THERMAL IMAGING FOR LANDMINE DETECTION
L.J. Carter, M.J. O’Sullivan, Y.J. Hung, and J.C-C. Teng
University of Auckland, New Zealand
ABSTRACT
EXPERIMENTAL PROCEDURE
A thermal imaging technique for landmine detection is
described. The technique uses microwave radiation and
infrared detection, and works best for soil moisture
levels of around 5%. Thermal images obtained in the
laboratory show that the technique can be used to locate
buried landmines and completely nonmetallic minelike
objects.
MOISTURE
DETECTION
AS
A
BASIS
FOR
MINE
A novel technique for landmine detection has been
developed at the University of Auckland [1]. This
presumes a difference in moisture content between the
mine, which would be likely to be dry, and the
surrounding soil, which (especially in the case of
agricultural land) would be likely to have some
moisture content. The ground is irradiated from above
by microwave energy, producing a small warming of
the surface layer. The resulting heat flows in the
vicinity of the mine give rise to a cooler spot on the
surface above the mine, which allows an infixed device
to detect the position of the mine. This paper gives
some results of recent study of this technique, using an
infrared camera to produce thermal images of the
ground surface after irradiation.
COMPUTER SIMULATION
Reference 1 gives a simple analysis of the heat flow in
irradiated ground containing a mine-like object. A more
complete understanding may be obtained by a threedimensional numerical simulation. The physical
processes which determine the temperature signature in
irradiated soil near a landmine are heat and mass
transfer (air, water vapour and liquid water) in a porous
material (the soil matrix). This is a small scale and low
temperature version of the processes which are
important in a geothermal reservoir. At the University
of Auckland there is a research group with considerable
experience in developing and using computer
simulators (MULKOM/TOUGH2 [2]) to model the
behaviour of geothermal fields and they have carried
out some preliminary simulation studies on the heat
flow near a landmine in irradiated soil. To test the
method a cylindrically symmetric grid (see Fig. 1) was
set up with the landmine placed on the axis of
symmetry. The results so far are encouraging but more
work is required testing various assumptions about the
amount of heat generated by irradiation, the magnitudes
of thermal parameters and the top air/soil interface
conditions.
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The experiments were all of the same general form. The
mine, or mine-like object, was buried in sandy soil in a
round metal container with open top. This was placed in
a commercial 700 W microwave oven and subjected to
a short period of irradiation. The container was then
removed from the oven and placed under an infrared
camera; and the camera output was recorded over an
ensuing period of up to twenty minutes. Figure 2 shows
the experimental setup. The parameters studied
included soil moisture content, mine type, irradiation
duration, and mine depth.
RESULTS
No mine
Figure 3a shows an image in the visible spectrum of the
surface of the circular container of sand, after
irradiation for 90 seconds, but with no object buried.
Figure 3b shows a thermal image of the same surface.
Shadings of grey across the surface correspond to small
temperature variations caused by some unevenness in
the microwave illumination of the soil.
Soil moisture
Soil samples with moisture contents by weight in the
range 1% to 10% were prepared. For each sample a
non-metallic mine-like object consisting of a small
plastic container (50 mm diameter) filled with
petroleum jelly was buried in the soil at a standard
depth. Each sample was irradiated for 90 seconds, and
the resulting thermal images were examined. It was
found that there was an optimum soil moisture content
at about 5%. This gave the best contrast between the
‘cold spot‘ and the surrounding soil. Images were
obtained for drier and wetter soils, but the contrast was
poorer. For very dry soils, there would be a smaller
moisture-contrastbetween the mine and the surrounding
soil; and for very wet soils, more microwave energy is
reflected at the ground surface and so there is a smaller
degree of heating produced deep in the soil. The
following results were produced using sandy soil with
5% moisture content.
Mine-like object
Most antipersonnel mines are buried at depths between
2 and 4 cm. The non-metallic minelike object was
buried at a depth of 3.5 cm and subjected to microwave
irradiation of 90 seconds. A thermal image developed
“Detection of abandoned land mines”, 12-14 October 1998, Conference Publication No. 458 0 IEE 1998
after a few minutes, and then persisted for some time.
