Assessment of fractures in some columns inside the crypt of the Cattedrale di Otranto using integrated geophysical methods

We investigate the problem of internal fracturing of the columns of a building of cultural importance (the Crypt of the ‘‘Cattedrale di Otranto’’, Apulia, Italy), using a multi-disciplinary approach, integrating different non-destructive techniques (Microclimatic and Geophysical). In fact historic buildings, over centuries, may accumulate a high concentration of water and/or moisture that increases significantly the deterioration rate, diminishing the mechanical resistance. It is well known that the distribution of moisture within stone strongly depends on ambient conditions, i.e. temperature and relative humidity. No suitable environmental condition in the Crypt and the presence of wet buried structures in the ground involved in the process of deterioration have been indeed assessed in previous surveys. Actually we perform a ground penetrating radar (GPR) investigation, in reflection mode, in order to reveal the existence of fractures inside the columns, because of the important rising of water from pavement. The reconstruction of the fracture distribution is fundamental to plan the safeguard and the restoration of the columns themselves and also the stability of the Crypt and of the uppermost Cathedral. Such survey was performed on seven columns using GPR technique. Moreover the electrical resistivity tomography (ERT) and GPR traveltime tomography (TT) methods were applied on the most damaged column. The results obtained with these different methodologies are in good agreement, however, the 2D electrical resistivity tomography seems to be less resolutive than GPR. While the GPR traveltime tomography is more time expensive than GPR reflection mode method. The GPR method, in reflection mode, allows to reconstruct the main fractures inside the columns also in virtue of its penetration power. We think that the joining of different methodologies permits to better understand the complex physical mechanisms in the phenomenon of deterioration of the cultural heritage, in order to achieve useful information for possible, future, restoration works.