16 Sep 2021
Tests at Robert Gordon University confirm ways to remotely identify explosives in soil environments.
Advances in the design and utility of unmanned aerial drones have been an asset in landmine location, potentially allowing suitable optical sensing technology to be mounted on a drone and survey the territory of interest from the air, without direct human involvement.
In a presentation to SPIE Security+Defense, Shruti Karnik of Aberdeen's Robert Gordon University (RGU) described the latest trials using Raman spectroscopy as a nondestructive, remote sensing and potentially drone-borne technique able to detect traces of explosives characteristic of hidden landmines.
"Landmine detection targets need to achieve a high probability of detection while maintaining a low probability of false alarms," said Karnik. "Detection of landmines in soil is affected by different factors, including the type of mine, the type of explosive, the type of casing, geographical features of the land, vegetation, ground water, etc."
Raman spectroscopy has become one of the most actively studied techniques for identifying landmines, due to its ability to detect the particular spectral fingerprints of explosive substances and do so at potentially high rates of selectivity. However, the presence of contaminants and other environmental factors is known to be a potential hurdle for Raman methods.
Different Raman variants have been used to increase Raman's sensitivity, such as surface enhanced Raman scattering (SERS) in which particular molecules adsorbed on the active sensor surfaces can significantly enhance the inherent Raman scatter from a sample. Another is spatially offset Raman spectroscopy (SORS), applied to the specific detection of explosives and toxic chemicals by the UK Border Force in 2018.
Helping humanitarian efforts
"Our work is focused on the detection of TNT-based explosive in the landmines," said Karnik. "TNT, or trinitrotoluene, can be leached or leaked through the landmines, either due to faulty casings or other environmental factors, and nitroaromatics associated with TNT can often be found in soils near buried landmines."
The RGU project therefore assessed TNT-based nitroaromatic-contaminated soil and water samples using Raman spectroscopy, and considered three scenarios: contaminated dry soil, contaminated wet soil, and contaminated water. This was designed to mimic explosive-rich natural environments such as dry land, deserts, groundwater and wetlands.
In each scenario the RGU team found that with optimized sample preparation its Raman analysis could successfully detect the nitroaromatics associated with TNT, with the water-contaminated samples providing the strongest Raman signal strength, and the dry soil samples providing the weakest signal.
Although the project intends to explore ways to further reduce the inherent interference of the soil matrix, the study already demonstrates that Raman-based detection of nitrotoluene can provide important indications of environments infected by leaked TNT mines, and RGU envisages complementing the technique with other modalities such as Lidar or laser-induced breakdown spectroscopy (LIBS), which can be mounted alongside Raman instruments on arial drones.
"Detecting these chemicals in different water resources, terrains, types of soils and environmental areas, will safeguard the environment and protect flora and fauna" noted Karnik. "It will definitely help the humanitarian de-mining process."• SPIE Security+Defence Digital Forum is taking place online this week, in tandem with similar events dedicated to remote sensing and optical systems design.