Exposing buried danger

Researchers in the US are developing a landmine detection system that uses high-frequency seismic waves to displace soil. A non-contacting radar sensor then measures the results to reveal buried mines.

A landmine-detection system under development at the Georgia Institute of Technology offers potential advantages over existing technologies and could ultimately help prevent the thousands of injuries and deaths land mines cause annually.

The system uses high-frequency seismic waves to displace soil and objects in it by less than one ten-thousandth of an inch. A non-contacting radar sensor then measures the results, creating a visual representation of the displacement that reveals the buried mines.

This seismic-wave system presents potential advantages over existing electromagnetic-wave techniques used in metal detectors and ground-penetrating radars (GPR). Although metal detectors and GPRs can locate mines successfully, they have more trouble locating the small, plastic anti-personnel mines that have become more prevalent. Metal detectors and GPRs can also be confused by ground clutter such as rocks, sticks or scraps of metal, resulting in false alarms.

Because plastic mines have very different mechanical properties from soil and ground clutter, the seismic waves are capable of detecting and distinguishing these mines from common ground clutter. Researchers have demonstrated this advantage in laboratory and limited field tests.

‘When a wave hits a land mine, resonance builds over the top of the mine, triggering a vibration which is bigger than the wave that excited it, and the vibration persists longer,’ said Waymond Scott Jr, a professor in Georgia Tech’s School of Electrical and Computer Engineering and principal investigator on the project.

Sponsored by the US Office of Naval Research, the US Army Research Office and the US Army Night Vision & Electronic Sensors Systems Directorate, the mine-detection project involves researchers from various departments at Georgia Tech. This multidisciplinary team started work in 1997 with computer modelling and lab experiments. Field-testing began in fall 2001, and during the past two years, the researchers have conducted tests at six sites.

In November 2002, the researchers travelled to a US government testing facility in a temperate climate where they detected six different anti-tank and anti-personnel mines. ‘Our results there were comparable to what we saw in the lab, which was very significant. That was a big hurdle for us,’ Scott said.

Field tests at government facilities give the researchers greater credibility because conditions are more realistic. Mines tend to have been buried for several years, complicating detection.

‘It’s much easier to detect a mine that’s been buried recently because you’ve disturbed the soil,’ said George McCall, a senior research engineer in the Georgia Tech Research Institute’s Electro-Optics, Environment and Materials Laboratory. ‘After a land mine has been in the ground for a while, the soil becomes weathered and more compact. This makes it harder to find, so it’s a better test for our detection system.’

In February 2003, the researchers travelled to another government testing facility where the ground was frozen. This test broadened the scope of environmental conditions under which the mine-detection system had been used.Testing in a variety of sites is important because different environmental conditions affect how far and how fast seismic waves travel through the earth. That, in turn, affects how waves interact with buried mines and what kind of signal processing is required to image the mines.

The field tests have also given the researchers a chance to develop another aspect of the seismic mine detector, an audio representation of buried mines.

‘When the system passes over a mine, you hear a resonance that’s easy to distinguish from the incident signal – it’s a hollow sound like what you hear when you tap on a wall to find a stud,’ Scott explained, adding that the operator would listen to this resonance via a headset, or the unit would have a speaker. ‘In some cases, this audio representation was clearer than the visual representation.’

Because of this discovery, Georgia Tech will collaborate with CyTerra Corp to evaluate the feasibility of incorporating the Georgia Tech seismic sensor into a handheld mine detector the company is producing for the US Army. CyTerra’s current handheld system combines a metal detector with ground-penetrating radar.

‘No single sensor has proven capable of detecting mines well with acceptable false alarms in all environmental conditions,’ Scott said, noting that what works best in a given situation depends on the type of mine and where it’s buried. ‘A fusion of multiple sensors will most likely be necessary to get good performance in all conditions. Our seismic sensor is ideal to fuse with other types of sensors like GPRs and metal detectors.’

In June 2003, researchers conducted their eighth field study, this time to test techniques for making the mine-detection system faster. They were able to shorten the system’s acoustical signal to one-sixteenth of a second, while still detecting mines. Also, researchers attempted to scan continuously, rather than moving 2 centimetres at a time to take measurements. This accelerated the measuring process by nearly 25 times and still yielded good data, researchers said.

Also, researchers added two radar sensors to the current system to demonstrate that interactions between multiple sensors are not problematic. Adding more sensors makes the system faster, they explained.

Bottom line, researchers say the time required to measure a square meter can be sliced from several hours to less than a minute. Faster measurements are crucial as the team develops a prototype for more extensive field tests.