Enemy combatants lurking in tunnels have attacked US troops throughout history, including during both world wars, Vietnam, and more recently in Iraq and Afghanistan.
Detecting these secretive tunnels has been a challenge that has been answered by the US Army Engineer Research and Development Center's engineers at the Geotechnical and Structural Laboratory in Vicksburg, Mississippi.
The lab developed the Rapid Reaction Tunnel Detection system, or R2TD, several years ago, according to Lee Perren, a research geophysicist at ERDC who spoke at the Pentagon's lab day in May.
R2TD detects the underground void created by tunnels as well as the sounds of people or objects like electrical or communications cabling inside such tunnels, he said. The system is equipped with ground penetrating radar using an electromagnetic induction system.
Additionally, a variety of sensors detect acoustic and seismic energy, he added.
The detection equipment data can then be transmitted remotely to analysts who view the data in graphical form on computer monitors.
The system can be carried by a soldier or used inside a vehicle to scan suspected tunnel areas, he said.
R2TD has been deployed overseas since 2014, he said. Feedback from combat engineers who used the system indicated they like the ease of use and data displays. It takes just a day to train an operator, Perren said.
Jen Picucci, a research mathematician at ERDC's Structural Engineering Branch, said that technology for detecting tunnels has been available for at least a few decades.
However, the enemy has managed to continually adapt, building tunnels at greater depths and with more sophistication, she said.
In response, ERDC has been trying to stay at least a step ahead of them, continually refining the software algorithms used to reject false positives and false negatives, she said. Also, the system upgraded to a higher power cable-loop transmitter to send signals deeper into the ground.
The improvements have resulted in the ability to detect deeper tunnels as well as underground heat and infrastructure signatures, which can discriminate from the normal underground environment, she said.
Picucci said ERDC has shared its R2TD with the Department of Homeland Security as well as with the other military services and allies. For security reasons, she declined to say in which areas R2TD is being actively used.
Besides the active tunneling detection system, a passive sensing system employs a linear array of sensors just below the surface of the ground to monitor and process acoustic and seismic energy. These can monitored remotely, according to an ERDC brochure.
While current operations remain classified, ERDC field engineers have in the past traveled in to Afghanistan according to members of the team.
In 2011, for example, ERDC personnel set up tunnel detection equipment to search the underground perimeter of Camp Nathan Smith, Afghanistan, said Owen Metheny, a field engineer at ERDC who participated in the trip.
His colleague, Steven Sloan, a research geophysicist at ERDC in Afghanistan at the time, said the goal was to ensure safety at the camp.
"We make sure nobody is coming into the camp using underground avenues that normally wouldn't be seen and wouldn't be monitored," Sloan said. "We check smaller isolated areas -- usually areas of interests and perimeters."
The researchers did a lot of traveling around Afghanistan.
"We travel to different regional commands and help out in the battle spaces of different military branches," Sloan said. "We use geophysical techniques to look for anomalies underground. We look for things that stick out as abnormal that might indicate that there is a void or something else of interest. As we work our way through an area we look for how things change from spot to spot."
"I really enjoy my job," Metheny said. "I'm doing something for my country and helping keep people safe. Plus, where else could a bunch of civilians get to come to Afghanistan and look for tunnels?"