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1
Anti-Personnel Landmine Detection Based on
GPR and IR Imaging: A Review
Alauddin Bhuiyan and Baikunth Nath
Computer Science and Software Engineering,
The University of Melbourne, Melbourne, Australia 3010
Abstract: Ground penetrating radar (GPR) and Infrared (IR) camera have become two established
sensors for detecting buried anti-personnel mines (APM) which contain no or a little metal. This report
reviews the detection techniques of APM using GPR and IR, and describes particular situations where
each technique is feasible. We provide an analysis for fusion based detection and classification of APM.
We discuss the GPR and IR data acquisition, signal processing and image processing methods. We also
include a comparative study of these two sensors with respect to signal processing and target detection
procedures. The report discusses the strengths and weaknesses of each of the sensors based on data
capturing efficiency, overcoming environmental difficulties and sensor technology. Finally, we
emphasize that a geometrical feature based sensor fusion, combining GPR and IR, for detection and
classification of APM may be the most effective technique.
Keywords: Ground Penetrating Radar, Infrared Imaging, Mine Detection, Sensor Fusion
1. Introduction
Landmines are causing enormous problems (for example, they endanger life and make the land
uninhabitable) in a large number of areas throughout the world. Based on the estimation of International
Red Cross, there are about ten billion mines buried in eighty countries [1]. Detecting and classifying
minimum-metal/non-metal mines buried in soil offers considerable challenges. The insidious nature of
mines has stimulated significant research – spanning over more than half a century – on techniques for
detecting and identifying the buried-mines [2]. The most significant tools used for mine detection are
GPR and IR camera.
There are mainly two types of landmines, anti-tank mines (ATM) and anti-personnel mines (APM), which
kill or maim people around the world. ATM is usually large in size and contains metal, so it can be
detected by conventional metal detectors. APM on the other hand is smaller in size (60-120mm in
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diameter and 40-70mm in thickness) (Fig.1). APM are mostly made of plastic making them more difficult
to detect since the dielectric properties of their plastic content are similar to soil. For this reason, the
reflected signal from the mine is usually weak and is masked by its background.
Most mine detection techniques, based on sensor data and image, consist of signal processing, image
processing and decision processes. In this report, we will focus our discussion on GPR and IR data
acquisition, signal processing and image processing techniques and classification methods for APM. We
provide a comparison between the two techniques for justifying their fusion. The proposed GPR and IR
geometrical feature level fusion is likely to provide better detection and classification of APM.
Fig.1. Typical Anti-personnel mines OZM-4 (left) and PMD-6 (right)
In recent studies [3-6], single/hybrid sensors have achieved significant improvement on APM detection.
However, there are limitations on soil conditions, ground penetration depth, false alarm rate, classification
and orientation of the mine. Classification and orientation of the buried mine is very important for safe
and efficient demining. To determine correct classification and orientation we must require better imaging
technique based on GPR and IR sensors.
For optimum performance, an understanding of the limitations of the sensors induced by the environment
like soil type, soil structure, humidity, temperature and vegetation is necessary. Signal processing is the
most essential part for GPR data as is the image preprocessing for IR. For a proper analysis of the image
to help classify the target, understanding of image processing is essential for both GPR and IR techniques.
We discuss particular situations where the two techniques can be most useful and emphasize that no
single sensor has the potential to increase mine detection probability and decrease the false alarm rate for
all types of mines. Here, we propose a multi-sensor fusion system framework to combine both GPR and
IR techniques. This proposed system is likely to decrease the mine detection time while maintaining high
probability of mine detection.
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This rest of this article is organized as follows. In Section 2, GPR based APM detection techniques are
described. The use of IR imaging for APM detection is given in Section 3. The two techniques are
compared in Section 4 to provide the reasons for proposing sensor fusion. In Section 5, we review some
of the existing sensor fusion techniques, and provide geometrical feature based sensor fusion model.
Finally, conclusions on the fusion model are drawn in Section 6.
2. Ground Penetrating Radar Techniques
In this Section we describe the GPR system, GPR data acquisition method and data processing
techniques. Using GPR data we construct image for use in detecting APM. We also discuss some of the
signal processing techniques on GPR data, and image processing for enhancing the constructed image to
facilitate target detection.
2.1. GPR systems
GPR consists of an active sensor, which emits electromagnetic (EM) waves through a wideband antenna
(Transmitter) and collects signals reflected from its surroundings (Receiver). The commonly used
frequency band of the GPR, EM wave is between 100 MHz and 100 GHz [7]. However, for APM detection
a center frequency of 1 to 2 GHz seem to be a good choice for most types of soil [8]. GPR imaging from
data is based on three most important parameters which are the wave speed, polarization and amplitude of
the propagating electromagnetic wave field. The height of the antenna above the surface is also the key
factor, increasing the height influences the image quality negatively as the spatial resolution is decreased
[9].
