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 -. Until now, UWB ground penetrating radar (GPR) systems have been intensively investigated for mine detection . 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 . However, as explained in , 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 . 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 . 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 G G kd\ GROUND w}jG kdYW G kGOkPd[W 10 m (a) w}jGS kdYW GS kd\ GS kd[W 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 G 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  J.D. Taylor (Ed.), Ultra-Wideband Radar technology. CRC press: USA, 2001.  J.D. Taylor (Ed.), Introduction to UltraWideband Radar Systems. CRC press: USA, 1995.  D.J. Daniels, Surface Penetrating Radar. IEE: London, UK, 1996.  H. L. Bertoni, L. Carin, and L. B. Felson (Ed.),. Ultra-Wideband, Short-Pulse Electromagnetics. Plenum Press: NY, USA, 1993  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.  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.  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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