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Design and Investigation of the Use of TEM Cells in the Characterization ofUnderground Targets

Abstract : Landmines contamination is a widespread problem in many countries. Indeed, thousands of people around the globe are living in daily threats due to these hidden traps. The clearance process of the contaminated areas for humane purposes is known as humanitarian demining. Current demining methods are mostly manual and rely on metal detectors and ground penetrating radar (GPR). In some cases, the metal amount that exists in the landmine is very small and is not sufficient for being sensed by the metal detector. On the other side, the GPR can detect and locate the buried landmine in the soil relying on the fact that the reflected signal is related to the permittivity of the materials at interface. The GPR system needs complicated processing of the received signals to identify the presence of mine or not. One of the GPR disadvantages is its inability to detect the landmines at shallow depth. In addition, its detection efficiency is limited in the case of wet soil. The high cost and effort necessitate the work on related research topics that can enhance the existing demining techniques or even develop a newer one.This Ph.D. research work aims to study the effective permittivity of the buried landmine with its surrounding soil. Knowing more information about the interaction between the electromagnetic wave and the buried target can be very useful to enhance the GPR detection process. In this work, the different methods of microwave material characterization were described in detail in order to provide the literature with a good material reference. The characterization of landmines implies the search for a suitable test fixture that can be used in effective permittivity extraction. Since the traditional transmission lines are not compatible in use with landmines characterization due to their material insertion complexity and size limitation, the transverse electromagnetic (TEM) cell was selected as the test fixture owing to its advantage in the size and material insertion easiness. In this research work, the use of the closed TEM cell for material characterization is attempted for the first time. A novel design for closed TEM cell is proposed, this design allows the implementation of the different material characterization techniques. The proposed TEM cell can be used to characterize both homogenous and inhomogenous (including landmines) materials. The effective permittivity of two different types of landmines was extracted using the proposed closed TEM cell technique. Correspondingly, the GPR system configuration and its use in permittivity extraction of soil are studied and implemented numerically. The optimal configuration of the GPR system is obtained after a parametric effects study for different cases and scenarios. The main disadvantage in the simulation study of the GPR system is the long run time, especially with the presence of the buried landmines which has a complex shape. Thus, make use of the homogenization concept, we can replace any inhomogeneous material by its alternative homogenous material which has the same response (has the same effective permittivity). So, instead of simulating the landmine with its surrounding soil we can simulate its homogenous material equivalence that has been studied in the TEM cell. In this way, we can reduce the time of the simulation. Furthermore, a novel approach for landmines detection is proposed in this work based on the effective permittivity signature. This method is dedicated to shallow depth detection of landmines and its detection efficiency increases in the case of wet soil, and that is covering the current GPR technique limitations.Finally, this work opens new prospects for humanitarian demining research. And its promising results can lead to enhance detection technology, in other meaning a reduction in the loss of life.
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Ali Al Takach. Design and Investigation of the Use of TEM Cells in the Characterization ofUnderground Targets. Optics / Photonic. Université Grenoble Alpes, 2019. English. ⟨NNT : 2019GREAT092⟩. ⟨tel-02984775⟩

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