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By Richard Craig
I have to confess that I am not an expert on land mines. This issue marks my third anniversary of working in this area. What I've learned has been connected with research on how to deal with them. The mines I've met have had the detonators removed-and I'm just as happy to keep it that way. Land mines, however, present an issue that won't go away on its own and provide opportunities for physicists to apply their peculiar skills to help address this life-and-death issue.
Some refer to land mines as the perfect soldiers. They are inexpensive-a few 10s of dollars each to buy and deploy. They don't eat; they don't fall asleep on duty; they don't require maintenance. Land mines are selective in the sense that they don't, generally, detonate spontaneously or when encountered by something less than their target. By design, land mines aren't shock-sensitive so they're difficult and expensive to defeat.
Most field soldiers with whom I've spoken don't like them at all. They view land mines as an evil component of warfare. To them, land mines are undiscriminating weapons that kill and maim friends and foe alike.
Yet another perspective is that of civilian inhabitants of former war zones. For these people, land mines are a part of their everyday life. Land mines render their living and working areas dangerous. This is especially true in the case of civil wars for which the purpose of the land mines is, often, genocidal. Bosnia/Herzegovina and Croatia are one such example: Land mines, laid as part of ethnic cleansing activities, continue to contaminate the traditional living and farming areas of noncombatant populations.
The Present: low tech, high numbers
The numbers are overwhelming. The United Nations estimates that more than 110 million mines contaminate greater than 20 million square miles in 64 countries. (See Table 1.) Asia, Africa, and the Middle East are affected, as are western South America and some countries in Central America. It estimates more than 800 civilian deaths per month. Because antipersonnel mines are designed to maim rather than kill-an injured soldier removes three from the enemy force-many others are crippled.
There are various estimates for the rates of humanitarian land-mine removal but, until very recently, the rate of mine placement exceeded that of removal manyfold. The cost of land-mine removal is 2 to 3 orders of magnitude greater than that of placement.
The primary issue for humanitarian demining-and for military demining-is to find, remove and dispose of the mines without harm to self or others. The primary difference between military and humanitarian demining is the time scale: military demining has to be done "right now" whereas humanitarian demining is part of the peace process.
One approach for front-line military demining is simply to run a flail-equipped tank in front of the advancing troops. A flail is like an oversized lawn roller that explodes the mines in-place. Various nations have been developing similar hardened-vehicle technology for large-scale demining. These generally require or result in the removal of all vegetation and leveling of the area being demined.
The ORDATA II database lists 800 varieties of land mine; this includes metal, plastic, wood, and ceramic mines. There are anti-tank mines, bounding mines-antipersonnel mines that pop out of the ground and then explode, for increased effective radius-and "toe poppers" with just a few grams of explosive. For most modern mines the casing is plastic and little to no metal is used in construction. Consequently, induction-based metal detectors, which worked so successfully on WWII metal mines are of very limited application, although their capabilities have been pushed far beyond those available in 1950. Sappers-those who remove mines-are loath to depend on low-detection probability techniques because false negatives are the source of about half of all land mine injuries.
The people who presently remove land mines mostly do so by hand. They use a probe to search the ground, inch-by-inch. In some cases, the probe is a pointed stick, in others, a bayonet or other long knife; Great Britain has a standoff probe with an extension handle; the US uses a composite probe with acrylic handle; for frozen soil the high-tech solution is a titanium probe. Composite probes are desirable so that, if a mine detonates, and the probe turns into a projectile, the consequences to the user are minimized.
Except for the mechanical probe, there may be no single technology capable of finding land mines under all conditions. And, as noted, a different mechanical probe is required for differing conditions.
Mine detection, largely, depends on the contrast between a physical property of the mine and the surrounding earth. Mine concealment has been based on fabricating mines from materials that match the earth. A continuing issue for all technologies is "clutter"-objects that have signals similar to the target but that are false positives. When real-time imaging is possible, this provides the operator with the basis for interpreting away much clutter.
In pursuit of high-tech options
The scientific community already has invested considerable R&D to find, remove, and dispose of land mines. Various approaches are being developed, each with advantages and disadvantages either from a technical aspect, feasibility angle for implementation, or cost issue. Each is significant in its own way. Because of the variety of environments in which these must operate, the international community is moving towards an integrated suite of technologies as the most reliable and comprehensive method of detecting land mines, instead of relying on a single device.
