Technological Innovation in Humanitarian Demining
Kosta Tsipis
Introduction
In 1993 the U.S. State Department publication "The Hidden Killers, The Global Landmine Crisis" pointed out that there are about 120 million landmines still buried in 62 countries, potentially lethal remnants of armed conflicts over the past half century. Even though the combatants in these wars did not generally intend to harm the civilian population, abandoned landmines now kill or maim about 30,000 persons globally every year, 80 percent of them civilians.
Four hundred million mines were deployed between 1935 and 1996; of these 65 million were emplaced during the last 25 years. Various nations currently manufacture 7.5 million mines annually. In 1993 alone, according to U.N. estimates, 2~5 million mines were laid. In the same year, only 80,000 were removed. It is estimated that 100 million mines are currently stockpiled around the world ready for use.
Mines are durable objects that can remain active for decades. They are manufactured in large numbers by many nations including the U.S., Russia, China, Italy, and a hundred other suppliers. Their cost varies mostly between $3 and $15 each. A few may cost as much as $50 each. A mine usually consists of a casing (metallic, plastic, or even wood); the detonator, booster and main charge; the fuse; and sensors that range from a simple pressure plate or a trip wire to more sophisticated triggers. a collapsing circuit, pressure-distorted optical fiber, pneumatic pressure fuse, or various influence sensors -- acoustic, seismic, magnetic, thermal. The most common triggering mechanisms depend on pressure --5 to 10 kg of force -- applied on the top of the mine.
In addition to the human toll landmines claim in many, mainly poor, underdeveloped areas (in Cambodia an incidence of one amputee per 250 people has been caused by land-mine accidents). their negative effects are multidimensional. Landmines can, over the long term, disrupt normal economic activities, such as travel and transport, and deny land to farmers, in turn often causing malnutrition, hunger, or migration of agrarian populations to urban centers. Clearance is not only a safety issue, but an economic and social issue as well.
Demining Operations; Current Practices
Demining operations differ sharply according to their purpose. Tactical demining, including minefield "breaching," aims at rapidly clearing a corridor for combat use through a minefield during battle, often in hours. "Tactical countermine" operations aim at the removal of most mines by military personnel from areas occupied by the military over days or weeks. "Humanitarian demining," the subject of this paper, involves the peacetime detection and deactivation over a considerable time of all mines emplaced in an area.
Because most mines have metallic casings or contain at least a few grams of metal (usually the firing pin and associated spring) the standard method of detecting mines either buried or hidden in overgrowth is a pulsed-induction eddy-current sensor that can unambiguously detect the presence of less than a gram of metal buried in nonmetallic soils to a depth of 10-20 cm. The pulsed-electromagnetic induction (PEMI) detector applies a pulsed magnetic field (T ;.5 msec) to the soil. The magnetic field propagates into the soil and diffuses into buried conducting materials. Eddy currents are induced in the conducting material which in turn produce an opposing magnetic field, (T ;200 5sec) as the applied field collapses. This opposing field disturbs the magnetic field produced by the detector. Perturbations in the detector field indicate the presence of a metallic object buried in the soil and are signaled by an audible sound. In effect, such detectors can detect reliably most of the smallest antipersonnel mines buried close to the surface. But they cannot detect totally, or almost completely, metal-free mines.
But this method suffers from a major disadvantage. Metal detectors detect not only mines but all metallic objects in the ground. Since quite often mines are laid in or near battlegrounds, metallic detritus -- shrapnel, bullets, pieces of metal, screws, wires, etc. -- causes a false alarm ratio often higher than 1000 to 1. Therefore, one of the major technical challenges in humanitarian demining is discriminating between false alarms and real mines.
