Dielectric Spectroscopy: A Fishy Way to Detect Explosives

September 2009
Topics: WMD Defense, Sensor Technology, Counterterrorism
In their efforts to develop new techniques to detect and identify explosive materials, researchers are borrowing a trick from the electro-sensory organs of certain species of fish.
Microwave signal

Underwater Inspiration

Improvised explosive devices (IEDs) contribute to a large percentage of U.S. deaths in Iraq. While a variety of counter-IED technologies are currently being used, the effectiveness of these technologies is limited by a number of factors: the diverse nature of IEDs, the short ranges and time frames in which the explosives operate, and the willingness of the insurgents employing IEDs to achieve martyrdom. To overcome those challenges, new tools to sense and defeat IEDs are needed. MITRE has found inspiration in creating such tools in a surprising place.

It has long been known that some species of fish use amazingly sensitive electro-sensory organs to navigate in murky water, locate prey and other members of their species, and avoid predators. The elephantnose fish locates dead larvae, its favorite snack, by waving its trunk-like chin over the gravel-covered riverbed like a beachcomber sweeping a metal detector across the sand. MITRE's Biologically Inspired Sensing project is borrowing the elephantnose's electro-sensory secrets in order to detect and identify explosive materials.

Dielectric Spectroscopy

Where most explosive detection technologies use infrared spectroscopy to sniff out the vapors or residue exuded by the explosive materials, MITRE's novel approach remotely classifies materials based on electrical characteristics. MITRE discovered that, at the lowest end of the electromagnetic spectrum, the electrical properties of different materials exhibit characteristic signatures. Exploiting these extremely low frequency signatures for remote material detection and identification may offer advantages in certain operational situations. For example, dielectric spectroscopy exploits the bulk electrical properties of the materials, meaning that vapors and physical residue are not required for classification. Also, because the detection process isn't sensitive to trace residue, it is more difficult to spoof the sensor.

Remote material characterization by dielectric spectroscopy is not without challenges. It remains unclear how unique spectroscopic signatures are at extremely low frequencies. Furthermore, at extremely low frequencies, dielectric properties lack the complex structure of high frequency spectra that facilitates material identification. Small changes in moisture content, temperature, or position can result in significant dispersion in a material's signature, complicating classification. And because the dielectric measurements are essentially capacitive, the phenomenon is measurable only at relatively close ranges. Nevertheless, in certain operational settings, dielectric spectroscopy could provide a new and significant capability to mitigate the increasingly sophisticated and deadly threat posed by IEDs. Other applications, including law enforcement, homeland security, and counter-narcotics are possible.

Some Answers and More Questions

MITRE recently conducted a demonstration on the use of dielectric spectroscopy to detect explosives in cars. In this demonstration, held at a mining company in West Virginia, MITRE identified a store of commercial explosives and a commonly used improvised explosive in a compact-size car—a major step in the journey from laboratory to operational capability. MITRE anticipates follow-on studies this year to further explore the sensor trade space and signature phenomenology.

Nature often provides the most elegant solutions to the most vexing technical challenges. Working closely with our sponsors, MITRE has navigated dielectric spectroscopy through the initial hurdles of proof of concept. We are optimistic that the somewhat esoteric ability of a tropical fish to navigate and avoid predators may provide the warfighter greater safety and maneuverability in the battlefields of tomorrow.

——by Nick Donnangelo

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