Development of a UWB GPR System for Detecting Small

Development of a UWB GPR System for Detecting Small
Development of a UWB GPR System for Detecting Small
Objects Buried under Ground
Young-Jin Park*, Kwan-Ho Kim*, Sung-Bae Cho*, Dong-Wook Yoo*, Dong-Gi Youn**, Young-Kyung Jeong**
*
Power Telecommunication Network Research Group
Korea Electrotechnology Research Institute (KERI), Euiwang-City, 437-802, Korea
Phone: +82-(0)31-420-6183, Fax: +82-(0)31-420-6199
e-Mail:[email protected]
**
Microline Co., Ltd.
Shinnae Techno-Town 801, Sangbong, Seoul, 131-220, Korea
Abstract
A ground penetrating radar (GPR) using short-pulse is developed in order to detect small and shallow metal
objects buried under the ground. A bistatic mode in which the GPR system uses separate transmitting and
receiving antennas is applied. A modified fat dipole antenna is developed for the transmitting and receiving
antennas,. The prototype of the system is tested in real environment and 2D visualization of raw data is achieved.
It is shown that the developed system has a good ability in detecting underground metal objects, even small targets
of several centimeters.
1. INTRODUCTION
Recently, impulse radio technology, also called
ultra wideband (UWB) technology is receiving much
attention for applications to wireless communication
and high resolution radar. Its main principle is to use
impulses which result in very accurate timing
information and ultra wide bandwidth in frequency
domain. These features are so useful for the UWB
technology to be widely applicable for the detection of
unknown or known small and shallow objects buried
under the ground [1]-[4]. Until now, UWB ground
penetrating radar (GPR) systems have been intensively
investigated for mine detection [5].
This paper reports a new application of UWB
radar for the detection of gas pipelines buried in the
ground. The UWB GPR is used to draw a map of gas
pipelines buried under ground by connecting global
position system (GPS) system to the GPR. Compared
to conventional radar systems, such as a FMCW radar,
the complexity of the system is very reduced, but its
performance is better.
In the following sections, the design procedure of
the UWB GPR system is described. Also, the image
processing procedure of the raw data for 2D
visualization is shown. Especially, a novel UWB fat
dipole antenna is designed and presented for the
system.
2. UWB GPR SYSTEM CONFIGURATION
In Fig. 1, the detection scheme of the entire UWB
GPR radar is shown. The radar system is composed of
roughly three main parts, a transmitter unit with a
portable impulse generator and a UWB transmitting
(Tx) antenna, a receiver unit with a UWB receiving
(Rx) antenna and a high speed sampling digitizer, and
a data processing unit for 2D visualization.
For the system, a bistatic mode is applied. That is,
Tx and Rx antennas are separate. The gap between Tx
antenna and Rx antenna will be kept constant. In
moving the transmitter at a foot’s pace, the receiver
saves the signal scattered by underground objects. As
it is shown, the whole system is simple, not so
complex.
recorder
A/D
conversion
FFT
receiver
timing
Tx/
Rx
moving
objects
Fig. 1. Detection scheme of the developed ground
penetrating radar (GPR) for the detection of
underground metal objects.
2. 1 Determination of the specification of system
Usually, the gas pipelines are buried under ground
within 3 m and made of metal. Thus, for the system,
first, the maximum target depth of 3 m is decided and
then the operating frequency is chosen to be between
100 MHz and 400 MHz on 3 dB line. It is true that
with higher operating frequencies, the UWB GPR has
better localization and identification of targets [3].
However, as explained in [6], the limitation of the
operating frequency is determined to be about 400
MHz since the ground in our country usually contains
more moisture, thus the attenuation is much more
serious./ /
power spectral density (dB)
DSP
display
impulse generator is developed. Figure 2 displays the
impulse shape. It has about 2.5 ns pulse duration. For
the impulse measurement, a digital oscilloscope with 5
GS/s is used.
As is well known, a UWB antenna is considered as
a band pass filter in a UWB system [1][2]. In order
words, a derivative form of the original impulse shape
is radiated. In Fig. 3, the power spectral density of the
derivative of the pulse shape in Fig. 2 is shown. The
figure shows that the frequency bandwidth is between
150 MHz and 500 MHz on 3 dB line (fractional
bandwidth is more than 150 % on 10 dB line).