Discrimination is currently accomplished in a very slow and dangerous fashion. The metal detector locates the buried metallic object to within five cm. Such detection takes about a minute per square meter of terrain. Demining personnel then probe the spot with a rod (metallic, plastic, or wood) about 20-25 cm. long to determine whether the detected object has the characteristic size and shape of a mine or is instead a mere piece of scrap metal. Depending on the soil type and condition (hardened, overgrown, etc.), discrimination by probe can take anywhere between two and 20 minutes. Once a mine is confirmed, it now takes about 10 minutes to dig it out, another 10 to explode it in situ (creating additional metallic clutter), and 10 more minutes for the de-miners to walk away from the explosion and back. All detected objects must be identified, or even dug up to assure that no explosives have been left in the ground.
This is clearly too time-consuming a method of discrimination. With current equipment and practices the process can take about 30 minutes for every metal detector signal. Moreover, the use of a probe to determine the nature of an object detected by a PEMI detector does not tolerate carelessness or boredom. The resulting average casualty rate for this work is one injured or dead deminer per 1000 mines detected. But rates as high as an injury per 100 mines have been encountered.
An additional problem is that many types of mines are designed and constructed with very little metallic content; some are completely metal-free. Metal detectors are of little use for these types of mines.
The laboriousness and riskiness of the current canonical method of discriminating mines from false alarms, and the existence of non-metallic, or low-metal content mines, have led to new technologies for use in humanitarian demining. Some of these are evolutionary versions of older approaches, some are quite novel. Here I describe first detection/discrimination methods, based on the mine's explosive contents, in its solid or vapor state:
- Thermal neutron activation of the element nitrogen in explosives
- Back scatter of X-rays from plastic landmines based on the lower-Z contents of such mines compared to the Z of average soils.
- Nuclear Quadrupole Resonance properties of nitrogen nuclei in crystalline structures like explosives.
- High speed gas chromatography that detects explosives vapors (or particles?) emanating from buried mines.
- Arrays of organic polymers that can sense and identify vapors.
A second class of detection technologies is based on the fact that a buried mine represents a discontinuity of dielectric or conductivity properties in the soil. This approach uses:
- Ground penetrating radar or its cousin
- Microwave Impulse Radar to detect mines on the surface or buried in the ground.
A third class of detectors, based on the differences in dielectric and diamagnetic properties of materials, uses:
- Magnetoquasistatic detectors
- Electroquasistatic detectors
These two types detectors detect and discriminate metallic and non-metallic mines respectively from the clutter presented by the ground and its contents.
An entirely different approach is to detonate mines without detecting them, instead of using detection and discrimination methods to locate mines (which are subsequently destroyed). This capital-costly "brute force" approach involves the use of vehicles equipped with rollers or treads that detonate anti-personnel mines by riding over them. Application of this is limited by terrain, the potential presence of antitank mines that can destroy the vehicle, and the difficulty of assuring that all mines in a given area have been destroyed (on uneven ground, the equipment may not apply the needed pressure everywhere).
Before either approach -- detection/discrimination, or brute force neutralization -- can be used, it is necessary to find, and determine the boundaries, of a minefield. Perhaps even more important is the confident identification of areas that are free of mines. This is the second major hurdle for humanitarian demining: developing rapid and efficient area search methods that will reliably determine the presence or absence of mines. Currently finding minefields and determining their approximate boundaries, as well as declaring areas as mine-free depends on visual observation, history of mine accidents or records of laid mine fields. Specially trained dogs or simple metal detectors are now used for area surveillance, a slow and risky method.
Several technological avenues are being followed in pursuit of a satisfactory method for remote, rapid, and reliable minefield detection.
- Several passive IR systems relying on thermal images of mines or "scars" in the soil resulting from excavation to bury mines. Some of these systems are airborne (fixed-wing aircraft, helicopter) and some are vehicle-based.
- Multi-spectral and hyperspectral systems based on the resulting imagery.
- Airborne active laser systems based on detecting the reflected light.
- A helicopter-borne system using an active laser (1.06 5m) and a passive long wavelength IR sensor (8-12 5m). The system collected reflectance and polarization image data, and thermal emission data. The system incorporated real-time imaging and data analysis that automatically detected minefields.
Although this latter system was the most successful, it detected 99% of conventional surface-laid minefields but 66% of scattered mineflelds and 34% of minefields with buried mines.
