Less than half a mile from the U.S. Port of Entry at Otay Mesa in San Diego County, Mexican drug smugglers operated an elaborate 2,400-foot tunnel that connected a small warehouse 175 yards south of the border near Tijuana's Rodriguez International Airport to a larger 50,000-sq ft warehouse on Siempre Viva Road on the U.S. side of the border.
When found, in 2006, the tunnel was equipped with rails, electric lighting, ventilation ducts, and a sump pump to handle groundwater drainage. The Mexican entrance had an 85-foot deep cement-lined vertical shaft with a pulley system to lower drugs on a gurney attached to a rope, according to the Drug Enforcement Administration. At its deepest point, the tunnel, which averaged 6-feet high by 4-feet wide, ran more than nine stories below commercial warehouses and busy border roads in U.S. territory.
No one knows how much marijuana or other drugs entered the U.S. through the tunnel, although at the time of its discovery pallets containing approximately 2-tons of marijuana was seized on the Mexican side and another 200 pounds on the U.S. side.
Immigration and Customs Enforcement, which investigated the case along with the DEA and Border Patrol, did not know how long the tunnel had been in use. It's also not the first lengthy tunnel found in the area. A similar 1,000-foot passageway was discovered nearby in 1993, although it was unfinished when found.
"Since the early 1990s, we've had over 150 tunnel attempts along the southwest border, 137 of which have actually crossed the border," said Jose Garcia, deputy special agent in charge for ICE Homeland Security Investigations in San Diego, which includes leadership of the ICE Tunnel Task Force. The TTF is composed of elements of the Border Patrol, Drug Enforcement Administration, and, to a lesser degree, the California Bureau of Narcotics Enforcement. Garcia said tunnel detection at the border is a full time job, in the sense that the TTF crew handles no other types of investigation.
"We spend most of our time following leads and tips," Garcia said.
Found tunnels are eventually remediated, which involves filling or otherwise impeding passage through the crude shafts and closing exit points located on United States soil.
Some of the larger ones are first plugged before they can be filled up with a material ICE declined to identify. Plugging a tunnel is not as simple as it sounds. A 6-foot diameter hole must be drilled through the earth into the actual tunnel so a concrete obstruction can be created.
The Hole Truth
As border security has tightened since the terror attacks of 9/11, drug smuggling, and to some extent human trafficking, has gone increasingly underground. Of the 150 or so tunnels found, none were discovered using detection technologies, simply because no reliable tunnel detection technology exists.
Ground penetrating radar, or GPR, has been in industrial use for over 40 years and has become the gold standard for such varied work as scanning roadbeds for subsurface voids, locating unmarked graves, mapping underground utility lines, and searching archaeological sites.
GPR is also used for tunnel detection but the depth penetration limits of GPR is something around 40 feet in the best soil conditions, making it of limited utility. Some discovered border tunnels are over twice that deep. GPR also does not work well in damp clay-like soil, where penetration is only a few inches. GPR technology is also prone to false alarms that waste time and money.
"For us, GPR is better at detecting buried tunnel openings than finding tunnels themselves," Garcia said. "It's been quite successful finding openings where a tunnel may be exiting or starting."
The limits of GPR are well-known, even to drug smugglers. They simply dig their tunnels deeper than the technology can detect. Garcia said one place GPR has been used reliably is to find voids in walls where drug cartels hide money.
Several other tunnel detection technology exist, either in practice or in theory. One tunnel detection 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. 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 underground 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 said 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. A standing joke in the Task Force is the water trucks are the most reliable tunnel detection technology they have.
Tunnel Vision
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. Better tunnel-detection technology than driving into a hole may be at hand.
A government engineer at the Department of Energy's Idaho National Laboratory has designed and built a hand-held 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 an 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," said INL engineer Phillip West, the developer.
West calls the new unit the Look-Ahead Sensor, or LAS. He said 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 said 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 eight 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 said.
In addition to detecting natural or man-made voids, LAS uses a sophisticated algorithm to calculate the distance to the void from the surface. West said one day he hopes the LAS can be installed on autonomous Border Patrol robot scout vehicles, to prowl the Mexican border like Martian rovers.
Dig This
The device uses a method called acoustic driving point impedance, principles that were originally pioneered at INL to aid the oil and gas industries with well-bore physical properties logging. The need for forward-looking properties detection is obvious when drilling for oil or gas. Environmental and cost considerations preclude drilling blindly forward. A forward-looking sensor was therefore proposed by INL to determine earth conditions ahead of the drill.
Later, the Department of Energy funded a different project to develop a related technology that could detect subsurface voids simply by standing on the surface in the immediate vicinity. That project included the development of the Orbital Vibrator-Physical Properties Logger, which West used as the basis for his forward-scanning, void-detecting sensor.
West said an orbital vibrator is best described as an out-of-balance motor in a shell. OVs have been used for many years in seismic surveys as an acoustic source that transmits to remote receivers, he said. His tunnel detector device, however, requires no remote components. The entire system consists of only the LAS and a laptop computer.
According to West, once lowered into a water- or oil-filled housing, the LAS device is operated at various frequencies, producing sufficiently fast vibrations that earthen properties can be characterized and measured. The vibrations are actively induced in the ground in the ~60 Hz to ~200 Hz range. These vibrations propagate through the ground with some attenuation and geometric dispersion. If the acoustic energy encounters a change in media, such as a void, transmission characteristics change also. The software reads these changes and produces a graph illustrating underground conditions.
"The acoustic energy passes through the water or oil into the surroundings to which the device is in contact as if it were directly coupled," West explained.
Specifically, according to a West paper that appeared in the Proceedings of the IEEE conference on Technologies for Homeland Security (2009), the device incorporates two orthogonal, internally-mounted motion detectors in the form of geophones that provide motion data. The recorded orbital motion of the device as it tries to orbit while restrained by the fluid characterizes the transmission of acoustic energy. This characterization, resulting from the radial transmission and returning reflections of acoustic energy, is indicative of the surrounding media's energy-absorbing properties. In mining, by evaluating the reflective data, properties of the earth surrounding the well, such as rock types, can be measured. In tunnel detection, the presence of anomalies indicates the presence of underground cracks, voids, or tunnels.
The device West ended up with is 23-inches wide by 14-inches long by 5-inches deep, with a 6-inch diameter earth contact pad. The unit weighs about 25-pounds, including the onboard lithium battery pack.
Even though LAS technology has been known in government circles for over three years, it has yet to find its way to the southwest border to see whether it can help border authorities find the unseen. Garcia said he is always ready to enlist new weapons in his tunnel detection fight.
"ICE is always interested in new and emerging technology that might assist the TTF in detecting and discovering tunnels used by transnational criminals and the drug cartels," he said.
Douglas Page
Douglas Page writes about science, technology and medicine from Pine Mountain, Calif. He can be reached at [email protected].