In terms of finding land mines, the best single technology is still a dog or similar animal. Some investigators are working on training other animals such as rats. Bees have been suggested for finding minefields. The difficulty with the use of animals is that the training cost and required infrastructure often outweigh the advantages. The U.S. DoD has a "Synthetic Dog's Nose" program to replace the biological sensor with something electronic; mass balance sensors with selective coatings are the heart of this program but others are looking at ion-mobility devices. This program is in the research phase; the limitation here is that most high explosives have vanishingly small vapor pressures; the technology depends on the vapors (primarily nitrobenzenes) released by trace contaminants and degradation products. Because vapor transport through soil is slow, the dog-and its synthetic analog-work best shortly after a rainfall.
Most research into mine detection involves a physical probe other than a mechanical probe. Ground-penetrating radar (GPR), in some form or other, continues to get considerable attention for a number of reasons. GPR is attractive because it provides the potential for imaging the subsurface when phase information is retained. GPR depends on the contrast in dielectric properties between the earth and the mine and, so, is somewhat improved by a slight amount of moisture in the soil, which also serves to limit the depth of penetration. The imaging and penetration criteria are also somewhat contradictory as the penetration varies directly with wavelength and the resolution inversely with wavelength. The particular difficulty with GPR is that the impedance mismatch between the air and soil is so great that a large surface reflection results.
One variation on GPR being explored is dielectric susceptibility probing in which the contrast between the complex dielectric response of the soil and that of foreign objects can be exploited.
Nuclear quadrupole resonance (NQR) depends on the contrast in nitrogen concentration between the explosive and the soil. For nuclei with a quadrupole moment, such as 14N, molecule-specific fields result in splitting of the spin states in the absence of an applied field. The information contained in this splitting can be accessed by applying a broad-band RF pulse and observing the resulting weak, chemically specific, echo pulse. Among the common high explosives, NQR is much more successful in "seeing" RDX (hexogen) than the less expensive and more common TNT, for which both the frequency and signal size is much smaller. In either case, the NQR return pulse is very weak and subject to interference, so fieldable NQR devices require substantial power sources. A team of Russian and Germany physicists have recently proposed a means for enhancing NQR splitting signals via cross-relaxation from strong proton excitation. This variation requires a large, i.e., heavy, high-field magnet.
Land mines have different thermal conductivity and heat capacity than the surrounding earth. As a result, at certain times of day, a land mine presents a warmer or cooler area than its surroundings. This is the physical basis of infrared techniques for mine detection. When IR detection works, it can work very well-antitank mines can show up very clearly from an airborne platform-the issue with IR is that it may not work at all. This is not a confidence-building characteristic for a demining technology.
Acoustic-detection techniques are being explored. For these the principal hurdles is the large attenuation in soils and the very large impedance difference between air and soil. The latter essentially restricts the technology to in-ground transducers.
There are a number of techniques based on nuclear physics being investigated for locating land mines. Among these are x-ray and gamma-ray backscattering. Because photoelectric absorption of photons increases as a high power of the atomic number (Z), low-Z materials backscatter x-rays and gamma rays better than high-Z materials. Land mine components are principally H, C, N, and O, whereas soil is mostly made up of oxides of the third and fourth row of the periodic table. Consequently, photon backscatter presents an opportunity for land-mine detection, including the potential for imaging. One difficulty with this approach is that x-ray and gamma-ray backscatter cross-sections are relatively small.
Neutron elastic-scattering mine-detection techniques depend on large cross-section contrast between hydrogen and other elements. All organic explosives and all plastics contain hydrogen at concentrations greater than that in relatively dry soils. A number of groups around the world are using neutron scattering as a means of land-mine detection. For neutron scattering, ground moisture is the principal clutter; however, because a land mine forms a barrier to moisture transport, a small amount of soil water actually improves detection. One "gotcha" with neutron scattering is that some mines classified as plastic are, in fact, glass-fiber composite; apparently there is sufficient boron in the glass fiber to capture a large fraction of thermal and epithermal neutrons.
Prompt activation of explosive components-principally nitrogen-by neutrons is another technique being investigated by many groups under various names. Neutron-capture cross-sections are much smaller than elastic-scattering cross-sections so these techniques require much larger neutron sources and longer counting times than neutron-scattering techniques. On the other hand, the nitrogen excitation is quite specific so these techniques continue to demonstrate promise as confirmatory techniques.
For all the nuclear techniques, the principal hurdle is the antipathy towards things radioactive. The sources used can be small enough as not to project any significant risk to the user-especially when considered in the context of working in a possible mine field-but public perception still regards things nuclear in a negative light. Just as NMR imaging was changed to MRI for public relations purposes, for the same reasons, developers of NQR are silently dropping the "N" to QR.