0
-5
-10
-15
-20
-25
0
100 200 300 400 500 600 700
frequency (MHz)
Fig. 3. Power spectral density of the derivative form of
the impulse.
2. 2 Transmitting and receiving antennas
It is important to design a proper UWB antenna in
order to transmit impulses into the ground and receive
scattered signals from objects with minimum
distortion. In this paper, a modified fat dipole antenna
with a broad bandwidth, between 100 MHz and 350
MHz is developed.
50 ohm coaxial cable
without PVC coat
50
excitation joint
100 Ω
amplitude (V)
40
30
100 Ω
20
10
0
0
1
2
3
time (ns)
4
5
Fig. 2. Pulse shape of the impulse generator.
Now, for the above desired frequency bandwidth, a
BNC connector
Fig. 4. Modified fat dipole antenna. Substrate material is
FR4 (εr = 4.8).
VSWR
Figure 4 shows the planar fat dipole antenna. The
substrate of FR 4 (εr = 4.8) is used. To transmit more
power into the ground and prevent the received signals
from the external undesired signals, a parabolic
reflector is used for each antenna. Also, as it is shown
in the photograph, the edges of the antenna are
connected to the 100 Ω resistor in order to prevent the
ringing effects from the original scattering signals by
objects. A BNC connector and 50 Ω coaxial cable are
used for the Tx and Rx antennas. The PVC coat of the
coaxial cable is removed and the outer shield is
welded to one arm of the antenna for better excitation.
The gap of two arms is selected to be 0.1 times as long
as that of each arm. Mainly, with an aid of simulation,
proper dimensions of an antenna are determined for
the desired frequency bandwidth. In this paper, by
considering the impulse generator, the arm dimension
is chosen to be 240 mm × 500 mm. The attractive
advantages of the antenna are easy fabrication, low
cost, and light.
3
2
1
100
200
300
400
frequency (MHz)
2.3 Data processing algorithm
As mentioned previously, a high speed digitizer is
used for measuring the backscattered signal in the
receiver unit. In this paper, the digitizer has 5GS/s
sampling rate, an internal delay line, a 14bit resolution,
and ability in averaging up to 10,000 times, to obtain a
higher dynamic range. The receiver unit is designed to
gather backscattered signal in memory by way of a
GPIB cable.
For visualization, digital signal processing of the
raw data is necessary. In this paper, a commercial
delphi program for 2D visualization is used. Figure 6
shows the 2D GPR image signal processing algorithm
for the system. As it is shown in Fig. 6, the procedure
of 2D visualization is as follows. The envelope
detection on each A-scan is fulfilled by finding
absolute values of the Hilbert transform, and then the
A-scans results are arranged in adequate B-scan. For a
good A-scan, A-scan data at a point are obtained by
averaging several tens of A-scans results. For a B-scan,
some image preprocessing tools such as filtering,
smoothing and varying threshold, and removing
background noise are applied. The program for
visualization makes it possible to easily distinguish
other buried objects by assigning color depth table
obtained through B-scan data processing. The results
are displayed in next section. The algorithm is
explained details in [7].
3. MEASUREMENT AND RESULTS
Fig. 5. VSWR measurement of the antenna.
Figure 5 shows the VSWR measurement of the
designed antenna.
The whole system is set up and tested in a real
environment. Figure 7 shows an outside test-bed built
Number of A-scan <158
GPR data acquisition
No
Scan Number++
Raw data(A-scan) convert to binary data
Calculate maximum, minimum ,
average of Per A-scan
Sample Number of A-scan <500
Yes
End
Yes
No
Bitmap create
(in using color depth table)
Color depth table create
Sample Number++
Fig. 6. 2D visualization processing algorithm from raw data.
B-Scan
Display
A-Scan
Display
near the institute. In the ground, a metal plate is buried
at depth of 100 cm, a metal pipe of 40 cm diameter at
100 cm, a metal pipe of 5 cm diameter at 50 cm, and a
PVC pipe of 20 cm diameter at 50 cm. The distance of
200 cm between two targets is fixed. Metal pipes of
two different sizes are buried for testing the
performance of the UWB GPR. The total length of the
test bed is 10 m. It should be pointed out that in
addition to the buried targets, lots of small or large
stones and plant roots exist together in the test field.
Impulses are transmitted into the ground at every
5 cm and reflected impulses are recorded in memory.