There are two distinct types of GPR, time-domain and frequency-domain. Time domain GPR is further
divided into i) Impulse System GPR and ii) Ultra Wide Band (UWB) GPR. Impulse GPR emits a pulse
with a carrier frequency, modulated by a square envelope. Ultra Wide Band GPR emits a pulse without a
carrier frequency; this way the spectral band of the emitted energy is very large. Frequency domain GPR
is also divided into two categories such as Linear Sweep and Stepped Frequency. The frequency domain
GPR transmits a signal with a changing carrier frequency over a chosen frequency range. This carrier
frequency can be changed, either continuously (Linear Sweep), or with a fixed step (Stepped Frequency).
GPR provides information on both the existence and location of mines. The presence of an object is
detected by checking for interruption through the round trip path of the signal. While propagating from
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the transmitter towards a buried object and being scattered back to the receiver, the electromagnetic
waves of the GPR are subject to some losses. In particular, the deeper the object is buried, the higher the
losses introduced by the soil are. In order to compensate for these attenuations in function of R or, more
directly, of time t, a time varying gain (TVG) is introduced, by which a fixed gain of X dBs/s (/m) is
added to the raw signal s, so the amplified signal in time domain is [10]:
STVG(t) = s(t)10Xt/20
(1)
The distance between the sensor and an object is measured by using the time delay, �t, between the
emitting and receiving moments of the signal as
r = 2
v �t (2)
where v represents the velocity of EM wave in the medium, and r the distance of the object from the GPR
sensor. Since many parameters of EM waves, including the velocity, vary according to the content of soil,
soil parameter should be estimated prior to taking the measurement [7].
GPR detects any object below the soil surface if it differs from the surrounding medium in the
conductivity (metallic targets), the permittivity or the dielectric constant (plastic and non-conducting
targets), or the permeability (ferrous metals) [10]. The penetration depth of the wave into the soil usually
depends on two factors, the humidity of the soil and the wavelength of EM wave. The content of water in
the soil significantly reduces the depth of penetration of a wave with a relatively shorter wavelength.
Based on the reflection and penetration properties, GPR works best with low frequency EM waves in dry
sand. low frequency signals, however, tend to make low resolution maps of data, which decreases the
accuracy of mine detection [2].
2.2. GPR data acquisition
The data is generally acquired with a handheld or vehicle mounted GPR scanner by moving it along a line
over the places where the mine is buried. The scanner is moved with fairly constant velocity and traces are
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continuously recorded. The number of traces depends on the area of the minefield or the region and a
constant inline step needs to be maintained (average 1-2 cm). The positioning of the radar system is
crucial as the distortion of the image takes place due to positioning error. Although, there are some recent
developments of sensors which are detecting mines without imaging, we have mentioned earlier that
imaging is compulsory if we want to consider the orientation of the mine. The automated data acquisition
frame can be used to reduce positioning data error GPR antennas operate in transmitter and receiver pairs
to cover a wide search area [11].
A, B and C-scans: GPR data can be represented in three different forms, A-scan, B-scan, and C- scan,
according to the number of scanning dimensions. It can be defined with the 3D coordinate system, where
the x y-plane represents the ground surface and the z-axis represents the direction into (depth) the ground.
The A-scan signal is obtained by a stationary measurement after placing an antenna above a specific
position. The collected signal is presented in the form of a group of signal strength versus time delay. A-
scanned signal is a 1D signal. B-scan signal is obtained as the horizontal collection from the ensemble of
A-scans. The collected 2D signal is presented as intensity on the plane as scan width versus time delay. C-
scan signal is obtained from the ensemble of B-scans, measured by repeated line scans along the plane.
The collected C-scanned signal forms a 3-D signal [12]. In the 3-D coordinate system (Fig.2), the x and y
axes respectively represent the horizontal and the vertical positions in the plane of the target, and the z-
axis represents the depth of the target.
2.3. Preprocessing
The acquired data usually has some clutter or noise due to vegetation (e.g. grass, bushes) and the local
variations in the material in the ground. It is necessary to remove the clutter to enhance the quality of the
data. We can distinguish three kinds of clutter, which can be approached in a different way [13]. The first
type of clutter involves everything appearing on and beneath the ground. The interface between air –
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including the first layer of vegetation and the ground creates a second type of clutter, known as the
surface clutter. The third kind, the equipment clutter, is due to the imperfections of system used. Clearly,
this will vary a lot from one system to another. At present, the pre-processing step is performed in two
different ways: i) signal processing of the GPR raw data (A scan) and ii) construct an image using the
GPR raw data and then enhance this image.
