In April of last year, the U.S. military dropped the most powerful non-nuclear bomb ever used in combat on a tunnel complex in Afghanistan’s Nangarhar province. The airstrike targeted the Islamic State’s Khorasan branch. The use of the GBU-43/B Massive Ordnance Air Blast weapon, the so-called “Mother of All Bombs,” highlighted the growing threat posed by adversaries’ underground structures.
Gen. John Nicholson, commander of U.S. Forces-Afghanistan, cited the challenge of dealing with subterranean targets to justify the use of the bomb, also known as the MOAB.
“As ISIS-K’s losses have mounted, they are using [improvised explosive devices], bunkers and tunnels to thicken their defense,” he said in a statement released after the attack.
“This is the right munition to reduce these obstacles and maintain the momentum of our offensive.”
In other locations such as the Iraqi city of Mosul, ISIS built an extensive network of tunnels to move fighters, launch attacks and hide from coalition aircraft.
Meanwhile, drug cartels are using tunnels to smuggle contraband into the United States from Mexico. Sixty-seven of them were discovered from fiscal years 2011 through 2016, according to the Government Accountability Office.
“In a lot of cases they start [digging a tunnel] in a building on one side of the border and they end in a building on the other side of the border,” said Mark Kaczmarek, a program manager at the Department of Homeland Security’s science and technology directorate.
Advanced sensing technology is needed to find them. “It’s not like you just have a big hole in the ground on one side or the other that you can just see a pile of dirt showing up all of a sudden,” he said in an interview with National Defense.
Additionally, hostile regimes such as North Korea are believed to be hiding WMD technology and other weaponry in underground facilities that the U.S. military might need to locate.
To tackle these challenges, the United States government is developing new systems to find and investigate subterranean complexes.
“There’s a variety of technologies that you can use to detect voids in the ground … but geology plays a major role in that process,” Kaczmarek said.
“Certain types of geology will absorb certain types of radiation whether it’s acoustic, whether it’s electromagnetic, RF energy, etc.,” he explained. “You may find that one technology that you field may be suited for a particular area and its geology [but] may not be suited for another area and its geology. So if you have a place that’s highly rocky versus highly sandy versus clay — all that plays into how well any given technology works in trying to detect tunnels.”
The depth and size of a tunnel and the types of materials inside it are also important factors, he noted.
There are several different types of tunnels that U.S. forces need to be able to find, said Lee Perren, a research geophysicist with the Army Engineer Research and Development Center.
“We have very rudimentary tunnels — it’s just a hole in the ground, there’s really nothing in it. You can have interconnected tunnels, things that connect legitimate infrastructure — so they’ll go into a sewer system and cut through the wall and go over somewhere else so that they can get to where they’re trying to get to. Or you can have purpose-built, sophisticated tunnels that have all kinds of infrastructure in them,” he explained.
An elaborate cross-border drug smuggling tunnel was discovered inside a warehouse near San Diego. (Customs and Border Protection)
Locating ones that have already been completed sometimes requires different technologies than locating ones that are under construction, he noted during a media engagement in Fort Belvoir, Virginia, hosted by the Joint Improvised-Threat Defeat Organization, also known as JIDO.
“When you take those perturbations and start to look at all of those different things, there are a lot of different ways that one needs to go about detecting tunnels,” he said.
The Engineer Research and Development Center developed the border tunneling activity detection system-point, or BTADS-P. It includes an array of buried geophones that listen for construction activities.
“This one is very infrastructure rich,” Perren said. “It’s somewhat more expensive to [deploy] but it’s very good at finding localizations.” It can tell operators where the underground activity is taking place, and they can adjust the depth of the array as they conduct their search, he noted.
The Homeland Security S&T directorate paid for part of a system that was installed near San Diego, Kaczmarek said.
ERDC has also developed a less expensive linear sensing system that uses a buried line of fiber-optic cable to monitor and process acoustic and seismic energy associated with tunnel activity.
These passive detection technologies work very well if the adversary is in the act of building the tunnel. But if the tunnel already exists other types of capabilities are needed to detect them, Perren noted.
One such technology is the active seismic imager, or ASI, which is a modified Toolcat. “Essentially we have a Bobcat with a thumper,” Perren explained. “It pounds the ground and we have a stream of geophones behind it that listens for that sound to propagate through. … When there’s a hole in the ground the velocity structure changes and we can find that.”
Another capability is the electromagnetic induction system, or EIS. Using power and light wires, communication wires and transmitter-receivers it detects buried infrastructure that conducts electricity. It is man-portable and weighs about 30 pounds, according to ERDC officials.
Ground resistivity is a major area of interest. “We put electricity in the ground using electrodes,” Perren said. “When there’s a void it resists electricity. The electricity goes around it and that allows us to map its location.”
ERDC is trying to make the data gleaned from these systems understandable to non-experts, he noted.
“We’ve developed processing routines that take that huge amount of strange-looking data — squiggled lines, this and that — and turns it into something that is more likely to be usable by a commander” such as color-coded maps, he added.
Looking beyond ground-based systems, officials are exploring the use of drones to detect tunnels from the air.