Thus, the problem of rapid minefield surveillance remains active, and is being addressed. A fusion of synthetic aperture radar and a hyperspectral imager data is showing good promise.
To summarize the current state of humanitarian demining technologies: even though humanitarian demining has the dual advantages of time and low wage local labor ($1500 - $3000 per person per year), the currently used method is unacceptably slow, expensive., and dangerous. More important, it has only insufficient impact on the global landmine problem. More specifically, research and development in humanitarian demining needs to focus on four key areas:
- Efficient method to survey large tracts of land to identify confidently mined and mine-free areas, roads, etc.
- Improvement by an order of magnitude in the speed and safety of equipment and methods used in the current labor-intensive approach.
- Rapid and efficient methods to neutralize discovered mines.
- Development of advanced detection/discrimination technologies that can be mechanized and used with automated or even robotic systems-- which can replace the existing labor-intensive demining practices no later than five years from now.
The Next Steps
Humanitarian demining technologies in the near horizon easily fall in two groups:
technologies approaching maturity that can be applied to increase the efficiency, speed, and safety of the labor-intensive current demining methods and those, more promising perhaps, that won't be ready for field operations for half a decade or so.
In the first category I include the Meandering Winding Magnetometer (MWM) and the various configurations of the Interdigitated Electrode Dielectrometer (IDED), the air knife (already in multiple uses), the explosive foam "Lexfoam," and a family of smart probes -- acoustical, thermal, or magnetic. In the second category I put the Nuclear Quadrupole Resonance explosives detectors, the rapid gas chromatograph, and the polymer-black carbon composites arrays to detect explosives vapors, and the family of remotely controlled, automated, or robotic vehicles that can perform both the detection and the neutralization of landmines. The wide area surveillance system that uses fused SAR/hyperspectral imager data naturally falls in this second group.
The MWM can reduce false alarm rates by a factor of 10 within a year or so and consequently reduce the time spent for detection to 5-20 sec/m2 of searched terrain. The air knife, which uses high pressure air as a hand-held probe to uncover buried metallic objects (false alarms or mines), could replace the simplest manual probes and speed up discrimination of mines from metallic fragments by a factor of 5-10 while improving safety at the same time. The use of Lexfoam ($9 per pound) to blow up the exposed mine would speed up the overall demining process by a factor of 2 to 5.
I shift now to the more advanced, more promising, but still untested in the field, detectors of explosives. Their advantage is clear: only mines and other UXO would trigger the detectors, fusing the tasks of detection and discrimination into a single step and therefore speeding up humanitarian demining decisively. One key advantage of this approach is that its efficiency of detection is not dependent on the metallic content of the mine. Plastic and low metal-content mines can be detected and identified as well as metallic ones.
Nuclear Quadrupole Resonance (NQR) depends on the fact that some atomic nuclei, such as nitrogen, are not spherically symmetrical, i.e. they possess electrical quadrupole moments. Depending on what kind of crystalline structure nitrogen nuclei find themselves in, their non-sphericity produces a unique set of energy states characteristic of the crystalline structure when precessed magnetically. Using this property, an explosive in its solid phase can be identified by its nitrogen absorption radio frequency lines. The explosive RDX can be readily detected by NQR, but TNT (the most common explosive in mines) and PETN are more difficult to detect. Although TNT detection will require longer detection times, many seconds, or even several minutes, NQR detectors can be used in discriminating mines from false alarms.
Two-sided nuclear quadrupole resonance explosive detectors have been tested in airports where they detect quantities of RDX comparable to those in a mine in six seconds. But applications of NQR to mine detection will first require the satisfactory solution of several problems: a) A way must be found to improve the detectability of TNT by exploiting more absorption lines, and by improving the electronics and the detector coil; b) The possibility of interference from stray radio signals at the relevant frequencies must be reduced; c) Some method must be found to deal with the inhomogeneities caused by the one-sided detection geometry that mine detection dictates.
