The Imaging Capabilities of Ground Penetrating Radar for the Detection of Buried Anti-Personnel Landmines
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The Imaging Capabilities of Ground Penetrating Radar for the Detection of Buried Anti-Personnel Landmines Dissertation zur Erlangung des akademischen Grades Doktoringenieur (Dr.-Ing.) von Dipl.-Wirtsch.-Ing. Alexander Teggatz geb. am 24. April 1979 in Magdeburg genehmigt durch die Fakult¨at f¨urElektrotechnik und Informationstechnologie der Otto-von-Guericke-Universit¨atMagdeburg Gutachter: Prof. Dr. Abbas Omar (FEIT/IESK) Prof. Dr. J¨urgen Nitsch (FEIT/IGET) Dr. Martin Dohlus (DESY/MPY) Promotionskolloquium am 19. M¨arz 2008 ii Schriftliche Erkl¨arung Ich erkl¨are hiermit, dass ich die vorliegende Arbeit ohne unzulssige Hilfe Dritter und ohne Benutzung anderer als der angegebenen Hilfsmittel angefertigt habe. Die aus fremden Quellen direkt oder indirekt ¨ubernommenen Gedanken und Informa- tionen sind als solche kenntlich gemacht. Insbesondere habe ich nicht die Hilfe einer kommerziellen Promotionsberatung in Anspruch genommen. Dritte haben von mir weder unmittelbar noch mittelbar geldwerte Leistungen f¨urArbeiten erhalten, die im Zusammenhang mit dem Inhalt der vorgelegten Dissertation stehen. Die Arbeit wurde bisher weder im Inland noch im Ausland in gleicher oder ¨ahnlicher Form als Dissertation eingereicht und ist als Ganzes auch noch nicht ver¨offentlicht. Alexander Teggatz iii iv Acknowledgement I gratefully would like to acknowledge the outstanding support of all people who, by many different means, provided help, advice and encouragement to me. First and foremost, I would like to thank my supervisor Prof. Dr.-Ing. Abbas S. Omar for the opportunity to become a member of his group at the Institute for Electronics, Signal Processing and Communications at the Otto-von-Guericke University. Prof. Omar supported me continously with his profound knowledge and his big experience. His helpful suggestions and our interesting discussions illustrate his enthusiasm and help to encourage young scientists to contribute to the scientific community. To the same extend, I sincerely thank the leader of the research group for Ground Penetrating Radar Dr.-Ing. Andreas J¨ostingmeier. Without his constant support, his technical and personal guidance, his patience and his willingness to correct count- less technical publications this work would have never been completed. Moreover, I would like to thank Dr.-Ing. Tobias Meyer, Dipl.-Ing. Michael Anis, Dipl.-Ing. Nikolaos Spiliotis, M. Sc. Ayan K. Bandyopadhyay and other members of the research group for their frequent suggestions, the provided knowledge and experiences and, especially, for the support of all measurement activities. Of course, I am grateful to my parents for their love and support which means so much to me. Finally, I would like to thank Anja, my beloved wife and source of inspiration, who I owe so much for making this effort possible. Without her love, encouragement and her understanding this manuscript would not exist. v vi Abstract Anti-Personnel (AP) landmines are considered as a problem of global proportions and it is estimated that about 60-70 million AP landmines are scattered within at least 70 countries all over the world. Many of the mines are made without metal so that detection methods based on electromagnetic induction (EMI) often tend to fail. A promising concept for the detection of buried nonmetallic objects is the ground penetrating radar (GPR) which originates from geophysical techniques. The investigations in the context of this thesis cover different important aspects of GPR. A novel approach for the 3D field simulation of a complete GPR environment will be proposed which not only allows to study the fundamental principles of GPR but will also be utilized for the systematic verification of antenna concepts in the context of GPR applications. Finally, the important problem of focusing the raw data of GPR measurements will be addressed and two different focusing concepts will be investigated using both, field simulations and measurements. Anti-Personen (AP) Landminen werden als Problem globalen Ausmaßes betrachtet. Es wird gesch¨atzt, dass es weltweit ungef¨ahr60-70 Million AP Landminen verteilt in mehr als 70 L¨anderngibt. Viele Minen werden inzwischen ohne Metall hergestellt, so dass ihre Detektion auf Basis von elektromagnetischer Induktion (EMI) h¨aufig versagt, w¨ahrenddas sogenannte bodendurchdringende Radar (engl. Ground Pene- trating Radar, GPR) f¨urdie Detektion von vergrabenen nichtmetallischen Objekten gut geeignet ist. GPR hat seinen Ursprung im Bereich der Geophysik. Die Untersuchungen im Rahmen dieser Arbeit betreffen unterschiedliche wichtige Aspekte von GPR Systemen. Zun¨achst wird eine neue Methode zur 3D Feldimula- tion eines kompletten Systems vorgestellt. Diese erlaubt nicht nur die Untersuchung grundlegender Prinzipien von GPR, sondern auch f¨urdie systematische Verifizierung verschiedener Antennenkonzepte, die im Rahmen von GPR Anwendungen verwen- det werden. Schließlich wird das Problem der Fokussierung der Rohdaten einer GPR Messung diskutiert. Daf¨urwerden zwei unterschiedliche Fokussierkonzepte sowohl mit Feldsimulationen als auch anhand realer Messungen untersucht. vii viii Contents Nomenclature xv List of Figures xxiv 1 Introduction 1 1.1 The Global Landmine Crisis . 