Fig.2. A 3-D coordinate system defined on a section of ground.
2.3.1. Signal processing
The signal processing is performed by different filtering techniques and deconvolution algorithm. The
following signal processing techniques are commonly used by researchers in APM detection community.
These techniques are briefly described for completeness.
� Subtraction of the average A-scan. By calculating an average A-scan over the whole B-scan, and
subtracting this from each individual A-scan, one only succeeds in eliminating the perfectly horizontal
background clutter signals. This is the most widely used way to eliminate the problem of the air-ground
reflection although in many cases it is not very efficient [14].
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� Application of a horizontal high pass Butterworth filter. This filter reduces the different background
signals effectively, but it also modifies the diffraction hyperbola substantially, which reduces the
efficiency of this method [14].
� Application of the Discrete Wavelet transform. This transformation is applied in the horizontal
direction and the coefficients of the lowest octaves (which correspond to horizontal energy) are set to
zero, and then the inverse transform is applied [14].
� Windowed average subtraction. This is a version of the general average subtraction method. Here the
average A-scan is not calculated over the whole B-scan, but only over the A-scans within a window
around the signal being considered. This subtraction can adapt to slowly varying signals so that slightly
oblique surface reflection can be eliminated [14].
� Removing DC offset and DC offset drift. During data acquisition A-scans are suffered from DC
(Direct Current) offset or DC offset drift which are removed by applying second order polynomials [15].
� Application of ARMA model. A system identification based clutter removal algorithm using the
ARMA model that describes clutter. ARMA model basically considers abrupt changes between clutter
and target signal [16]. Figure 3 (taken from [2]) illustrates the changes in image features after clutter is
removed.
� Application of Wiener filter. An adaptive version of Wiener filter (adaptive means that it calculates its
parameters according to the data present locally in the image) records data which are in the form of A-
scans, B-scans and C-scans and can remove the noise for each of the forms [17, 18].
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� Adoption of basis function. The clutter reduction method is based on basis function decomposition of
the SF-GPR time-series from which the clutter and the signal are separated.
� Finite Impulse Response filter (FIR). FIR enhances the signal part of an A-scan and reduces the
background energy [18].
Fig.3. Parametric model background removal (left) and detected hyperbolas in mean-removed
background (right).
� Finite-Difference Time-Domain (FDTD) technique. The EM (Electromagnetic) field values of every
last time step and other variables are stored. Parallel implementation of the FDTD algorithm on a using
cluster of workstations (COW), which is a time domain method, produces data in the whole frequency
domain by performing only one calculation in time domain [19]. When resonance frequencies are used,
FDTD simulation can be applied to detect anti-personnel mine [20].
� Application of F-K filtering. F-K filter can eliminate the artifacts like high amplitude, large saddle
corners, low temporal frequencies and high spatial slopes of a frequency-wave-number domain. For GPR
signal processing, F-K filter is applied to remove artificial time shifts, gaps in coverage and variations in
signal characters [21].
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� Robust Linear Prediction (RLP) and Adaptive Shifted and Scaled (ASaS) algorithms. Both RLP and
ASaS are based on a flexible data model applicable to rough ground surface clutter removal algorithm [1].
� Karhunen-Loeve Transform (KLT). KLT can eliminate the strong GPR background and enhances
greatly the signal-to-noise ratio of the data [22].
� Deconvolution algorithm. Considering the received signal is a convolution of the impulse (frequency)
response of transmitter and receiver (which can be estimated beforehand), the deconvolution method is
applied to GPR data [23]. By applying deconvolution, the original signal is separated from the received
signal that leads to feature extraction of a mine.
The above mentioned signal processing techniques have been applied by different researchers on several
different occasions. Not all these techniques are optimized. Moreover, one technique can be most efficient
in a particular circumstance while may not be in another. In the next paragraph we state the most efficient
signal processing techniques based on their applications and image enhancement capability.
ARMA modeling provides quantifiable improvement in feature extraction applied to the detection of non-
metallic anti-personnel landmines. The Discrete Wavelet Transform and Windowed average subtraction
offer the best results in eliminating the various background signals and preserving the diffraction
hyperbola. The Wiener filter has been shown as one of the best filters for B-scan data processing as the
background and measurement noise get completely attenuated without distorting the signal itself [18].
