Using Plants to Detect Buried Explosives

VCU researchers examine spectral signatures of plants

Long after wars and military conflicts have passed, landmines and unexploded ordnances remain, posing a threat to many innocent civilian lives. An estimated 110 million landmines or unexploded ordnances exist worldwide, and that number is likely to rise with the onset of conflict brewing in Syria and other nations. 

While detection technologies help, it becomes particularly challenging to identify the danger in regions of overgrown vegetation where the surface is no longer bare. 

In an effort to overcome this complex issue, Virginia Commonwealth University researchers, led by Donald Young, Ph.D., chair and professor of biology in the VCU College of Humanities and Sciences, and Julie Zinnert, Ph.D., a biology and research biologist with the U.S. Army Corps of Engineers, and a team of VCU graduate students are exploring how plants may be used to detect buried explosives. 

So why use plants? 

Photographs showing plants grown in soils with no RDX (left), low RDX (middle) and high RDX (right). Photo by Stephen Via

Photographs showing plants grown in soils with no RDX (left), low RDX (middle) and high RDX (right). Photo by Stephen Via

Because they are rooted in the soil, plants have nowhere to go and can be considered indicators of the local environment, said Zinnert. She added that explosives in soils from landmines and other unexploded ordnances are taken up by plants and incorporated into leaves, affecting plant reflectance. This may disrupt the process of photosynthesis, which plants need in order to survive. 

Studies have shown that if there is an undetonated landmine buried in the ground beside a plant for a lengthy period of time, the explosive compounds could leak into the environment, ultimately contaminating the plant. Many of these explosives – whether homemade or military-grade – do not stay contained in their casing. 

“Most explosives will affect the physiology or functioning of the plant and that’s something we can detect with its spectral signature,” said Zinnert. 

According to Zinnert, every object has what is known as a reflectance value, or spectral signature. The spectral signature is like a fingerprint – every object has its own. 

The goal of the work by the VCU team is to determine if a technique called hyperspectral imaging can be used as an accurate tool for detecting buried explosives. Hyperspectral imaging measures reflectance values from the blue to near-infrared region of wavelengths to learn unique things about a plant’s physiology and its spectral signature. 

By comparing known uncontaminated plant signatures to those of plants in contaminated soil the team hopes to determine which area(s) of a plant’s signature can tell researchers if buried explosives are present.

Understanding wavelengths 
The human eye is able to see and process only a certain portion of the electromagnetic spectrum, which refers to a range of frequencies and wavelengths an object may possess. The unaided human eye can see three bands in the spectrum including blue, red and green. Because plants absorb blue and red wavelengths and reflect green, so we see plants are green. 

Hyperspectral reflectance signatures of control plants (blue), plants grown in low RDX (red) and high RDX (green). Image by Julie Zinnert, Ph.D.

Hyperspectral reflectance signatures of control plants (blue), plants grown in low RDX (red) and high RDX (green). Image by Julie Zinnert, Ph.D.

Spectral imaging further breaks down the bands of the electromagnetic spectrum, allowing the human eye to see what it ordinarily could not. Take for example low-energy wavelengths, infra-red and microwaves. We have to use special glasses to see infrared wavelength. 

In one recent study, the VCU team compared plants exposed to soil contaminated with explosives with plants in uncontaminated soil. They observed a notable change in the infrared portion of the spectral signature in plants that had been exposed to contaminated soil. 

Natural environmental stress vs. Environmental contamination 
Last year, the team published findings in the peer-reviewed journal, Plant and Soil, where they compared effects on plant physiology of plants exposed to natural environmental stress, such as drought and salinity, and to environmental stress, such as exposure to the explosives RDX and TNT. Using hyperspectral imaging, they found that explosive-treated plants have a significantly different response than plants exposed to natural stress. 

“We took the spectral signatures from a previous study and compared with the explosive- and naturally-stressed plants and found that they differ,” Zinnert said. “These findings help us discriminate between the types of stress using the spectral signature – and may help us fine-tune remote detection of explosives versus natural stressors moving forward.” 

Moving this research toward a usable tool are graduate students, Stephen Via and Paul Manley, who are attempting to build a moderately inexpensive sensor unit. While there is current technology available to detect explosives, it’s done using equipment that costs thousands of dollars and cannot be afforded by civilians living in landmine-infested regions. 

Under the mentorship of Young, Via and Manley are trying to use some freely available hardware and software to make an affordable detector – possibly in the form of an Android Phone App. The idea is that a civilian user may be able to go to the plant and use the smartphone camera to take an image of the leaf. This would be based on the spectral response and likely some sort of algorithm, which would be computed for the user ahead of time. 

“The app may one day be able to inform the user whether the plant is contaminated with explosives or not – basically some sort of message to indicate ‘go ahead’ or ‘stop,’” said Via. 

Zinnert added that similar technologies based on spectral signature change already exist. For example, special glasses have been developed to determine the stress factor in a field of crops. A farmer can look out over his field of crops while wearing the glasses to determine what part of his field needs attention – whether it is due to drought or something else, she said.   

Next steps 
Before they can move forward, the team needs to gather more data. More species must be tested in the lab to see if spectral responses hold true across the multiple different types of species of plants. 

“We see far-reaching possibilities with this,” Via said. “Obviously this could have military applications, but I know personally we are really hoping this can benefit the humanitarian front.” 

Zinnert said, “With science, it’s sometimes difficult to see how your work could impact the future, because it can be so technical and specific. This work has a really good endpoint – we want to help people … this could help save lives one day.”