To provide the border patrol with a reliable technological tool, MITRE designed a prototype system to boost the ability of ground-penetrating radar to spot tunnels. By running radar sensors through horizontal boreholes burrowed underground, border agents could get a much clearer view of the clandestine activity beneath their feet.

Underground Assistance

Each of the smuggling tunnels near Nogales was uncovered through the border patrol's diligent intelligence gathering. While there is no replacing old-fashioned detective work, technology to reliably detect underground tunnels can only help make border agents' tough job easier. But the technology tested so far has not proved up to the task. For example, other sensors, such as current seismic sensors, have problems picking up signals in the midst of a noisy urban environment whereas the technology developed at MITRE does equally well in urban and isolated rural environments.

For some time, the Border Patrol has relied on ground-penetrating radar sensors placed on the surface of the border. But smugglers have taken advantage of the limited ability of the radar to peer through the surface clay and sand composition of the terrain along the majority of the border by burrowing their tunnels deep enough underground—up to 90 feet—to escape detection. Underground sensors, however, have not proven the better alternative.

"Conventionally, putting sensors underneath the earth has faced huge cost issues," says MITRE sensors system engineer Weiqun Shi, the principal investigator for the project. "But what breaks the rule here is the explosive development of horizontal, directional-drilling technology." Thanks to the miles and miles of fiber-optic cables telecommunication companies have been burying in the last few years, the technology for boring out horizontal, directionally drilled holes has become effective, affordable, and environmentally friendly.

The plan is to dig horizontal boreholes at the border. A robot crawler carries the radar antenna through the borehole as the omni-directional radar antenna peers 360 degrees through the soil, searching for signals that might indicate a smuggler's tunnel.

"There are so many advantages to this solution," says Shi. "You can begin drilling your horizontal hole far from the urban area you want to monitor. So you not only avoid tipping off the smugglers, but you avoid any environmental impact. Also, you place the sensors out of the range of surface noise, so the sensor signals are much more stable."

On Campus Testing

And what sensor signals exactly should border agents be alert for? To be certain, Shi and her team needed to take test readings of different types of tunnels through a variety of soils. So (with permission from the MITRE grounds keeping crew) they proceeded to dig test-bed trenches around MITRE's Bedford, Mass., campus.

In the trenches, Shi's team buried pipes to simulate smuggling tunnels and then covered them with a variety of rock and soil types. "We had about 10 different tunnels of varying sizes," she says. "Because we wanted to see how the sensors perform with different types of tunnels, we had tunnels made of concrete, of plastic, all of different diameters and buried at different depths." The team then dug horizontal boreholes and inserted high-density polyurethane tubes to provide an unimpeded pathway for the borehole crawler. When all was ready, the team dispatched the crawler to prowl each tube hauling along its radar antenna. The team collected the radar signals from each test site and compared them to signals gathered from surface radar and laboratory simulations.

Once calibrated and analyzed, the results from the field testing will provide Shi with a clearer picture of the system's range and capabilities. She'll also have the beginning of a library of signals that will eventually help border agents more precisely understand the composition and location of tunnels they locate.

A Solution Without Borders

With the field tests completed, the team members will fine-tune the technical aspects of the project. They are developing an algorithm to more effectively process subsurface radar signals, as well as a dashboard to control the borehole crawler and to display collected data in near-real time. The field tests will also lead to improvements in the crawler's antenna array, cables, couplers, and sensor transducer, all which must be streamlined and strengthened to survive the rigors of subsurface patrol duty.

Shi also wants to experiment with using scanning radar in place of the system's omni-directional radar. "Scanning with a focused radar beam provides not only a higher-resolution image, but a stronger lock on the location of the image."

Once proven and expanded, the system holds great promise for providing an effective, low-cost, and reliable solution to long-term border surveillance. But the system's applications do not end at the U.S. border. They could provide perimeter protection for military bases and embassies. And the system could also assist in monitoring underground nuclear facilities and missile-testing sites.

MITRE's next step is to begin testing the system in the field. "We're currently searching for suitable sites along the border," says Shi. If the tests prove successful, soon the only subterranean threats the residents of Nogales will need to be on the lookout for are gophers in their gardens.

—by Christopher Lockheardt

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