2.3.2. GPR image enhancing
One of the popular methods of enhancing a GPR image of buried target is to use a simple filtered time-
domain back propagation algorithm. Then the inversion method is applied to retrieve more details of the
inhomogeneous characters of the buried object [24].
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A combination of opening and closing operations can remove noise and smooth the texture in an image.
This is called the alternating sequential filter (ASF). The ASF performs well with repetition rather than a
single operation. When the contrast between the background and mine target is not (usually) high enough,
the Morphological contrast enhancement and Histogram equalization algorithms may be applied [7].
The dielectric properties of plastic mines are similar to the soil and the reflection from the mine is usually
weak and masked by background. This is known an inverse scattering. To resolve inverse scattering
problem, the inversion method, reduced-order modeling technique, the forward-backward time-stepping
(FBTS) method and the gradient-based optimization method may be used [24].
2.4. GPR image processing and target classification
In order to get better detection ability and more accurate indication of the position and size of the buried
object, imaging algorithms may be applied on the preprocessed (contrast enhanced) images. For instance,
a diffraction stack is used in the spatial and time domain based on an estimation of an image value at a
certain position by stacking the data associated with appropriate arrival time for the specific location. It is
a flexible technique to handle irregularly positioned data [15].
The linearized forward scattering model is used to invert preprocessed data and obtain the three
dimensional distribution of the unknown dielectric permittivity contrast. There are several algorithms to
obtain a three-dimensional image from GPR data such as the fast multistage effective inversion algorithm,
advanced real-time algorithm and multi-component imaging algorithm [24]. We can apply Focusing
algorithm when the real target responses coherently sum together in the imaging process and the non-
target features do not focus coherently [25].
Mathematical morphology has been applied primarily for binary image processing. Basic of the first level
functions, such as dilation and erosion, are performed by structure elements such as shape and size. Gray-
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scale morphology can provide more complicated processing, such as gradient extraction, contrast
enhancement and region segmentation (watershed algorithm) as well as noise removal and smoothing
which are typical applications of binary morphology [7].
Target detection and classification: Target detection and classification algorithms can be applied after
either image enhancement or image processing algorithms or both. The APM detection and classification
is based on feature extraction, segmentation and matching algorithms. For feature extraction, Karhunen-
Loeve transformation (KLT) and Kitller-Young Transformation (KYT) have produced promising feature
extraction results in the mine detection area [7]. The amplitudes of the reflections from mines are used as
features for object classification. The final image shows a series of hyperbolic arcs from which mine
classification may be possible [26].
The Semi-automatic segmentation (cue-based analysis) is another option for mine classification where the
user defines the image information (cues) when a software system interprets these cues (such as distance,
vector, entropy, intensity and global image parameters) to generate a list of image processing functions.
These image processing functions finally enhance, segment and analyze the image for classification [17].
Target detection can also be performed in stages such as feature estimation, feature extraction, object
model formulation and final classification [18, 27, 28]. Geometrical features (dimensions, orientation and
center) based on morphological operations of dilations and erosion has also been used to maximize the
probability of detection [29]. Feature estimation step reduces the data into a feature vector which
represents the target. Feature extraction extracts out of the estimated feature vector only those features
that are pertinent to the classification problem. The object model formulation stage determines a set of
parametric or non-parametric models for each object class considered. The final classification step then
compares the feature vector extracted out of an unknown data sample, with the different class models to
determine the likelihood of being part of a mine object.
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3. Infrared Techniques
Infrared techniques based on Infrared Imaging and processing of those images to detect APM. Here, we
describe in detail about IR image acquisition method. We analyze different image pre-processing
techniques which are used for IR image enhancement. Finally, we provide a discussion on different image
processing and target detection techniques.
3.1. Infrared camera
The use of thermal IR technique is based on the thermal radiation contrast of objects, with respect to their
backgrounds. All objects at temperatures greater than absolute zero emit electromagnetic (EM) radiation
at wavelengths from 3µm to 100µm is referred to as the thermal IR radiation. The magnitude of spectral
radiations of an object depends on its temperature. For landmine detection, landmines are thought of as a
thermal barrier in the natural flow of the heat inside the soil, which produces a perturbation of the
expected thermal pattern on the surface. The detection of these perturbations (anomalies) will put into
evidence the presence of potential landmine targets [30].
Infrared camera detects infrared energy (heat) difference as low as 15mK and converts it into an
electronic signal, which is then processed to produce a thermal image on a video monitor and perform
temperature calculations [31, 32]. The IR camera consists of detectors which are transducers and convert
the energy of EM radiation into an electrical signal. There are two types of IR detectors, the photon
detector and the thermal detector. The photon detector or counter essentially measures the rate of quantum
absorption, whereas the thermal detector measures the rate of energy absorption [7].
