Preliminary study of embedded structural anomalies in architectural structures by microwave subsurface tomography

A non-destructive technique (NDT) based on microwave reflectometry has been applied to perform subsurface tomography images. The localization in depth of anomalies in architectural structures such as, for example, inhomogeneity in the masonry texture (cavity of air, metals, and humidity) or on timber structure by means of NDT approach is crucial not only in the diagnostics of the Cultural Heritage, but in general for all civil industry. The target of our research is the development of a robust and user-friendly system to investigate the materials used in architecture until 20 cm in depth, with a resolution less than 1 cm. The system consists by a vector network analyzer and an wide band antenna (ridged antenna) operating in the range 1.5-6 GHz. The measurement of the reflection coefficient variation (amplitude and phase) due to the dielectric dis-homogeneity into the materials under investigation is acquired. The equivalent time-domain response is reconstructed using an Inverse Fourier Transform of the baseband signal and by a simple model based on TEM approach is determined the constrains on the size of the anomaly (maximum resolution) and on its position into the host medium (maximum non unambiguous range). The material can be investigated by scanning-step by step-the surface by the probe antenna. For each step is obtained the reflection coefficient in the frequency range, and subsequently transformed in the time-domain by out-line processing. Thanks to the penetration of the used microwave radiation, it is possible to sectioning the material under test in slices transversal to the surface up to 20 cm in depth. The interdistance between two consecutive slices is related to the distance between the two corresponding scanning lines. The performance of the proposed technique together with its resolution limits are discussed as regards with other approach proposed in literature. The results on architectural structure are presented. The potentiality of the system as diagnostic tool based on NDT approach for the framework of Cultural Heritage will be discussed together with its limits.