Figure 4 shows the thermal image at 17 minutes and 27
seconds after the end of irradiation, shortly before the
'mine' was excavated. The pale patch (blue in the slides)
corresponds to an area of significantly lower
temperature than the rest. This may be compared with
Figure 5 , which shows the thermal image immediately
after removal of the 'mine'. The position of the mine at
about 'four o'clock' is clearly shown by the circular
image of the hole from which the mine has been
extracted. The light grey colouration shows that the
ground under the mine was warmer than that above the
mine. It should be noted that in all of these experiments,
the view of the surface in the visible spectrum showed a
uniform sandy surface, with no clue as to the location
of the buried object.
thermal barrier to heat rising in consequence to the
surface. The thermal im,age is therefore a kind of
'shadow' of the mine, presented at the surface.
Whether this technique could be useful in the extreme
variability of conditions found in a real minefield
remains to be shown, perhaps by the development of a
field prototype. Further work remains to be done in this
area.
CONCLUSIONS
A thermal imaging technique using microwave
radiation and infrared detection has been demonstrated.
The technique works best for soil moisture levels of
around 5%. It has been used successfully in the
laboratory to locate burield landmines and completely
nonmetallic minelike objects. The technique is quite
slow, but may nevertheless be useful in the field.
Type 72 landmine
An inactive Chinese type 72 antipersonnel mine was
buried at a depth of 3 cm and subjected to 90 s
irradiation. Figure 6 shows the thermal image shortly
before excavation, indicating that the mine should be
found in a position between 12 and 1 o'clock. The
accuracy of this conclusion is illustrated in Figure 7,
which shows the circular hole left after removing the
mine. Some cool sand dug out from above the mine
shows pale on the surface, and the light grey in the hole
shows again that the soil under the mine was warmer
than that above it.
REFERENCES
1.
Carter, L.J., Bryaint, G.H.B., Le Fevre, M., and
Wong, W.C., "Moisture and landmine detection",
Proceedings of EUREL/IEE International Conference
Edinburgh, IEE Conference Publication Number 43 1,
83-87, 1996.
2.
Pruess, K., "TONJGH2 - A general purpose
numerical simulator for multiphase fluid and heat
flow", Lawrence Berkeel29400, Berkeley, California, August, 1991.
Type 72 mine, 30s irradiation
The experiment was repeated, this time with the mine
buried at a depth of 1.5 cm, and subjected to microwave
irradiation for only 30 seconds. Figure 8 shows the
resulting thermal image, and Figure 9 confirms the
mine's position, this time at almost six o'clock. There is
good correspondence between the image and the mine's
location.
DISCUSSION
The results indicate that it is possible to find buried and
completely non-metallic objects by this method. There
are some clear limitations, already identified in
reference 1:
Quite slow: the image forms in minutes;
Needs, ideally, a soil moisture of around 5%;
Needs quite a large amount of
probably about 1 kW.
power:
As already discussed in [ 11, none of these is necessarily
insuperable. The speed of the technique should be
compared with manual probing, which is itself very
slow and also dangerous.
The warmer soil below the mine suggests a possible
mechanism for the formation of the cold spot. The
microwave energy may be producing warming at depths
of 10 cm or more; and the mine may be acting as a
111
Figure 1:Grid used for computer simulation
Figure 2 Experimental setup
112
Figure 3b: Thermall image after irradiation, n o
buried object
Figure 3a: Sand container in visible spectrum
Notes:
.
The original colour images have
been converted here into grayscale. In the colour
version of these images (presentation slides) red = hot,
blue = cold, and shades of grey represent intermediate
temperatures, lighter being warmer, and blacker being
cooler. Actual temperature calibration varies slightly
from image to image, but as a general guide, take red
(hottest) to be about 40 degrees C, and blue (coldest) to
be about 20 degrees C.
.
The large grey circular object is the
round container of sand, which has been warmed to
more than room temperature.
.
The background (blue in the slides)
is the surface on which the sand container rests, and is
at room temperature.
.
The smaller patch (blue in the slides)
within the grey circular area in Figures 4, 6 and 8
corresponds to a cooler spot above the buried object.
113
--____--
Figure 4: Cold sput above buried mine-like object
Figure 5: Showin
Figure 6: Cold spot above Type 72
antipersonnel mine
Figure 7 Showing location of T*e
Figure 8: Type 72 mine, 30s irradiation
Figure 9: Confirming location of Type 72 mine
114
72 mine