The process of removing land mines from a current or former war zone is a dangerous, time-intensive and expensive proposition thanks largely to antiquated technology and methods of detection and extraction. An international team of researchers and engineers is hoping to change this with the help of a holographic radar system that sends continuous microwaves into the ground and creates subterranean images revealing what lies buried in a particular area.
Holographic radar systems have several key advantages over commonly used metal detectors, which are ineffective against explosives made from plastic and are unable to distinguish between a land mine and a piece of buried scrap metal. Holographic images are made line-by-line as the microwaves are transmitted into the ground.
A continuous-wave transmitter operates like a "radar flashlight" that concentrates on a small area rather than broadcasting signals across a vast expanse, says Tim Bechtel, adjunct professor of geosciences at Franklin & Marshall College in Lancaster, Pa. Bechtel is working as part of a team of researchers from Italy's University of Florence, Russia's Bauman Moscow State Technical University and elsewhere, to develop the technology, which is referred to as RASCAN. "It's a very narrow beam operating at a single frequency that illuminates a very small spot at one time," he adds. "It's not like snapping a picture." (View video below of a prototype RASCAN device built by the University of Florence.)
As the beam sweeps the surface being investigated, signals are gathered via an antenna and assembled into a more complete image, which can be viewed on a computer or video monitor. The colors that make up a given image represent different phase shifts between the object beam and the reference beam, or "how different the object is from normal soil," Bechtel says.
Land mines have caused more than 73,000 casualties across 119 countries in the past decade, according to the International Campaign to Ban Landmines. Despite treaties to ban landmines and several countries' efforts to eradicate existing explosives within their borders, it's thought that active landmines still exist in 70 countries.
United Nations specifications indicate that a person searching for landmines manually—typically with the aid of a spike thrust carefully into the ground—must probe every three centimeters to a depth of 10 centimeters. In dry soil, holographic radar is able to generate an image as deep as 15 centimeters and can cover a square meter in about three or four minutes, Bechtel says. The radar is designed so that depth and resolution can be adjusted, depending upon the user's needs. By setting the device at a lower frequency, such as four gigahertz, the radar can penetrate farther into the ground but with a lower resolution. Setting the device at seven gigahertz works for shallower targets and improves resolution.
The technology is still in its early stages and is not yet ready to be deployed in the field as part of landmine removal operations. For that, the antenna must be perpendicular to the ground and its tip must be no more than a few centimeters from its target.
So far, the only commercially available holographic radar system—RASCAN Radar—was developed by the Remote Sensing Laboratory of Bauman Moscow State Technical University. Researchers at the University of Florence are building a robot that can carry the RASCAN across mine fields, keeping the microwave antenna about 20 millimeters off the ground.
The researchers see their technology's utility extending beyond landmine searches to include finding cracks and signs of weakness in statues and other works of art as well as historic structures.