The processing of images given by thermal scanners can be carried out in two ways. The first is the
dynamic approach where an image sequence is processed in one batch, and the second is the static
approach where the processing is performed on one individual image. Dynamic thermograph consists of
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processing a sequence of images of the same scene, submitted to heating or cooling (be it artificial or
natural), so that the scene parts follow different temperature curves in time due to the variability in their
specific characteristics (material, volume, shape). The use of thermal infrared (IR) sensors for landmine
detection has been of interest to researchers since the 1950s [30]. The Dynamic thermal IR technique has
been used since 1980s for nondestructive evaluation [33].
IR images can be captured by active or passive sensing method. The passive IR system senses only
natural radiation from the object, while the active IR system provides an extra heat source and receives
the artificial radiation created by the heat source.
The overall IR detection technique can be summarized by a sequence of steps. The first step is the
acquisition of the polarimetric IR image data. The next step is preprocessing polarimetric images for
contrast enhancement. Finally, image processing algorithms are applied to facilitate target detection and
classification.
3.2. Acquisition of IR image data
Infrared camera is often used in a vehicle based multi-sensor platform for landmine detection. It may also
be used in airborne or in a moving platform for taking images [34-36]. Polarimetric infrared camera based
on a moving platform can acquire images while moving at a constant speed and maintaining a specific
angle with the target area. An automated airborne system was introduced for IR data capturing with
GPS/INS for the camera positioning accuracy [35].
The polarimetric system is operated in a free running mode in which image sequences are acquired
continuously. The method is described as follows. Using a laser distance meter the distance traveled by
the platform along the measurement bridge is measured and recorded. Every finite distance interval the
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laser distance meter gives a trigger pulse. These trigger pulses are recorded along with the images. The
exact position of each image in the sequence is estimated using interpolation of the trigger pulses [37].
The detection performance of Infrared camera can be enhanced by means of a Polarization filter. By using
this filter not only the differences in thermal properties (for buried and surface-laid landmines) can be
observed but also surface properties (for surface-laid landmines) can be acquired [38]. Another significant
characteristics for Polarimetric IR is that within the visual and IR bands, polarization gives extra
information about objects and their surfaces [39].
Predicting the thermal signature (the temperature differences of soil surface with mine and without mine)
which is the key concept of the thermal imaging of a buried landmine requires knowledge of the heat flux
into the soil, the thermal properties of the landmine, the soil above the landmine and of the soil
surrounding the landmine. Inherent to the cyclic nature of the incoming solar radiation, the thermal
signatures of underground anomalies follow a cyclic pattern [40].
3.3. IR image pre-processing
IR polarization filters were introduced into thermal imaging systems for improving the low target-to-
clutter ratio in infrared scene [41]. However, when the contrast between the object and the background is
not adequate more processing is necessary and direct segmentation does not work. Some of the significant
image pre-processing algorithms are:
� Application of image calibration. Noise is added to the desired image because of the vibration of the
camera. Camera calibration is one of the simplest ways to reduce this noise. For each angle orientation a
cold and a warm calibration is performed. These two calibration sequences give the gain and offset for
every pixel. The offset correction removes the reflected image, the emission from the filter and the offset
of each pixel [37, 42].
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� Radiometric Calibration. Provides the radiometric calibrated images after which the image grey
values are provided in physical units: W/m2/sr (Watt per square metre per steradian). The Calibration
procedure using two blackbodies (one heated and one at ambient temperature) with temperature sensors
on two sides of IR camera, assuming the blackbodies has a uniform temperature over their full surface
and has an emission coefficient of 1. The acquired images, I(x,y), are calibrated using eq. 3, after the full
scan of the area has been performed [30].
Ic(x,y) = Ir1 + )(),(),(
),(),(12
12
1
rr
bbbb
bb IIyxIyxI
yxIyxI−
−
− (3)
With Ibb1(x,y) and Ibb2(x,y) are average blackbody images, for blackbody 1 and 2, respectively. Iri (i=1,2)
are the blackbodies radiances, estimated using planck’s equation (eq. 4):
Iri = λλ
λ
λ
λ
d
e
hc
ikT
hc
1
122
1
5
2
−
� (4)
Where, Ti (i=1,2) are the measured blackbodies temperature; �1 and �2 are respectively the lower and
upper bound of the wavelength of the camera. The resulting image radiance Ic(x,y), is directly related to
the radiance of the scene.
