Corrosion under insulation (CUI) is a common cause of pipeline failure in the oil and gas industry. Its detection with conventional inspection techniques is challenging due to the presence of the insulation layer and a protective metallic cladding that prevent direct access to the pipe surface. Currently, several techniques are being developed to detect sections of wet insulation since water is a necessary precursor to corrosion. Among these, guided microwave testing (GMT) has been proposed as a cost-effective approach to screen an extended length of pipeline. The sensitivity of GMT is dependent on the strength of microwave reflections from wet volumes of insulation and has been demonstrated in the case of relatively abrupt transitions from dry to fully saturated insulation. This thesis investigates the performance of GMT in the presence of saturation gradients characterizing more gradual transitions. Due to the lack of detailed field observations about realistic saturation paths, the thesis proposes model experiments of saturation dynamics in a highly permeable material in a first attempt to determine saturation profiles resulting from capillary rise and seepage. The experimental observations reveal that the extent of the transition region is typically much smaller than the wavelength of the probing microwave signal and therefore has a limited effect on the amplitude of the reflected signal. i iii Table of Contents List of Figures Chapter I Introduction Chapter II Mechanisms of water absorption Chapter III Background of microwave excitation and propagation in coaxial waveguides