Surajit Sen and Ronald L. Woodfin
Land mines are scattered across many countries. Most of these countries are poor and developing countries with meager resources to develop technologically sophisticated solutions for mine detection and removal. These mines are leftovers of conflicts, both large and small. Some land mines have been in place for as much as half a century. One such nation is Egypt, where millions of mines remain from World War II. Others have been placed very recently, as in some central African nations. Most of these mines are buried in soil at depths of less than 15 centimeters. The removal of these land mines is a mandatory requirement for using the affected land. Until they are removed, people are in danger and millions of acres of potentially productive land lies fallow and/or unavailable for grazing. Thousands of people including children are killed or injured each year by these mines.
Mines are difficult to detect and remove. De-mining is mostly a manual process. Metal detection and hand prodding remain the widely used approaches for locating mines. We still lack technological expertise when it comes to low risk, non-invasive, stand-off detection of mines. At the current rate of removal, several centuries will elapse before minefields become usable. The Ottawa Treaty of 1997 designed to ban the introduction of new mines has no effect on the existing minefields. Some nations (e.g., the US) have not joined the Treaty; furthermore, the absence of inspections and enforcement makes the possibility of violations rather likely.
Any nation that has a land mine problem has already been disrupted to its core by conflicts that led to the placement of mines in the first place. The presence of mines leads to loss of agriculture, as well as infrastructure such as roads, bridges and sanitation systems. Often there has been displacement of population with related problems of unemployed and homelessness. While these structural problems are visible and quantifiable, the social effects of the tragedy inflicted by land mines in the post-war period upon these citizens who are trying to reconstruct their lives cannot be measured and are seldom publicized.
In this article, we discuss the magnitude of the mine infestation problem and attempt an assessment of the state of mine detection technologies that are currently under development or are already available.
2. Mines, Mines and Mines ...
The number of land mines that need to be cleared to restore land for civilian usage is not exactly known. There are more than 750 varieties of known land mines. The US GAO and the International Campaign to Band Land mines estimate that there are some 127 million land mines that must be neutralized in as many as 88 countries. Some of the heavily mined nations with estimates of mines in parentheses in alphabetical order are: Afghanistan (~ 107), Angola (1.5x107), Bosnia-Herzegovina (3x106), Cambodia (6x106), China (107), Croatia (3x106), Egypt (2.3x107), Eritrea (106), Ethiopia (0.5x106), Iran (1.6x107), Iraq (Kurdistan) (107), Mozambique (3x106), Rwanda (0.25x106), Somalia (106), Sudan (106), Ukraine (106), Vietnam (3.5x106).
3. Mine Casualty Data
Our understanding of the mechanical and electrical
properties of complex granular materials such as soil is limited (Bonner
et al. 2001, Liu and Nagel 1993, Rogers and Don 1994, Sinkovits and
Sen 1995, Muir 1954, Hoekstra and Delaney 1974, Wang and Schmugge
1980, Campbell 1990, Wensink 1993). High resolution imaging of shallow
buried objects in soil remains an unresolved problem. It is not a
surprise that small AP mines are most difficult to detect using available
technologies. As stated above, most humanitarian de-mining operations
rely upon the use of metal detectors and hand prodding. De-mining
operations occasionally employ specially trained dogs to sniff out
explosives. Besides, there are at least 20 different kinds of technologies
specifically aimed at detecting buried mines that are currently either
under development or are potentially available. However, all of these
technologies have their limitations and none of them can be used alone
as a reliable mine detection tool. Further, de-mining is not only
about digging out mines (King 1998). It also includes detection of
ground based trip wires and of clearing vegetation and other elements
that can potentially render many technologically sound methods practically
useless (King 1998).
5. The Global Budget for Humanitarian De-mining
According to ICBL, the total investment (including
equipment purchase, maintenance, salaries, R&D, etc.) on "humanitarian
mine action" in 1999 was $211 million. However, this amount is
meager when one considers the overall cost of de-mining, some $1-2
million/sq. km (Trevelyan 1998). The stated amount includes the costs
of operating an overall de-mining program in a typical third world
environment. Hence, it would be incorrect to associate the $211 million
figure with resources available for developing improved approaches
to the problem of mine detection, deactivation and certification.
6. A Brief Survey of the Technologies
There are some eleven distinct technologies that
are based upon sending electromagnetic energy into soil for mine detection.2
There are four technologies that are based upon reflecting electromagnetic
energy off the mine. These technologies radar, light detection and
ranging (LIDAR), Terahertz imaging and X-ray backscatter. There are
two technologies that rely on detecting an electromagnetic field.
These technologies are a conductivity/resistivity based approach and
metal detectors. In addition there are five different technologies
that somehow react with the explosive contained in the mine. These
technologies are electromagnetic radiography, gamma ray imaging, microwave
enhanced infrared, quadrupole resonance and X-ray fluorescence.