� Application of KLT and KYT. Karhunen-Loeve transformation (KLT) minimizes the object
representation error. It selects the main axis (maximum noise component of the variance) and applies the
first and second KLT transformations to reduce the noise. Use of Kitller and Young transformation
(KYT) leads to an improved mine image (Figure 4 adapted from [38]) by compensating the noise
component and finally have better discriminating ability for mine [43].
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� Gaussian filtering. The Gaussian filter or smoothing operator is a 2-D convolution operator that is
used to `blur' images. The pre-processing steps also include Gaussian filtering for removing noise in the
original image [44].
Fig.3. Infrared Image (left). Karhunen-Loeve Transformed Image (Center). Kittler-Young
transformed Image (right).
� Morphological filter. The morphological filter can efficiently remove noise by combining multiple
morphological operations, and can also provide more complicated processing such as gradient extraction,
contrast enhancement. Histogram equalization methods are also used for contrast enhancement [7].
� Application of Pixel fusion. Different IR intensity images taken from the same scene with different
orientations of a polarizing filter, and applying the pixel fusion for combining the polarization
information for image enhancements [41].
� Pre-whitening filter followed by a matched filter. By considering scaled 3-D Gaussian shapes of
mines, while modeling the colored noise as an auto-regressive process, detection of landmines is obtained
by applying a pre-whitening filter followed by a matched filter. The matched filter correlates the pre-
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whitened image sequence to the 3-D object model. Using the local maxima of the output position of the
mine is detected [45].
� Wavelet approach. In this denoising technique, the decision whether a coefficient originates from a
signal or noise is based on the comparison of its magnitude with a certain threshold [36].
� Autoregressive process. By separately estimating the parameters of the mine and the noise in an
alternating fashion, the background noise can be described and eliminated by autoregressive process [46].
3.4. IR image processing and target classification
Extracting mine-like shapes from the IR sensor image is a crucial task in the mine detection process. After
image pre-processing technique(s) the target/mine detection algorithms are applied.
One of the most popular methods for mine detection is Tophat filtering. Tophat filtering is a
morphological transform that is able to detect local maxima in an image within the region of a structuring
element. Detection performance can be improved by using more relevant information in the classification
process. One way to include this information is by means of features (size, shape and intensity) of regions
of interest (ROI) [37]. For target classification, Naïve Bayes classifier [47] and Novel classifier have been
used.
Hough transformation and Tophat filtering are used for feature-based detection of land mines to optimize
the target features. The feature combination methods based on the Mahalanobis distance and the Fisher
mapping have been used [48] for object classification.
Detection and classification of mines is also possible by perturbations on the thermal pattern of the soil
caused by the presence of buried objects. The procedure is based on the application of a thermal model of
18
the soil. The presence of unexpected objects is made evident by the alterations on the expected pattern,
constituting the detection stage of the procedure. Classification is possible by assuming the thermal
behavior of mine targets is driven by the thermal properties of the mine explosive [49].
The watershed transformation is used in morphological image segmentation and considered as a
topographic region growing method. The mine targets are considered as changes in the vegetation/soil
textures. Texture parameters extraction and clustering methods are used for classification of background
(vegetation, soil, etc. ) and mine-like objects [36].
4. Comparison of GPR and IR Techniques
It is clear from the above sections that collecting data and creating image from the data requires more
effort in employing GPR than IR. GPR data needs serious pre-processing at signal processing level to
create the image. While IR does not need such type of pre-processing, rather image enhancement is
necessary which also is required for GPR to perform a better detection.
Since it is easier to interpret visual images than data, IR technique has been widely used for mine
detection [7]. On the other hand, GPR data can be controlled by an operator as well as for processing and
imaging. GPR is also another popular technique for anti-personnel mine detection.
In case of GPR, for mine size effects on image; calibration of wavelength is necessary which is
proportional to mine size. This is also true for polarization; experiments have shown that circular mine
image taken with linear polarization distort the shape of the circular mine [50]. For IR only the mine size
has effect on thermal imaging, the larger the mines size the clearer the thermal signatures.
The height of the antennae above the surface has influence on the GPR image. As the height increases the
spatial resolution decreases [9]. This ultimately causes a blurred image and increases the detection
complexity. There is no such type of predicament in case of IR imaging.
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GPR data capturing is more complicated compared to IR imaging. In GPR technique, usually an operator
performs scanning by a hand held system. Both measuring conditions and operator skills pose weaknesses
for the GPR scanning method [51]. In contrast, IR imaging is highly dependent on surrounding
environment conditions. It is even get biased by operator’s body temperature, and motion and vibration of
the camera.
