Acoustic and Seismic Systems
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Acoustic and Seismic systems
Acoustic/seismic methods look for mines by “vibrating” them with sound or seismic waves that are introduced into the ground. This process is analogous to tapping on a wall to search for wooden studs: materials with different properties vibrate differently when exposed to sound waves. These methods are unique among detection methods that identify the mine casing and components in that they are not based on electromagnetic properties.
Description
Acoustic/seismic mine detection systems typically generate sound (above ground) from an off-the-shelf loudspeaker, although there are many possible configurations. Some of the acoustic energy reflects off the ground surface, but the rest penetrates the ground in the form of waves that propagate through the soil. When an object such as a mine is buried, some of the energy reflects upward toward the ground surface, causing vibration at the surface. Specialized sensors can detect these vibrations without contacting the ground. A variety of different kinds of sensors (laser Doppler vibrometers, radars, ultrasonic devices, microphones) have been tried. Researchers have field tested acoustic/seismic methods for landmine detection on approximately 300 buried antitank and antipersonnel mines and several hundred square meters of clutter locations at Army field sites in Virginia and Arizona. Initial tests focused on antitank mines and yielded high probabilities of detection and low false alarm rates. For example, in one test, the acoustic system identified 18 of 19 mines buried in dirt and gravel, yielding a probability of detection of 95 percent. There was only one false alarm in the test, even though the test site was seeded with clutter items that had confounded a GPR system. When the system was modified with advanced signal-processing algorithms, the false alarm rate dropped to zero.
Strengths
Acoustic/seismic sensors are based on completely different physical effects than any other sensor. For example, they sense differences in mechanical properties of the mine and soil, while GPR and EMI sensors detect differences in electromagnetic properties. Thus, acoustic/ seismic sensors would complement existing sensors well. Acoustic/seismic systems also have the potential for very low false alarm rates. In experiments to date, false alarms from naturally occurring clutter, such as rocks and scrap metal, have been extremely low (although such hollow clutter items as soda bottles and cans would cause false alarms because the resonance patterns of these objects are similar to those of mines). An additional strength is that, unlike GPR systems, these sensors are unaffected by moisture and weather, although frozen ground may limit the sensor’s capability.
Limitations
The greatest limitation of acoustic/seismic systems is that they do not detect mines at depth because the resonant response attenuates significantly with depth. With current experimental systems, mines deeper than approximately one mine diameter are difficult to find. Also problematic is the slow speed of existing systems. Speed currently is limited by the displacement sensor, which senses the vibrations at the surface caused by the sound waves. These displacements are very small (less than 1 µm) and are thus difficult to measure quickly in the adverse conditions of a minefield. The required scan time for locating antipersonnel mines may range from 125 to 1,000 seconds per square meter. However, a number of methods are being investigated to speed up the detection process. For example, an array of N sensors will speed the system by a factor of N. Small prototypes of such arrays have been developed and can be expanded and improved with further work. An additional limitation of existing systems is that moderate to heavy vegetation can interfere with the laser Doppler vibrometers that are commonly used to sense the vibrations at the ground surface. A new type of sensor could be developed, however, to overcome this flaw.
Summary/Evaluation
Significant progress has been made in the past five to ten years in developing acoustic/seismic mine detection systems. Interactions between the seismic waves and buried mines and clutter are much better understood, as are the seismic sources and displacement sensors. The systems show great potential, but more research is needed to make them practical. The development of an array of displacement sensors that is fast, can penetrate vegetation, and can function in the adverse conditions of a real minefield would be especially useful.
References
- ↑ Jacqueline MacDonald et.al: Alternatives of Landmine Detection", RAND report, ISBN 0-8330-3301-8, Document Number: MR-1608-OSTP, Year: 2003
