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Popular version of paper 4aPA6
Presented Friday morning, November 30, 2006
154st ASA Meeting, New Orleans, LA
Land mines buried in the ground generate casualties during military operations and continue to endanger civilians, even years after the end of conflicts. They constitute a severe safety and health threat in former war zones, since there are about 127 million land mines buried in 55 countries. The conventional method for land mine detection cannot detect nonmetal mines. According to US Department of Defense intelligence reports, about 75 percent of the land mines in Bosnia were nonmetallic.
Alternative methods are being developed for detection of nonmetal mines. Among them are acoustic methods (Figure 1). They are based on the fact that the presence of a mine affects ground vibration. Mines are not completely filled objects; instead they are shell-like and are less stiff than the surrounding soil. As a consequence, a similar seismic excitation will lead to higher amplitude vibrations of the soil if it contains a mine than if the soil does not.
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| Figure 1 Acoustic land mine detection principle. Vibrations in the ground induced by a loudspeaker are detected by a laser Doppler vibrometer. Differences in the vibration characteristics of ground above and away from the mine enable its detection. |
The vibration patterns on the grounds surface are complex. They depend on not only the presence of the mine, but also on soil layers, soil makeup, clutter, and moisture levels. In many cases the mine can be hidden in an inhomogeneous soil including rocks, roots, grass, etc. where surface vibrations can vary significantly, even without a mine present. Acoustic detection of mines in these conditions may lead to a high level of false alarms.
Researchers have focused their efforts on finding robust and reliable vibration characteristics that can be used in the acoustic detection of mines. It was found that vibration of mines and the soil on top of the mine is highly nonlinear. This nonlinearity is being studied as an indicator of the mine's presence using acoustic excitation. The existence of a nonlinear response means that the waveform and spectrum of vibration depend on the vibration amplitude. Increasing the excitation amplitude over a mine generates a more distinctive response form than it does in soil alone. To be able to detect this distinction, it is necessary to record ground vibration using a sufficiently wide range of acoustic excitation power. To induce ground vibration with sufficiently high amplitude at the point where the vibration is measured, it is beneficial to have an efficient means of focusing acoustic waves at a target point. Such a possibility is provided by an emerging approach called Time Reversal Acoustics.
Time Reversal Acoustics (TRA) provides focusing in inhomogeneous media where other methods of focusing fail. The idea of TRA has been developed for numerous applications during the last decade at a variety of institutions and extensively at the University of Paris VII. A diverse range of TRA applications has been demonstrated: destruction of kidney stones, detection of material defects, and communication with submarines. The TRA focusing of seismic waves was tested for landmine detection in a joint research effort between Artann Laboratories, The University of Mississippi and US Army Night Vision Lab (Figure 2).
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| Figure 2 The system for acoustic detection of mines developed in National Center of Physical Acoustics, University of Mississippi. The system was used to demonstrate feasibility of TRA focusing of seismic waves at multiple points where ground vibration was measured. |
Nonlinear effects in TRA focused acoustic waves were investigated at the facilities of the US Army Night Vision and Electronic Sensors Directorate with the help of a system consisting of 6 loudspeakers, a sensor measuring the vibration of the ground, and a microphone. We conducted experiments exploring the changes of the various features of ground vibration at different levels of excitation. Significant differences in nonlinear effects in ground vibration above the mine and away from the mine were detected. Based on these findings we developed a technique named Varied Amplitude (VA) method that excites a small region of soil with successive seismic waves of varied amplitudes. Analysis of vibration waveform and spectrum at this point and subsequent measurement points build a map that can be used for landmine detection
Preliminary experiments demonstrated high sensitivity of the TRA VA method and also revealed a drawback which could limit its applicability: measurements are time consuming. Measurement time is equally important to the sensitivity of the detection method, because it limits the area that can be inspected. Previously, it was necessary to adjust the TRA focusing at every new point due to changes in the pattern of seismic wave propagation in inhomogeneous soil which increased the measurement time significantly. To overcome this difficulty we explored the possibility of inducing the ground vibration by TRA focusing of air-born waves, which do not need adjustment at every measurement point assuming an appropriate source configuration is used. Experiments have shown that air-born excitation of ground vibration is about half as effective as that by seismic waves, but it is sufficient to utilize the VA method of mine detection.
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| Figure 3 Results of the experiment on the TRA VA method of land mine detection. |
Results from our TRA VA tests are illustrated in Figure 3 based on the use of airborne acoustic waves. For this experiment, a plastic antitank mine, VS1.6, was buried at a depth of 15 mm in sand. Focusing was performed once, at a location away from the mine. The excitation signal was comprised of four successive impulses with decreasing amplitudes. Measurements were performed at different locations by translating the loudspeaker system over the mine. The blue line, illustrates the conventional linear acoustic method of mine detection and shows a profile of vibration amplitude along the line crossing the location of the buried mine. It is seen that the ground vibration amplitude increases slightly directly over the mine. Green and red lines demonstrate results obtained by the TRA VA method, presenting two different spectral and amplitude related parameters of the vibration as a function of the distance across the mine. One parameter shown by green line clearly reveals the edge of the mine while the other parameter shown by red line, clearly reveals the mine center. The values obtained away from the mine are significantly lower, which makes detection easier. Results of this work established that TRA based nonlinear methods and specifically the Varied Amplitude method provide sensitive and robust means for land mine detection.
[Work was supported by the U.S. Army RDECOM CERDEC Night Vision and Electronic Sensors Directorate.]