Standoff Acoustic Laser Technique to Locate Buried Land Mines

■ The worldwide proliferation of land mines leads to thousands of civilian casualties each year and threatens military forces who patrol hostile territories. To reduce these casualties, military and humanitarian organizations seek methods to detect the large variety of mines being deployed. Many mine detection systems currently under development can detect only metal or a specific mine feature, have limited standoff range, or are impractical for field operations. A promising approach uses acoustic waves to induce mechanical vibrations in both plastic and metal mines. The vibration field above these mines can then be measured remotely with a laser Doppler vibrometer. This article describes a method to advance acoustic land-mine detection by increasing standoff range from the minefield and by developing a more practical lightweight system. We take a novel approach to excite mines by using a parametric acoustic array (PAA) source to transmit a highly directive sound beam from a safe distance. We discuss the standoff system concept, the process of PAA wave generation, and the coupling of acoustic waves to the ground to excite mines. A proof-of-concept system, built at Lincoln Laboratory, deploys a commercial PAA and a commercial laser vibrometer. We tested the system at a land-mine facility and measured distinct vibration signatures from buried anti-personnel mines. The overall concept shows promise. The PAA tested in these experiments, however, was developed for home entertainment and has marginal power for land-mine detection, even at close range. A system suitable for standoff detection requires more acoustic power and substantial modification. We estimate that power gains up to 50 dB may be achievable, and we discuss alternatives to the commercial PAA design.