Non-contact intrawall penetrating radar for heritage survey: the search of the 'Battle of Anghiari' by Leonardo da Vinci

In this paper a high-frequency large-bandwidth synthetic aperture penetrating radar intended for the inspection of masonry walls, and its application to intrawall investigation of an important Heritage structure in Florence, Italy, are described. The radar system has been conceived and designed for non-contact operation, in particular for allowing walls covered by paintings to be inspected avoiding damages to the surface artworks. The radar signal was a Continuous-Wave Step-Frequency (CW-SF) waveform, sampling a 4 GHz bandwidth at 10 GHz center frequency, thus providing relatively high resolution images of the investigated structures. Cross-range resolution is provided by applying a synthetic aperture approach, obtained by mechanically moving the radar antenna along a 2 m length aperture. In order to assess the radar performances and its detection capability, laboratory tests on masonry facilities were preliminarily performed. Finally, an extensive measurement campaign was carried out on a famous 16th century structure: the 'Hall of 500', in Palazzo Vecchio (Old Palace), in Florence (Italy). This investigation was aimed at finding evidence of possible discontinuities in the masonry walls, where fragments of the famous fresco the 'Battle of Anghiari' by Leonardo da Vinci could be hidden.