GPR for river dyke - GSB

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2 Principle of using GPR for river dyke monitoring The main purpose of on site training is to demonstrate the ability of using GPR to identify for example, the disturbed and weak zones in the dyke, as well as re-bars in the concrete etc. The weak zones include loosely compacted layers, joints in different ground materials within the dykes, presence of highly compressive layers, etc. GPR is a non-destructive method that can perform quick surveys on top of a river dyke. The conventional GPR test mode uses the reflection wave, where two antennas with a fixed spacing are used, one for transmitting and the other for receiving signals. They are moved along the river dyke. Electromagnetic waves can be transmitted through the ground with the help of the transmitting antenna. The energy of the wave is reflected back towards the surface where the electric or magnetic properties change (due to the weak zones etc.) in the river dyke. A second antenna receives the reflected wave. The ground materials will attenuate the radar wave. The detection distance is therefore dependent on the types of river dyke materials. Generally speaking, materials such as concrete, sand, rock, gravel, sandy soil etc are good materials for radar wave to penetrate. However silty clay and wet clay etc are the difficult materials for radar wave to penetrate. For good materials the GPR detection distance can be longer than 10 m. Even though the detection distance is limited in clay material etc, the GPR reflection measurement on top of the river dyke is a commonly accepted method used for the dyke monitoring. This is because river dyke material changes at shallow distances may imply that disturbances may take place in the internal part and thus deform the shallow layered structure. As mentioned, the GPR survey is non-destructive investigation method, as well as being quick. Survey speed can be as fast as 2-3 km/hour. For an experienced operator, the interpretation of the survey results can be made on site without further data processing (inversion etc.). Hence, the GPR survey is low cost geotechnical investigation method. 3 NGI GPR system At NGI there has been a continuous activity in developing and testing radars for different subsurface mapping tasks since 1989. The work has included development of hardware and software for data acquisition and algorithms for data processing and interpretation. In parallel with the development work, NGI has carried out more than hundred service jobs, gaining a lot of practical experience. The NGI Ground Penetrating Radar (GPR) employs an Agilent E5062 network analyser as the transmitter and receiver. It uses step frequency radar signals instead of the impulse signals that most of the commercial GPR use. NGI GPR has been successfully used for over 100 field tests in around 20 countries since 1989. The NGI radar systems have been exported to USA, South Korea, France, Taiwan, Malaysia, India, Denmark and Bhutan.
Figure 2: GSB personnel operating the NGI radar system The advantages of using a network analyser based GPR system are: The NGI system can work within the frequency band 0.3 MHz to 3000 MHz, which has a wider bandwidth coverage than most of the commercial GPR systems. The radar signal bandwidth can be adjusted by software at the test site in order to best match the ground conditions to the antenna resonance frequency. Conventional GPR systems, which use impulse signals, need to change hardware to modify radar signals. The frequency step number can be adjusted by software to make the best use of signal power for target detection under different ground conditions. The receiver of the NGI system has a higher processing dynamic range than conventional GPR systems. The NGI system has a perfect frequency response, flat and no-dispersion within the assigned bandwidth. It is in itself a good tool for measuring ground material parameters and antenna response. The NGI system makes it simpler to employ frequency-domain signal processing method such as filtering, deconvolution, Hilbert transform, etc. Figure 3: Layout of measuring floor at GSB office building where horn antennas of length 10 cm are used
Page 1 and 2: GOVERNMENT OF THE PEOPLE’S REPUBL
Page 3 and 4: Summary The Norwegian Geotechnical
Page 5: 1 Introduction The Geological Surve
Page 9 and 10: Figure 5: Radargram for the test on
Page 11 and 12: new pitch road has been made. The t
Page 13 and 14: 4.2.3 GPR survey on Prottasha bridg
Page 15 and 16: Figure 15 shows the radargram for G
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Page 19 and 20: Kontroll- og referanseside/ Review
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GPR for river dyke - GSB

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