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 says one day he hopes the LAS can be installed on autonomous Border Patrol robot scout vehicles to prowl the Mexican border like Martian rovers.
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. Forward-looking properties detection is important 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 says 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 explains. West’s 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 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 explains.
Specifically, according to a West paper that appeared in the Proceedings of the IEEE conference on Technologies for Homeland Security from 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 more than 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 says 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 drug cartels.”