GPR for river dyke - GSB

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Contents 1 Introduction 5 2 Principle of using GPR for river dyke monitoring 6 3 NGI GPR system 6 4 Test results 8 4.1 Indoor test results 8 4.2 GPR tests on dyke 4.3 GPR tests on road 9 5 GPR theory revisited: Ground material property, wave speed and propagation attenuation 17 6 Conclusions 18 Appendices Appendix A: Wave propagation in ground medium Appendix B: Application on GPR for subsurface investigations Control and review page
1 Introduction The Geological Survey of Bangladesh (GSB) and the Norwegian Geotechnical (NGI) Institute are co-operating in an institutional co-operation project titled “Geodisaster Risk Reduction in Bangladesh”. In this project, one of the key Tasks is to build-up capacity at GSB in using advanced geophysical tools for investigation of river dykes in Dhaka. The main goal of the project is to reduce the risks from natural hazards in Bangladesh by increasing the capacity of GSB through collaborative pilot project studies. Ground penetrating Radar is a useful tool for geophysical survey, which can be used to detect locations and extents of weak zone areas of a river dyke. Hence building up the ability for GSB personnel to use GPR for river dyke controlling becomes an important topic of this co-operation project. GSB personnel were trained at NGI in Norway July last year (2010) on how to use the NGI GPR. In March 2011 an on site training in Dhaka, Bangladesh took place. GSB personnel have the background of performing seismic survey. GPR has similar record as seismic survey. The main difference between GPR and seismic methods is that seismic reflection comes from a boundary where there is an acoustic impedance contrast and in the case of GPR the reflection comes from a boundary where there is a dielectric constant contrast. So the geoscientists of GSB are trying to extrapolate their knowledge from their training and seismic survey experience to GPR because in both cases the data (GPR & seismic) have similar 2D view. However, in seismic reflection method, the depth of penetration is very large so the geoscientists think about gross lithology but in case of GPR they have to think very minutely about the lithology and other variation within a very shallow depth in the range of 10 m. For any geophysical methods, the detection resolution is a portion (for example, one tenth) of the test distance. Therefore, GPR normally gives detection results with much higher resolution than the seismic method. The other advantages of using GPR are that for detection using electromagnetic wave, the sensors (or antennas) do not need to have good contact with the ground, which enables a fast survey speed, and that the GPR signal is generated by electronics, and is coherent and repetitive, which allows to use advanced signal and image processing methods.Figure 1 shows the GSB and NGI team at the office in Dhaka. In this Figure Professor Lars Ottemoller from the University of Bergen, second from right, is also present to plan the Task related to the seismic component of the project. Figure 1: GSB – NGI team at the GSB office in Dhaka
Page 1 and 2: GOVERNMENT OF THE PEOPLE’S REPUBL
Page 3: Summary The Norwegian Geotechnical
Page 7 and 8: Figure 2: GSB personnel operating t
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
Page 17 and 18: In Figure 20, there is no continuou
Page 19 and 20: Kontroll- og referanseside/ Review
Page 21: Kontroll- og referanseside/ Review

GPR for river dyke - GSB

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