Contribution of Ground Penetrating Radar and Electrical Resistivity Tomography to identify the cavity and fractures under the main Church in Botrugno (Lecce, