There are more than 100 million land mines worldwide and, for every 15 of them, someone is maimed or loses a limb.A team at the US Army CECOM Night Vision and Sensors Directorate in Alexandria, Va., led by University of Mississippi professor James Sabatier, has developed a noncontact method of detecting land mines that uses laser feedback interferometry.It's an old problem, he said. "How does one detect acoustic resonant phenomena just below the surface of the ground and yet not make contact with the surface?"He said that, like a bell or a drum, a land mine is a compliant object that exhibits a specific acoustic behavior that distinguishes it from any other material. "There are no false alarms."In their technique, the researchers aim a loudspeaker at the ground and measure the vibration with a Vibromet 1000, a vibrometer from MetroLaser Inc. The instrument shines light from a series of 780-nm laser diodes on the vibrating ground, which modulates the backscattered light at a Doppler-shifted frequency allowing the vibrometer, via laser feedback interferometry, to calculate surface velocity. By raster scanning a patch of ground with an X-Y galvanometer mirror, the instrument can isolate the Doppler modulation of a land mine.The researchers are studying how gravel or grass environments affect the technology, and how fast the method can go. They have made the mirrors sweep up to 10 km per hour, locating mines with 100 percent accuracy. However, Sabatier cautioned, that's different from physically translating the galvanometer mirror, which causes vibration.Working with Planning Systems Inc. of Slidell, La., the team is also using synthetic aperture ground-penetrating radar, which is effective for locating deep metal mines. Using the methods in concert, mine-seekers can detect deep or shallow mines of metal or plastic. "The future success of this work is in the optics community," Sabatier said.