1 1.2 Methods of Landmine Detection . 3 1.2.1 Standards and Definitions . 3 1.2.2 Manual and Mechanical Methods . 3 1.2.3 Electromagnetic Induction . 4 1.2.4 Infrared and Hyperspectral Methods . 5 1.2.5 Acoustic-to-Seismic Coupling . 6 1.2.6 X-Ray Backscatter Methods . 6 1.2.7 Neutron and Nuclear Techniques . 7 1.2.8 Nuclear Quadrupole Resonance . 7 1.2.9 Biological Sensor Methods . 8 1.2.10 Ground Penetrating Radar . 9 1.3 A Brief History of Radar and GPR . 10 1.3.1 Development of Radar Systems . 10 1.3.2 Development of GPR Systems . 12 1.4 Objectives and the State of the Art . 13 1.4.1 Starting Point of the Investigation . 13 1.4.2 3D EM Field Simulation of a GPR . 14 1.4.3 Investigation of Antennas for GPR . 15 1.4.4 Focusing by Synthetic Aperture . 16 1.5 Organization of the Thesis . 18 ix CONTENTS 2 Fundamental Relations 19 2.1 Propagation of Electromagnetic Waves . 19 2.1.1 Maxwell’s Equations . 19 2.1.2 Permittivity and Permeability . 20 2.1.3 Plane Wave Assumption . 21 2.1.4 Dielectric Interfaces . 22 2.2 Theory of Radar Systems . 26 2.2.1 Radar Categorization . 26 2.2.2 Transmitted Waveform . 26 2.2.3 Transmitted Power . 27 2.2.4 The Radar Equation . 28 2.2.5 Range Measurement . 31 2.2.6 Resolution of a GPR . 32 2.2.7 Illustration of GPR Data . 34 3 3D EM Field Simulation of GPR 35 3.1 Introduction . 35 3.2 Definition of Material Properties . 36 3.2.1 Lossless Materials . 36 3.2.2 Conductive Materials . 36 3.2.3 Lossy Metal Materials . 38 3.2.4 Dispersive Materials . 39 3.3 Modeling of Realistic Soil Structures . 40 3.3.1 Texture and Structure of Soils . 40 3.3.2 EM Properties of Soil Materials . 41 3.3.3 Modeling of Surface Roughness . 44 3.4 Modeling of Realistic Target Objects . 46 3.4.1 Modeling of Artificial Test Objects . 46 3.4.2 Modeling of Typical AP Landmines . 47 3.5 Integration of Different Antennas . 48 3.6 Remote Control Technique . 50 3.7 Verification of the GPR Simulation . 54 3.7.1 1D Transmission Line Modeling . 54 3.7.2 Planar PEC and Dielectric Layers . 56 3.7.3 Estimation of the Object Height . 58 x CONTENTS 4 Investigation of Antennas for GPR 61 4.1 Introduction . 61 4.2 Standard Gain Horn Antenna . 62 4.2.1 Design and Development . 62 4.2.2 Antenna Characteristics . 62 4.2.3 GPR Simulation Results . 67 4.3 Orion-type Impulse Radiating Antenna . 70 4.3.1 Design and Development . 70 4.3.2 Antenna Characteristics . 71 4.3.3 Calibration Procedure . 72 4.3.4 GPR Simulation Results . 76 4.4 Log-periodic Cylindrical Dipole Antenna . 80 4.4.1 Design and Development . 80 4.4.2 Antenna Characteristics . 83 4.4.3 GPR Simulation Results . 86 4.5 Modified Double-ridged TEM Horn Antenna . 88 4.5.1 Design and Development . 88 4.5.2 Antenna Characteristics . 90 4.5.3 GPR Simulation Results . 93 4.6 Small Double-ridged TEM Horn Antenna . 96 4.6.1 Design and Development . 96 4.6.2 Antenna Characteristics . 98 4.6.3 GPR Simulation Results . 100 4.7 Modified Bujanov Loop Antenna . 103 4.7.1 Design and Development . 103 4.7.2 Antenna Characteristics . 105 4.7.3 GPR Simulation Results . 107 5 Focusing by Synthetic Apertures 109 5.1 Introduction . 109 5.2 Common Processing Techniques . 110 5.2.1 Fundamental SAR Processing . 110 5.2.2 Doppler Beam Sharpening . 111 5.2.3 Polarimetric SAR Processing . 111 5.2.4 Interferometric SAR Processing . 112 xi CONTENTS 5.3 Synthetic Aperture Radar for GPR . 113 5.3.1 Formulation of the Focusing Problem . 113 5.3.2 Implementation of the SAR Focusing . 120 5.4 Investigation of the SAR Processing . 122 5.4.1 Simulation of an Array of Dipoles . 122 5.4.2 Realistic Antenna Characteristics . 130 5.4.3 Verification of the Energy Beam . 134 5.4.4 Gaussian Amplitude Weighting . 135 5.5 Verification of the SAR Focusing . 138 5.5.1 Resolution Capabilities of SAR . 138 5.5.2 Using an Independent Imaging Grid . 140 5.5.3 Real-Time Implementation of SAR . 142 6 Focusing with Dielectric Lenses 143 6.1 Introduction . 143 6.2 Design of a Dielectric Lens . 144 6.3 Verification of the Lens Concept . 148 6.4 Adaptation of Additional Antennas . 156 6.5 Discussion of the Lens Concept . 158 7 Experimental GPR Systems 159 7.1 Introduction . 159 7.2 Laboratory GPR Investigations . 160 7.2.1 Configuration of the GPR System . 160 7.2.2 Influence of the Antenna Height . 162 7.2.3 Comparison of Different Antennas .