Finding Mines by Using our Minds
High-Tech Detection of Buried Land Mines. By
Carey Rappaport
Across the sands of Kuwait, an estimated seven
million land mines were sown-and subsequently abandoned-by both Iraqi and
coalition forces during the five months leading up to the Gulf War conflict.
Since then 4,000 munitions experts from six nations, supported by nearly
$1 billion in cleanup contracts offered by Kuwait,
have been moving slowly across the sand, an inch at a time, unearthing
and disarming everything they encounter . . . 'We all tell the same joke.
It goes: How do you eat an elephant? . . . One bite at a time.'
This passage, from Donovan Webster's book, Aftermath:
The Remnants of War, sheds some light on the enormity of the problem of
demining. All told there may be as many as 165 million buried and as yet
undetected land mines throughout the world. Even in wealthier countries
like Kuwait, where the best mine detectors are used, the demining process
is extremely dangerous and excruciatingly slow. In less-developed countries
like Cambodia, Mozambique, and Guatemala (where recent hurricanes have
pushed long-forgotten mines closer to the surface), deminers use knives
and sticks to poke the ground in hopes of finding a mine without setting
it off.
The best mine detectors, used in U.S. Army countermine
operations and by humanitarian demining efforts that can afford them, are
based on fifty-year-old technology comparable to metal detectors used by
beachcombers. There is tremendous potential for improving the reliability
of mine detection by employing state-of-the-art sensors, computer modeling,
and signal processing, combined with the phenomenal recent advances in
computer power.
Northeastern is heading a five-year, $5 million
Multidisciplinary University Research Initiative in demining, supported
by the Army Research Office. This effort involves seven other universities
and small businesses and integrates fields like electromagnetics, acoustics,
optics, signal and image processing, and mathematics. We are designing
sensors that measure many of the physical characteristics of soil with
and without mines, and we are developing carefully tailored signal processing
algorithms that indicate when a mine is present. The ultimate goal of the
humanitarian demining effort is to use these tools to help people around
the world clear hazardous land at a cost affordable even to developing
countries.
The difficulty with finding buried mines is that
there is neither a single kind of "target" nor one type of surrounding
background. In addition, the soil around mines often strongly absorbs electromagnetic
radiation and may also contain buried rocks, moisture pockets, tree roots,
and bits of metal scrap (especially in former battlefields). A rough soil
surface may randomly scatter much of the transmitted sensing signal and
can be a significant source of electromagnetic "noise."
Conventional radar is usually inadequate for detecting
land mines. Radar can spot a fast-moving target, such as an airplane, at
a great distance, but usually the plane is made of metal and is flying
through air-a medium largely transparent to radar. Stealth aircraft, with
their nonmetallic radar-absorbing skin, are hard to see with radar, as
are submarines, which are surrounded by water, a medium that hinders radar
signals. The large metallic antitank mines, one foot in diameter, are relatively
easy to find with radar, but the three-inch antipersonnel mines may have
as little as half a gram in total metal content (about as much as a small
BB). In sum, the need to find all sorts of targets, at depths varying from
just below the surface to four inches deep, in all terrains, from desert
to rice paddy to rain forest, makes mine detection a challenge.
Although Northeastern's research
applies new science, devices, and algorithms to the demining problem, we
are convinced that no single technology will be able to find each of the
647 identified types of land mines in all of the innumerable types of soil
backgrounds. Instead, the N.U. team is working on both developing new sensing
systems and finding the most effective and least expensive combination
of sensing modalities, with effort spent on integrating the information
from the various sensors. Just as we can tell a fine spring day by using
our eyes to see flowers blossoming, our ears to hear birds sing, our sense
of touch to feel the warmth of the sun, and even our noses to smell fresh
air, an integrated multisensor system will perform better than the sum
of its parts.
One of our major research projects is to improve
the detection capability of ground-penetrating radar by using multiple
transmitters and receivers to simultaneously focus many waves on points
under the surface of earth. By "looking" from different angles
at the same time, it is possible to emphasize the features of a mine-even
a nonmetallic one-and minimize the clutter of a rough ground surface and
uneven intervening soil. For us to accomplish this focusing, it is important
to know how radar waves behave in real soil. Part of our project is to
use computers to simulate and display the way waves move, bounce, slow
down, and get absorbed.
Another exciting research area is the novel use
of sound waves to "listen" to echoes from buried mines. In deserts,
where the sandy soil is usually quite dry, nonmetallic mines appear very
much like the background in electromagnetic terms, and so are hard to pick
out. Northeastern's demining team is using sonar rather than radar to find
solid objects embedded in loose soil. One major difficulty with ground-penetrating
sonar is getting sound waves into and out of the soil. While medical ultrasound
devices can be pressed firmly onto a patient's skin, deminers consider
it bad form to push detectors hard against the ground in a minefield. Since
direct contact is not feasible, we have developed an alternative: using
a laser beam to quickly heat the earth, causing rapid expansion, which
in turn generates a sonic shock wave. The scattered signals are detected
either with a microphone placed very close to the ground, or interferometrically,
using another laser beam to amplify the ground motion.
Infrared (IR) detection is another sensing modality
that has been used to find land mines. By measuring the subtle temperature
changes from solar heating between soil that contains a mine and soil that
doesn't, deminers can sometimes find shallowly buried mines. The N.U. team
puts a new spin on IR thermography by simulating the sun with microwave
heating. While our microwave deeply heats a small area of soil, we take
IR pictures of the soil patch and look for unusual heating patterns. Instead
of being limited to sunrise and sunset (when solar heating changes are
most noticeable), our system can be used all day.
In designing each of our sensors, our emphasis
is to aggregate as much of the available information as possible: use many
different probing signals, with differing frequencies; use the widest possible
view, with many inexpensive sensors coupled together; use the information
gained from mine-free terrain and from terrain known to contain mines to
improve confidence; take advantage of the fact that mines are stationary,
while detectors can move; and above all, keep the advanced technology simple
in principle, so that it can detect mines in dirty, frozen, or hard-to-traverse
terrain. After all, whatever is developed must be used by poorly paid,
minimally trained men and women in countries where a microwave oven costs
an average year's wages.
The global scourge of land mines is here for a
long time. We and others are making progress at finding better ways to
locate mines before some farmers' kids step on them. Again from Donovan
Webster's book: " 'How many [prosthetic] legs will be made this year?'
I ask the doctor at the Vietnamese hospital . . . 'That's hard to know,
this year easily thousands.' "
Carey M. Rappaport is an associate professor
of electrical and computer engineering and the associate director of the
Center for Electromagnetics Research.
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