A number of different techniques are grouped here under the heading "Acoustics"; these aim to exploit, in one way or another, acoustic energy for landmine detection. This complements GPR and MD as it uses an entirely different physical property of the mine (mechanical compliance and resonance) to achieve detection. One of the key advantages in some situations is that it may be possible to realise non-contact techniques. Researchers have investigated a variety of different kinds of sensors for this, including laser Doppler vibrometers, radars, ultrasonic devices and microphones.
"Conventional" Acoustics: In "conventional" acoustics a sound wave is emitted into a medium, the soil in the case of landmines and water or mud for sea mines. Typically low to mid audio frequencies are used, but for increased resolution higher frequencies have been used. For mines is water or liquid mud ultrasound (above 20 kHz) has also been tried. The sound waves will be reflected on boundaries between materials with different acoustical properties (different acoustic impedance), and the reflected wave, or impulse in the case of a pulsed system, is then detected. Such systems should be capable of good penetration through very wet and heavy ground such as clay, which makes them somewhat complementary to GPR for example (although they are also likely to experience problems at the air-ground interface unless direct mechanical coupling of the wave generator to the medium is used.)
In general detection of mines buried more than about one diameter below the surface has proved elusive.
One of the problems, particularly when using impulse systems, lies in isolating small object pulses from other, often dominant, signals, and coping with ground contours and irregularities. A method of "background signal subtraction" may be necessary.
Acoustic Spectroscopy: Acoustic systems have also been proposed in which the emitted wave contains a large frequency spectrum (using a swept frequency source for example), and one looks for mine signatures (resonances) in the received signal. Whether this is really practical as a mine detection feature is an open question for AP mines, though some promising results have been reported for large AT mines.
Acousto-seismic: Sound waves above the ground can penetrate into the ground surface (air-borne, or acoustic, excitation) due to the poro-elastic nature of the soil or the granular nature of road construction materials. The sound energy coupled into the ground causes seismic motion of the ground surface and subsurface, called acoustic-to-seismic (A/S) coupling. Landmines buried in the subsurface will resonate and induce distinct changes in the seismic motion due to scatter and reflection of the sound energy coupled into the ground. Solid-borne (seismic) excitation is also possible. The soil surface vibrations are sensed remotely using for example laser-doppler vibrometers or microwave vibrometers.
Non-linear acousto-seismic techniques are also being actively investigated and tested. They use the non-linear nature of the soil-mine interface to generate sum and difference frequencies from a combination of two or more incident acoustic waves of different frequencies. Initial results have been fairly encouraging.
Ultrasound systems (sonar) have also been proposed as tools to detect landmines buried in water, at the bottom of rivers or canals, etc. These have included systems based on commercial-off-the-shelf ultrasonic analysis systems.
Hand-held Operational Demining System
Project Abstract: The result of the HOPE-Project will be a...