Air-soil interface has a significant effect on both GPR and IR data capturing. In GPR scanning, it
introduces the air-soil interface clutter [52] and in IR imaging it causes clutter due to the uneven sunlight
absorption of the surface [53]. Rough surfaces lead to distortion in signals on GPR sensing because of the
irregular surface reflection. Since IR image is based only on the temperature measurement, soil surface
irregularity is not a strong barrier for IR imaging. Moreover, a specialized method of dual-frequency
microwave-enhanced infrared thermography (MEIT) is available to minimize the clutter, if any, due to
surface irregularities [54].
Soil moisture has strong effect on GPR signals. Experiments have shown that a slight increase in water
content in soil around non-metallic landmines improves detection in sand but not in clay [55]. On the
other hand, soil moisture has significant positive effect on thermal signature, largest when moist and
lowest for dry conditions [56].
GPR can provide object detection with shape (3D structure of the buried mine) and depth information
while thermal IR gives shape features only. The additional feature for the IR imaging is that it estimates
texture of background clutter and objects [57]. Migration algorithms (both in time and frequency domain)
are used in GPR mine detection to give an idea of the exact physical position (depth information) and
shape of the reflectors in the subsurface [14]. However, our study has shown that such a technique is not
feasible in IR imaging.
20
Wind is another consideration that does not affect GPR mine detection technique but has a significant
effect on IR imaging. It has been shown that increase in wind speed decreases the strength of thermal
signature which in turn decreases the mine detection rate [40].
It is established that to detect a significant thermal signature of a mine at the surface, for most soils a
10cm is the maximum burial depth [58, 59]. For the GPR based mine detection methods there is no such
information available about the maximum burial depth. GPR high frequency signals can provide depth
information of the buried mine, but is a limitation for the IR camera technique [60].
Both GPR and IR camera techniques have some advantages and limitations and neither of them is
markedly superior to the other. It is clear that neither of them can provide effective information when
used in isolation. A hybrid model combining certain features of both the GPR and IR, we believe may
provide better performance for detection and classification of APM.
5. Fusion of GPR and IR
No single sensor technology has the capability of reaching a good detection rate while having a low false
alarm rate in all types of soil and with all types of mines. Hence it is best to use complementary sensor
technologies for mine detection and to perform an appropriate sensor fusion with them [61]. Sensor fusion
is the process in which information from different sensors is combined. This can be especially
advantageous, when sensors measure independent physical properties: weaknesses of one sensor are
compensated by inherent strengths of other sensors, resulting in good performance of the complete sensor
suite in a wide variety of conditions [62].
Three fusion architectures are defined based on the amount of processing performed on the sensor data
before fusion. These are data-level fusion, feature-level fusion and decision-level fusion [63].
21
Data-level fusion: In the data-level fusion architecture a minimal amount of processing is performed on
data from each sensor before data are combined. Processing is usually limited to calibration, alignment,
etc. which are essential to make data useful. The fusion operation in this architecture may comprise
stacking, weighted averaging or Kalman Filtering. The mine detection decisions can be made based on
the fused data or on features extracted from the fused data.
Feature-level fusion: In this architecture features extracted from the raw data of each sensor are merged
together to form a fused feature vector. Because feature vectors generally have a much lower dimension
than the corresponding raw data, the computational demand at the fusion center and the communication
bandwidth requirements can be significantly lower than those in the data-level fusion. The penalty,
however, is possibly the lower accuracy of the fused decisions, because of the loss of information
occurring during feature extraction at each sensor.
Decision-level fusion: In the decision level fusion technique the classification decision is made
individually at each sensor and then fused. A variety of techniques have been developed for decision level
fusion [58], which may be binary decisions or detection probabilities. Decision-level fusion is, in
principle, the least accurate because of the additional loss of information that occurs when sensor data are
reduced to decisions. Nonetheless, other considerations such as communication bandwidth and low
computational demand at the fusion center make the decision-level fusion attractive in situations dealing
with distributed mine detection.
The choice of a suitable fusion level depends on the available sensor types. When the sensors are alike,
one can opt for fusion at data-level. Feature-level fusion is appropriate when features obtained from
different sensors can be combined in a way that provides sufficient information for mine detection. When
the sensors are very different, decision-level fusion is more suitable and also computationally more
efficient [57].
22
The above study showed that for GPR and IR the suitable fusion methods are feature-level and decision
level. The feature-level sensor fusion was applied so far by [64] where the processes start with the regions
of interest with their features as measured by individual sensors. This includes three steps. First step is
object association where the features (object area and intensities) of objects from different sensors are
combined to form an associated object. The second step is to classify the features from these associated
objects by feature classification algorithms. The third and final step is the performance evaluation.