Engineers at the MIT Lincoln Laboratory have been working on a new technology for JIDO. It includes an Airborg 10K unmanned aerial system developed by Top Flight Technologies. The platform carries an antenna and electronics payload and weighs about 15 pounds, said Christian Austin, an engineer with the lab.
If the UAS is surveying an area and it detects a large change in the magnetic field, that indicates there is something subsurface, he explained.
“It would fly around in circles and it would generate sort of a heat map of what’s in the area — what you’re seeing below the ground — or maybe it would fly lawnmower tracks [and] the output of the system would be some sort of heat map from the algorithm,” he said. A commander could then decide what action to take against any suspected tunnels.
The experimental system is currently in the demonstration phase, Austin said.
Officials emphasized that there’s no silver bullet technology for dealing with the tunnel challenge.
“There’s lots of different subsets to this that we’re going after,” Perren said. “This problem set is huge, so there isn’t going to be one solution. ... We have to advance them all.”
Many of these systems have already been deployed in the United States and overseas for use by Customs and Border Protection and the U.S. military. The Engineer Research and Development Center is working with JIDO to improve their capabilities and decrease their limitations, Perren said.
“There are geologic differences between the Korean Peninsula, the things in the Middle East, the things in Afghanistan. So we’re looking at that and trying to incorporate how that affects the technologies,” he added.
Perren declined to identify the specific locations where the systems are deployed.
Just as the military needs different sensing technologies for different geographic regions, Homeland Security faces a similar predicament in the United States. The geologies of the Desert Southwest can be very different than the Pacific Northwest or the Northeast border areas, Kaczmarek noted.
The S&T division at DHS developed a tunnel detection performance-modeling tool to help identify the best technologies to use in a particular area based on its geology and local noise environment. It would enable the department to determine if a technology that a vendor is selling would be appropriate for the area where Customs and Border Protection wants to use it, Kaczmarek said.
“From that model we started investing in a couple of technology areas which I won’t go into particularly,” he said. “But we are building a prototype system here to try and see what the parameters are that might best suit a follow-on acquisition that CBP may have.”
Kaczmarek declined to discuss specific details other than to say they are “active systems” that search for voids in the ground, as opposed to passive systems that detect noises or vibrations.
Prototype work is ongoing. In fiscal year 2018 “we look to hand that over to CBP to have them do further operational testing on it and help to inform any potential acquisitions they may look at into the future,” he said.
Tunnel detection isn’t the only challenge that officials are trying to address. Once a tunnel is discovered, commanders must decide how to deal with it. In many areas — such as a U.S. territory or cities where there would be a high risk of civilian casualties — it might not be desirable or politically feasible to use the Mother of All Bombs or other high explosives to destroy it. In those situations, security forces must turn to other options.
In the past, soldiers and DHS agents have had to enter tunnels with no knowledge of the hazards that might lie within them. But advances in robotics and related technologies could enable commanders to send machines down first to find out what’s inside.
JIDO officials have expressed interest in robots that could explore subterranean environments to find IEDs. Homeland Security is interested in similar technology for border tunnel investigations. Robots could help DHS officials map the area before sending a person down to do follow-on investigations, Kaczmarek noted.
“From an officer safety point of view they would like to potentially go inside a tunnel with the device … to make sure that there’s no booby traps in there… [and] there aren’t any obstacles in there that could be dangerous to an agent going in there the first time,” he said.
The S&T directorate hopes that requirements for robotic systems will be generated in fiscal year 2018 so it can move forward with projects.
“Certainly within two years we want to at least hopefully have some kind of prototype out that they can start testing with and help influence the further development that we may be doing,” he said.
There are a number of challenges associated with deploying a robot underground, including communications, Kaczmarek said.
“If it’s not fully autonomous and able to navigate on its own, you have to guide it” using a camera or some other means, he said. Operators would need to be able to receive video footage and other data in real time, he noted.
Left: A GBU-43/B Massive Ordnance Air Blast weapon strikes an ISIS-K cave and tunnel April 13, 2017. (Defense Dept.)
Right: Airborg 10K unmanned aerial system (Top Flight)
Determining the right sensor to put on the robot is another consideration. An infrared sensor might be the best option if officials are trying to detect whether there are any people still in the tunnel. For mapping activities a video camera might be optimal, he explained.
Navigation might be difficult in some cases. “There could be obstacles on the ground that are a challenge to the type of vehicle that may be able to go in there,” he said. “Is it going to be able to climb over something? If it tips over what does that do to it? Do you have to go in and retrieve it?” he asked.
Using an unmanned aerial system could be advantageous, he noted. “We certainly may pursue those too. … When we’re talking about the ground obstacles that could be in a tunnel, if you have a flying vehicle it might make it simpler to navigate.”
U.S. officials expect continued collaboration between the Defense Department, Homeland Security, federally funded research centers, academia, industry and foreign partners such as Israel when it comes to tunnel detection.
“We stay connected with industry … so we can understand the state of the science and the state of the commercial market,” Kaczmarek said. “We can take advantage of commercial technologies that maybe only need to be adapted slightly to our mission set [and] we don’t need to develop something from scratch.
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