Detection of fractures from GPR data: the case history of the Cathedral of Otranto

Ground penetrating radar (GPR) surveys were carried out in the preliminary stage of a project of structural monitoring of the 42 columns inside the crypt of the 'Cattedrale di Otranto' (Lecce, Italy). Detailed knowledge of the structure of the internal columns was a key reason for their restoration since they had several points of deterioration (i.e. fractures). The aim of this work was to test the reliability of both a standard GPR processing (in a time domain) and a linear inverse scattering algorithm (in a frequency domain) in order to detect and achieve information on the damaged zones inside the columns. First, the reliability of both the techniques was preliminarily assessed by processing synthetic data resembling the measurement conditions of the experimental cases. Then the experimental data were processed by means of the two techniques. Our comparative analysis, for both the numerical and the experimental analyses, indicates that the linear inverse scattering approach is better suited for the detection of local fractures compared to the classical time-domain processing, while increasing the computational cost at a reasonable rate.