Mine detection using electromagnetic radiation is
based on the difference between the electromagnetic properties of
the target and the ground. We first mention the approaches that rely
upon the reflection of electromagnetic energy off the buried mine.
Usually, shorter wavelengths that afford higher resolutions attenuate
rapidly in soil. The strength of each technology relies upon penetrability
versus resolution for specific soil conditions. The radar-based technology
relies on the microwave part of the spectrum and hence can penetrate
some distance into the soil. However, because of the rather large
wavelength, ground penetrating radars offer limited spatial resolution.
They are also unable to penetrate water-saturated soils. The LIDAR,
terahertz imaging and X-ray backscatter approaches use shorter wavelengths
and hence suffer from significant limitations in soil penetration
(typically a few centimeters).
Among the electromagnetic radiation based approaches
that involve interaction with the explosives, the one based on quadrupole
resonance appears to hold promise. Many of these approaches do not
have the drawback of getting too many false positives due to clutter
and debris content of the soil. The quadrupole resonance approach
is already used to detect explosives at the airports. In this technique
a long wavelength pulse causes nitrogen nuclei to emit a pulse of
energy that is characteristic of the molecule (e.g., nitrogen in TNT
emits a unique pulse). The primary limitation of the quadrupole resonance
approach is that the detector head must be very close to the target
and the procedure is slow. In addition, it may not be easy to identify
the signatures from specific suspect molecules. Quadrupole resonance
is a mature technique and the Naval Research Laboratory has played
a major role in developing this approach. Electromagnetic radiography
scans the ground with long wavelength microwaves and excites target
molecules at certain atomic levels, which in turn results in a spectrographic
signature of the target substance. The electromagnetic radiography
approach appears to be in a relatively early stage of development.
In the microwave enhanced infra-red approach, the thermal signature
and infra-red spectra of chemical explosives can be detected. One
limitation of this approach is that it cannot detect metallic mines
because microwave energy cannot penetrate metal. In addition, the
speed and standoff distance at which this method can operate are concerns.
In illuminating the ground with X-rays, one causes a series of changes
in the electron configuration of the target atoms that results in
X-ray fluorescence. This approach detects molecules of explosives
that are emitted from the mine and the amount of fluorescence depends
upon the target molecule. Standoff and penetration remain serious
issues in the application of this technology to mine detection. Finally,
gamma-ray imaging is a fifth technology being explored under this
category. In this approach, an electron accelerator produces gamma
rays that interact with the chemical elements in the explosives to
generate a unique signature. Due to the short wavelength of this approach,
proximity to the target is essential.
In the category of approaches that detect electromagnetic
fields, metal detectors are most widely used for de-mining. These
detectors generate a magnetic field that reacts with the electrical
or magnetic properties of the target. This reaction causes the generation
of a second magnetic field, which is received by the detector. Metal
detectors are not very reliable when detecting low metal mines and
must be operated at close range. In the conductivity/resistivity based
approach, a current is applied to the ground using a set of electrodes.
Then the voltage is measured between various other sets of planted
electrodes. The voltage measured is affected by objects in the ground
including landmines. This technique was originally developed to locate
minerals, oil deposits and groundwater supplies. The need to place
the electrodes in or near the ground is a concern for landmine detection.
In addition to the eleven technologies referred to
above, there are four passive electromagnetic technologies that do
not actively illuminate the targets but are based on detecting energy
emitted or reflected by the mines. These technologies spot either
a contrast between the energy emitted or reflected from the mine and
that of the background or the contrast between the disturbed soil
immediately surrounding the mine and the top layer of the soil. Infra-red,
millimeter wave and microwave based technologies typically provide
good stand-off. Multispectral infra-red approaches gather information
in several infra-red wavelength bands at the same time. These approaches
are, however, strongly sensitive to temperature variations during
the day. A fourth passive approach that detects energy produced by
the circuitry in advanced mines that contain sophisticated fuses is
also under development.
Acoustics Based Approaches
A long history of theoretical and experimental work
dating back to the 1950s shows that a mine sized object in soil causes
persistent measurable changes in the local elastic properties of the
ground, which can be detected by acoustic probing.
The acoustics based attempts to mine detection fall
into three categories, "ground sonars," i.e., Rayleigh wave based
forward propagation and echo technique, low frequency (typically in
the range between 150 and 300 Hz or so) resonance based attempt in
which a selected low frequency is transmitted such that it resonates
with the natural vibration of the soil-shell interface of a buried
compliant object and impulse backscattering based approach, in which
signals are sent through the granular contacts for directly imaging
buried metallic and non-metallic objects using backscattered signals.
In the ground sonar approach, the shallow depths
of soil (meaning a floppy three dimensional network of air channels
and soil grains) is insonified with low frequency vibration pulses.
This can be accomplished by using speakers. The buried mines, which
possess mechanical impedance contrasts relative to the undisturbed
soil, generate backscattered waves that reach the surface. Eventually,
the entire soil column above the object will be set into vibration.