Current thinking in the land-mine detection community is multiprobe-hyphenated-instruments either to provide complementary strengths for differing soil conditions or to provide on-board confirmation. Generally, the confirmatory instruments are integrated into autonomous remote or heavily hardened vehicles.
Water is a problem with most proposed techniques; tropical areas and riparian and littoral zones present particular difficulties both for humanitarian and military mine detection and clearing.
Once the issue of finding a land mine has been solved, the first step in the removal process is uncovering it. The present approach is to brush the covering soil off-very carefully to take into account the possibility of booby trapping. A "air shovel" has been developed-basically a vacuum cleaner operated in reverse-to carefully remove the soil around a found mine.
Uncovered, the mine can be destroyed in-place, although this usually requires a fairly substantial explosive charge because land mines are designed to be shock-insensitive. Moreover, exploding in place is unpopular because of the essential lack of control. Several groups are working on means to destroy, nonexplosively, mines in place. One such method is to use, in effect, a torch to cut through the mine casing and ignite the high explosive. In most instances, the high explosive will burn-deflagrate-rather than explode.
The Future: our challenge
The land-mine problem isn't going to go away. Some of the approaches presently being studied may help to reduce the problem but, even if each is able to provide a contribution under its most promising physical conditions, there will still be conditions under which none, yet addressed, are effective. Consequently, the world will not be clear of mines by 2010 or 2110 for that matter.
As a science, where can physics help? All phases: Finding land mines; removing land mines; disposing of land mines. The opportunities in detection are several-fold: First, identifying a physical probe or suite of probes that is superior to those under consideration; second, finding a way to improve the performance of the existing probes and third, engineering that high-tech probe into a low-tech instrument that is patently acceptable to a community that is comfortable with mechanical probes. Engineering multiprobe systems into a remotely directed vehicle can be nontrivial if the modalities tend to interfere, such as a GPR antenna interfering with electromagnetic induction. The powers-that-be in the demining community have a dream date: Mine detection at a distance with a minimum standoff of 10 meters. Others are specifying aircraft standoff. Most of the present probes are limited by applicable physics to much shorter distances.
A group at Pacific Northwest National Laboratory has improved the signal-to-statistical-noise ratio of neutron backscattering mine detection by factors of 2 or more simply by adding time correlations. An enabling technology for this improvement was the development of a time-tagged fission source by Oak Ridge National Laboratory. Colleagues of mine at PNNL have applied physical principles to reduce the ground reflection problem in one version of imaging GPR by using slant-angle illumination so that the large surface reflection is, in large part, scattered away and reflections from subsurface objects is enhanced. Investigators at Coleman Research have been integrating swept-frequency GPR, electromagnetic induction, and infrared imaging into a single portable system. Investigators at Quantum Magnetics have been improving the sensitivity of NQR to TNT.
The demining community will accept higher-cost devices; it will not accept a device that is perceived to reduce the reliability, compared to a mechanical probe, regardless of speed or other advantages. Perhaps the low-field magnetic resonance spectroscopy work coming out of Berkeley recently can provide a basis for low-power, low-field resonance chemical probing akin to NQR. The need for helium cooling would restrict the application only slightly. Yet another variation on the dielectric probe theme, not yet considered for land-mine detection, to my knowledge, although it has been successful in examining storage tanks for leaks, is electrical-resistance tomography; this would require that the soil have some reasonable conductivity. Neutron-scattering land-mine detection could be improved substantially if a small, truly inexpensive neutron generator became available. This would also provide a source that might be "turned off", improving public perceptions, as well.
Recently, a middle-school teacher suggested to me the use of microwaves to locate mines by detonating them in-place. With mine densities smaller than one per acre, this may not be practical for locating mines but for stand-off destruction-especially if deflagration can reliably be induced instead of detonation-perhaps there is a kernel of an idea in this.
Finally, there is an interesting socioeconomic issue for humanitarian demining in developing countries. Presently, mine clearance there is often a closed shop; those doing the work are among the best-paid workers in their countries. They are loath to see outsiders come in to replace them or allow others to do so. Any technology to be used in these areas must be engineered to be adaptable to the existing infrastructure.
As a community, how can physicists help? The physics community has repeatedly demonstrated the talent, creativity and attitude needed in the pursuit of feasible, reliable solutions to real-world challenges, including those of land-mine detection. The bottom line is that this problem isn't solved and it is more than just a technical challenge. We can develop technologies that will be used to save lives. Lives of women and children often recruited to clear mine fields.
Richard Craig is a physicist at Pacific Northwest National Laboratory in Richland, WA. He received last year's Christopher Columbus Foundation Award for his development of a timed neutron detector of plastic land mines
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