0.5m
1m
1m
”Œ›ˆ“G—“ˆ›Œ
”Œ›ˆ“G——Œ
kd\Š”
GROUND
w}jG——Œ
kdYWŠ”
”Œ›ˆ“G——Œ
kˆ”Œ›Œ™GOkPd[WŠ”
10 m
(a)
w}jG——ŒS
kdYWŠ”
”Œ›ˆ“G——ŒS
kd\Š”
”Œ›ˆ“G——ŒS
kd[WŠ”
Figure 8 shows images of raw data. Figure 8(a)
shows the image before the removal of the background
image. Figure 8(b) is the image with a lower threshold
value than that in Fig. 8(a). The black and white lines
in the middle are from the strong direct wave and the
surface reflection. The black and white ones at the
bottom show the reflection at the maximum depth.
Figure 8(c) illustrates an image after removing the
background image. As shown in the figure, three
different metal objects are clearly distinguished.
However, the PVC pipe’s image is not clear. Also
several small scatters are found. By comparing the
transmitting and receiving signals, the maximum
depth of 3 m is derived.
1st reflection
by ground
metal
plate
metal pipe
(D=5cm)
metal pipe
(D=40cm)
(b)
(a)
10 m
”Œ›ˆ“G—“ˆ›Œ
gX”
clutters
(b)
Fig. 7. Test bed in real environment for testing the
performance of the developed UWB GPR. A thin
metal plate, two metal pipes, and a PVC pipe are
buried. D stands for a diameter.
In Tab. 1, the parameters used in measurement are
summarized.
Tab. 1. Summary of parameters in measurement
Parameters
Survey Range
Pulse Repetition Frequency (PRF)
Station Spacing
Sampling interval
Number of B-scan
Samples per A-scan
Separation of Tx and Rx antenna
Frequency Bandwidth
Value
10 m
10 kHz
5 cm
100 ns
158 points
500 points
0.8 m
100~300 MHz
(c)
Fig. 8. Images in processing raw data. (a) Image with
the background image. (b) Image with a lower
threshold value. (c) Image after removing the
background image.
According to the resolution, the depth resolution
in the vertical plane and the space resolution between
two objects can be considered. The depth resolution of
about tens of centimeter is observed. The reason of
lower depth resolution is that the lower frequency
range is used. In other words, to improve the depth
resolution, the frequency bandwidth of the impulse
signal becomes broader at the cost of the reduction of
maximum depth. One of the solutions will be the
better A/D (analog to digital) converter with higher
sampling speed. Also, the space resolution appears not
to be satisfactory. It is assumed that the reason of the
space resolution problem is due to less B-Scan
numbers.
However, the measurements show that the
developed radar has a good ability in detecting buried
metal objects, even small targets of several
centimeters.
4. CONCLUSION AND FURTHER WORKS
A UWB GPR radar is developed to detect
underground metal objects. The design procedure of
the system is explained. Measurement results show
that the UWB GPR has a good ability in finding
underground metal targets. For further works, the
performances will be tested, compared to conventional
radars. Also, some steps for improving resolutions will
be taken.
REFERENCES
[1]
J.D. Taylor (Ed.), Ultra-Wideband Radar
technology. CRC press: USA, 2001.
[2]
J.D. Taylor (Ed.), Introduction to UltraWideband Radar Systems. CRC press: USA,
1995.
[3]
D.J. Daniels, Surface Penetrating Radar. IEE:
London, UK, 1996.
[4]
H. L. Bertoni, L. Carin, and L. B. Felson (Ed.),.
Ultra-Wideband, Short-Pulse Electromagnetics.
Plenum Press: NY, USA, 1993
[5]
A. G. Yarovoy, et al., “Ground penetrating
impulse radar for detection of small and
shallow-buried objects,” Proc. Of IGARSS ’99,
Vol. 5, pp.2468-2470, June/July, 1999.
[6]
Y.-G. Jung, et al., “Development of ground
penetration radar using impulse technology,”
Proc. Of the KIEE Summer Annual Conf. 2002,
June 2002, pp. 10-12.
[7]
Y.-G. Jung, Dong-Gi Youn, Sang-Bo Min, and
Kwan-Ho Kim, “A study on the image
techniques for UWB-GPR system,” accepted
for the publication in Proc. of Asia Pacific
Microwave Conference 2003 ( APMC ’03).
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