The surface vibrations can be sensed by a spatially distributed array
of sensors/receivers or via more sophisticated analyses such as one
involving how light rays incident on a vibrating surface will get
scattered as in laser Doppler vibrometry. The measurements are typically
done in "near-field," meaning within a few centimeters from the target.
Typical depths that can be probed by this technique do not exceed
15 cms. A different approach currently under investigation, proposes
to send two differing frequencies from a transmitter and bounce them
off a buried object such that the difference frequency is received
by a receiver. The key idea is that the frequency difference can be
crafted in such a way that a specific material of known geometry can
respond to the transmitted signal while other objects would not. The
method has the potential to discriminate different materials in soil.
However, it is not appropriate for imaging and its usefulness in de-mining
operations is unclear. The propagation of mechanical impulses in soil
exhibit very different behavior compared to sound propagation. Unlike
sound propagation in soil, which disperses as it travels horizontally
or vertically through soil, impulses travel as weakly dispersive energy
bundles. The velocity of an impulse depends upon the amplitude of
the impulse and impulses backscatter efficiently from any object that
possesses a density contrast with respect to that of the soil grains.
The backscattered signals can be received at the surface using appropriate
ground contact sensors, which in turn can allow one to reconstruct
an image of the buried object.
Explosives in mines possess a much higher concentration
of nitrogen and hydrogen than in naturally occurring chemicals. In
this approach, a continuous or pulsed neutron source that emits bursts
of neutrons in sent into the ground. A detector is used to characterize
the outgoing radiation, which are predominantly gamma rays that result
from interactions of neutrons with soil and substances such as explosives.
The main limitation of the neutron activation approach is that it
cannot be used in stand off mode. The neutron source and detector
must be directly above the target. It is also unclear as to how deep
the neutrons can penetrate and as to whether the approach would be
capable of detecting small antipersonnel mines. The neutron activation
detector is likely to be used as a confirmatory detector.
All biological systems such as mammals and insects
exploit the possibility of direct sensing of explosive compounds.
This is, of course, the most direct route to exploring whether an
object is an explosive and hence potentially dangerous. The commonly
encountered difficulty in biological systems concerns translating
relevant information from the dog, rat, bee or some other animal to
the de-miner. Dogs are perhaps more reliable than others and are used
routinely in de-mining operations. However, even with meticulous training
and significant experience the information flow from the dog to the
de-miner is not perfect. In addition, biological systems are very
different than machines. The animals must be kept healthy, have fixed
duty cycles and efforts must be made to keep them undistracted.
There have been several attempts to artificially
accomplish the detection of explosive molecules by analyzing air samples
in the vicinity of explosives. These attempts have exploited three
distinct themes, the surface acoustic wave (SAW) devices, chemical
resistor devices and ion-mobility spectroscopy. The SAW devices capture
samples of the materials being sought and classify them by molecular
mass. These devices capture the molecules of interest on a membrane.
The membranes vibrational response spectrum is altered by the
captured molecules. Appropriate signal processing techniques allow
classification into molecular groups, from which the identification
follows. The chemical resistor devices capture samples and classify
the samples based upon how they affect the resistivity of the sampling
probe. These devices are able to distinguish between closely related
molecules with considerable precision. However, both the SAW devices
and the chemical resistor devices need a substantial amount of any
sample for reliable performance. In ion mobility spectroscopy, the
samples are classified according to molecular mass, size and shape
as all of these characteristics affect the drag forces on a molecule
in a moving stream of gas. All of these chemical sensors can potentially
be sensitive devices for mine detection. However, it is necessary
to miniaturize these devices appropriately and improve their sensitivities
for use in the context of de-mining. Some of that work is currently
In conclusion, the issue of automated detection of
land mines in various kinds of soils and terrains remains an outstanding
challenge to scientists and engineers. In many ways, this challenge
is related to the fact that we still have much to learn when it comes
to describing the propagation of electrical and mechanical energy
in complex materials such as soil.
A suite of cost effective and reliable technologies
is likely to be a crucial factor in humanitarian de-mining. To this
end, a balanced collaboration between scientists, engineers, de-miners
and social scientists is required. Humanitarian de-mining, a subject
of great importance and enormous complexity, could profit from such
We are indebted to Professor Sharmistha Bagchi-Sen,
Dr. Irving Lerch and Dr. Andrew M. Sessler for their comments on this
article. We acknowledge Professor Bernd Crasemann and Professor Daniel
C. Mattis for their interest in this work. SS acknowledges the National
Science Foundation grant NSF-CMS-0070055 for research support.
Surajit Sen, Ph.D
Associate Professor, Department of Physics
University at Buffalo - The State University of New York
Buffalo, New York 14260-1500
(716) 645 6314, Fax: (716) 645 2507
Ronald L. Woodfin, Ph.D
P.O. Box 55, Sandia Park, New Mexico 87047
(505) 281 2702, Fax: (505) 294 9282
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