The fact that trained dogs can detect mines unerringly indicates that mines emit vapors that characterize them uniquely, though this may depend on how long a mine has been buried. In all probability these are explosives vapors that either escape from the interior of the mine or come from traces of explosives "smeared" on the outside of the mine during manufacture, storage, or emplacement. It is not clear that these vapors emanate in real time from the mine or are vapors from particles that have stuck to dirt or vegetation directly above the mine.
Arrays of sensors, each with some specificity to a particular molecule or compound, are quite commonly used in the food and perfume industries to identify constituent compounds of the product. One such sensor is under development at Cal Tech. It uses physical-chemical properties of carbon-black organic polymer composites to develop vapor-sensing elements, each sensitive to different molecules. The collective response of an array of such elements can identify the type of vapor. DARPA is actively pursuing an array sensor for explosives detection intended of airport use, but probably adaptable for humanitarian demining work.
An electronic vapor detector that claims detection sensitivity that is 1 order better than what dogs achieve (i.e. 10-20 picograms of TNT) has been developed. The detector initially collects particles on which the vapor is attached and then performs the vapor recognition by rapid gas chromatography and chemiluminescence. In principle, this technology could be adapted to a probe that could be inserted in the ground near a suspected mine to "sniff" the vapor aura of the buried mine. It is not clear to what degree explosives vapor remnants in old battlefields will generate unacceptable clutter for the electronic vapor detector. Detailed field measurements of the presence and behavior of explosives vapors will have to be conducted in support of the development of such a detector.
Conclusions and Recommendations
Several of the technologies described in this paper appear to promise decisive improvement in humanitarian demining operations. If these technologies are to mature into useful, affordable, field equipment, I believe we have to follow four general guidelines.
First, since there is no indication that a single entirely new, revolutionary approach to humanitarian demining lies just beyond a near horizon, efforts should focus on incremental improvements of the various demining operations, starting with the field use of better tools in the current deminer-intensive method, and gradually introducing new, sophisticated mine detectors. Since no single "silver bullet" will solve all demining problems in all cases, a spectrum of new detection and neutralization technologies should be developed, field-tested and applied flexibly.
Second, efforts and funding should focus on technologies that lead to systems that are easy to operate and maintain in countries infested with land-mines. Power sources for demining instruments must be portable, detectors must be rugged, and associated electronics must be impervious to humidity, dust, and temperature extremes.
Third, the magnitude and complexity of a systematic humanitarian demining campaign are so large that its goals cannot be achieved by earnest, even ingenious, efforts that remain un-coordinated. A coherent, systematic progression from measurements of physical and chemical properties of mines, followed by experimentation, equipment development and laboratory testing, then field testing in realistic conditions, modification, engineering development, production, distribution to users around the world, training of operators, and creation of a central but easily accessible data-bank of mine and soil properties and of the latest results of demining research have to be carefully organized, guided, supervised, and evaluated.
Fourth, the entire effort to develop demining equipment of gradually increasing sophistication and efficiency must be centrally coordinated, guided, and overseen. A central agent is needed to set research priorities, assign technical tasks, coordinate their implementation, and evaluate the results. In addition, such an entity could act as an advocate for humanitarian demining within the U.S. government. This proposed coordinating agency will need high-level technical and scientific advice. Such a need can be satisfied by the establishment of a properly constituted Science Advisory Board that will advise and provide information about relevant scientific and technical developments in academic and high-technology industrial laboratories.
Meanwhile, in parallel, stable, long-term, adequate funding for these tasks must be secured. This latter task implies the need for a persistent effort to inform and educate decision makers, opinion makers, and through them, the tax-paying publics of developed democracies in parallel with the scientific and technological efforts. I believe that several of the technologies examined here will work well in the field and therefore can be politically attractive to governments wishing to assume leadership roles in humanitarian activities.
Kosta Tsipis is with the Program in Science & Technology for International Security
Massachusetts Institute of Technology,Cambridge, MA 02139 USA
tsipis@athena.mit.edu