| The problem of tunnel detection has become a critical task for the protection of national borders, sensitive areas, as well as civil applications, e.g. mining industry, search and rescue, environmental engineering, and exploration geophysics. To be effective, the detection technology must provide the decision maker with persistent and actionable information of the underground scene. Furthermore, the set up of the equipment must involve minimal human intervention, and applicable over a relatively wide area, from shallow region, to hundreds of meters of depth.To accomplish these objectives, we introduced a new approach, named RF tomography, which is based on a set of low-cost, randomly deployed, earth penetrating transmitters and receivers. Transmitters radiate coherent signals below ground that impinge upon dielectric/conducting anomalies, thus exciting a scattered field. Receivers collect this scattered field, and relay this information to a base station. Using the principles of inverse scattering, we developed a scheme for imaging underground anomalies from scattered data, thus detecting, locating, and imaging underground voids. Using RF Tomography, the spectral content of the waveform is significantly reduced, and sub-wavelength resolution and high quality images are easily achievable.This thesis is developed as follows. (1) Introduction and definition of a set of criteria for tunnel detection. (2) Background information regarding state-of-the-art techniques employed for underground imaging, corroborated with description of advantages and disadvantages. (3) Description of the principles of RF tomography, underlining the systems engineering aspect. (4) Construction of a linear model of the electromagnetic wave propagation into the ground, by selecting suitable dyadic Green's functions and approximations on the scattered field. (5) Derivation of several image reconstruction methods based on the characteristics of the ground, the geometry, and the desired trade-off between accuracy and speed. (6) Derivation of an inversion procedure purely based on Fourier relations. (7) Proof of the actual feasibility, reliability, and effectiveness of an RF tomographic system via simulations. |