The decision-level sensor fusion techniques are Bayesian approaches, application of Dempster-Shafer
theory, fuzzy probabilities, rules and voting techniques [65, 66]. Here, the individual sensor results are
obtained that act as confidence values and are input to the Decision-level sensor fusion algorithms.
Decision-level sensor fusion can also exploit the GPR and IR advantages in sensing efficiently into
different layers by surface-fusion (extension of decision-level sensor fusion). Thermal infrared has the
limitations for adding information in the deep layer and GPR has difficulties suppressing clutter in the
surface layer. So, the surface-fusion should place emphasis on the IR sensor at the surface layer and at the
deep layer in GPR [57].
US Army’s new dual sensor, HSTAMIDS (handheld standoff mine detection system), which combines an
electromagnetic induction (EMI) sensor and GPR, is an example of sensor fusion tools. It shows a
significant improvement on low metal APM detection which is 93% in average (by experienced
operators) and 80% (for trainee) in a simulated mine field (had the proximity of a real mine field) [5, 6].
But it still suffers from high false alarm rate 23%. In addition, the sensor EMI and GPR are not
compliments to each other. Therefore, its performance as a sensor fusion module may be not convincing
in a real mine field.
23
MINEHOUND is another example of fusion (decision-level) developed by UK Department of
International Development and ERA technology which combines MD and GPR. These sensors can work
individually or together and provide audio output. At first, MD detects all metal threats and latter GPR
mode confirms a present of a threat. The results of the trials showed that it is achieved 100% detection of
the mines encountered and an improvement of false alarm rate of better than 5:1 compared with a basic
metal detector [3, 4]. But, the concept of fusion is invalidated if we consider only APMs (with no metal).
Again, its performance may be varied with high soil moisture, depth and mine size as MD and GPR will
not be the compliment of one another.
The above mentioned techniques of sensor fusion still need improvement and suffer from large false
alarm rate. Not any of the techniques can efficiently classify APM or provide any information about their
orientation. We are proposing the geometrical feature based fusion of GPR and IR which may eliminate
these limitations.
We are planning to set up an experiment to acquire data by non coincidental scanning of mine field by
GPR and IR sensors (implement GPS for determining the scale of the surface area). We will construct 3D
image from GPR data. From GPR 3D image we will calculate the geometrical features of APMs such as
size, shape and dimensions. The IR images will provide us the 2D features of APMs from which we will
find the size and shape features, and can be estimated the maximum and/minimum axis of the target. We
will also measure the actual size in terms of length or width if it is cylindrical shape and diameter and
perimeter if it is circular shape. The extracted common geometrical features from individual sensor will
be fused in the fusion module. The rest features (3D) along with estimate the invariant features of object
like elongation and compactness will be consider for classification of mine.
To calculate the orientation we have to rely on the GPR 3D image. At first we have to create a database
from the commonly used APMs. On the 3D image we will have to apply the image rotation operation.
24
Every time we will apply the rotation operation and compare the resulted image with our image database.
Finally, the orientation will be calculated from angular difference of matched image and the original
image (image constructed from GPR 3D data). We may also verify the orientation with the upper surface
of the buried APM from IR image (applying 2D rotation).
The overall technique which we are proposing is depicted in the following diagram:
Figure. The overall system for the detection and classification of APM.
6. Conclusion
We note that each of the methods reviewed has some limitations under certain conditions of environment
and mine type. It is, therefore, unlikely that any single technique will provide a breakthrough necessary to
substantially improve mine detection effort and time. To achieve substantial decreases in mine detection
time while maintaining high probability of detection as well as a low false alarm rate, an efficient
approach is a necessity. Rather than focusing on individual sensor operating in isolation, we should
emphasize that a design of an integrated, multi-sensor system may overcome the limitations of any single-
25
sensor technology. A multi-sensor hybrid system that combines GPR and IR should be used for achieving
a better performance in the detection and classification of APM.
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Minerva Access is the Institutional Repository of The University of Melbourne
Author/s:Bhuiyan, Alauddin;Nath, Baikunth
Title:Anti-personnel landmine detection based on GPR and IR imaging: a review
Date:2006-04
Citation:Bhuiyan, Alauddin, &Nath, Baikunth. (2006). Anti-personnel landmine detection basedon GPR and IR imaging: a review, Technical Report, Computer Science and SoftwareEngineering, The University of Melbourne.
Publication Status:Unpublished
Persistent Link:http://hdl.handle.net/11343/34827