THE UNIVERSITY OF HULL An Evaluation of the Performance of Multi-static Handheld Ground Penetrating Radar using Full Wave Inversion for Landmine Detection Being a Thesis submitted for the Degree of Doctor of Philosophy in the University of Hull by Suki Dauda Sule, MSc, B.Eng. (Hons) June 2018 Acknowledgments I would like to begin by thanking my first supervisor, Dr. Kevin Paulson for his support and guidance before and throughout my research. His enthusiasm, optimism and availability have been critical to the completion of this work despite the challenges. To my second supervisor Mr. Nick Riley whose persistent constructive criticism and suggestions always helped to point me in the right direction. My special gratitude goes to my sponsor, the Petroleum Technology Development Fund (PTDF) of Nigeria for providing me with an exceptional full scholarship, one of the best in the world, without which this research would not have been possible. I’m also grateful to the humanitarian demining research teams at the University of Manchester led by Professors Anthony Peyton and William Lionheart for their support. I thank the Computer Simulation Technology (CST) GmbH technical support for the CST STUDIO SUITE. I will not forget the administrative support of Jo Arnett and Glen Jack in processing my numerous requests, expense claims and other academic requisitions. To my colleagues, my laboratory mate and other PhD students in the Electronic Engineering Department for their moral support and encouragement. I’m very thankful to Pastor Isaac Aleshinloye and the Amazing Grace Chapel, Hull family for providing me with a place of spiritual support, friendship, opportunity for community service and a place to spend my time outside of academic study productively. I need to thank my parents, Mr. and Mrs. S. S. Sule for their immense support in many ways and encouragement throughout my studies generally and my postgraduate study at the University of Hull. Special thanks also go to my sisters Suninba and Saundiba, as well as Aisha and my brother Saminu, who have also provided the motivation for me to succeed. I won’t forget my extended family members, uncles, aunties and cousins who I can’t mention by name but who have all contributed to my achievements in one way or the other. Lastly, I want to appreciate my dear wife Ndepana for her encouragement and support and my wonderful daughter Sarah Azaria for arriving and inspiring me in the final stages of the project. I cannot conclude this acknowledgement without a declaration of gratitude and praise to God who I firmly believe in and who is the source of my life and my driving force. 2 Abstract This thesis presents an empirical study comparing the ability of multi-static and bi-static, handheld, ground penetrating radar (GPR) systems, using full wave inversion (FWI), to determine the properties of buried anti-personnel (AP) landmines. A major problem associated with humanitarian demining is the occurrence of many false positives during clearance operations. Therefore, a reduction of the false alarm rate (FAR) and/or increasing the probability of detection (POD) is a key research and technical objective. Sensor fusion has emerged as a technique that promises to significantly enhance landmine detection. This study considers a handheld, combined metal detector (MD) and GPR device, and quantifies the advantages of the use of antenna arrays. During demining operations with such systems, possible targets are detected using the MD and further categorised using the GPR, possibly excluding false positives. A system using FWI imaging techniques to estimate the subsurface parameters is considered in this work. A previous study of multi-static GPR FWI used simplistic, 2D far-field propagation models, despite the targets being 3D and within the near field. This novel study uses full 3D electromagnetic (EM) wave simulation of the antenna arrays and propagation through the air and ground. Full EM simulation allows the sensitivity of radio measurements to landmine characteristics to be determined. The number and configuration of antenna elements are very important and must be optimised, contrary to the 2D sensitivity studies in (Watson, Lionheart 2014, Watson 2016) which conclude that the degree (number of elements) of the multi-static system is not critical. A novel sensitivity analysis for tilted handheld GPR antennas is used to demonstrate the positive impact of tilted antenna orientation on detection performance. A time domain GPR and A-scan data, consistent with a commercial handheld system, the MINEHOUND, is used throughout the simulated experiments which are based on synthetic GPR measurements. Finally, this thesis introduces a novel method of optimising the FWI solution through feature extraction or estimation of the internal air void typically present in pressure activated mines, to distinguish mines from non-mine targets and reduce the incidence of false positives. 3 Table of Contents Acknowledgments ............................................................................................................. 2 Abstract ............................................................................................................................. 3 Table of Contents .............................................................................................................. 4 List of Figures and Tables .................................................................................................. 7 List of Acronyms .............................................................................................................. 10 Chapter 1 Introduction .............................................................................................. 12 1.1 The Humanitarian Demining Challenge ........................................................... 12 1.2 Landmine Detection Methods .......................................................................... 14 1.2.1 MD and GPR Sensor Fusion ....................................................................... 15 1.2.2 GPR Signal Processing ............................................................................... 18 1.3 Research Motivation and Background ............................................................. 20 1.4 Thesis Aim and Objectives ................................................................................ 23 1.5 Thesis Methodology ......................................................................................... 25 1.6 Thesis Contributions ......................................................................................... 26 1.7 Publications ...................................................................................................... 28 1.8 Thesis Structure ................................................................................................ 29 Chapter 2 Landmine Detection Using Handheld GPR ................................................ 31 2.1 Introduction ...................................................................................................... 31 2.2 Landmine Detection Methods .......................................................................... 31 2.2.1 Nonlinear Seismo-acoustic Technique ...................................................... 31 2.2.2 Gamma Rays .............................................................................................. 32 2.2.3 Passive Infrared (IR) Polarization .............................................................. 33 2.2.4 Active Thermal Sensing ............................................................................. 33 2.2.5 Nuclear Quadrupole Resonance ............................................................... 34 2.2.6 Ultra-wideband (UWB) Radar ................................................................... 35 2.2.7 Electromagnetic Induction (EMI) .............................................................. 35 2.2.8 Summary ................................................................................................... 36 2.3 Sensor Fusion.................................................................................................... 37 4 2.3.1 Types of Sensor Fusion .............................................................................. 37 2.3.2 Current Handheld MD and GPR Systems .................................................. 39 2.4 GPR System Overview ...................................................................................... 41 2.4.1 Physics of Operation ................................................................................. 43 2.4.2 Range and Path Loss ................................................................................. 44 2.4.3 System Loop Gain ...................................................................................... 45 2.4.4 Velocity of Propagation ............................................................................. 46 2.4.5 Depth and Plan Resolution ........................................................................ 47 2.4.6 Clutter........................................................................................................ 48 2.4.7 Antennas for GPR ...................................................................................... 48 2.4.8 System Operating Frequency and Domain ............................................... 49 2.4.9 Signal Processing ....................................................................................... 50 2.5 Conclusion .......................................................................................................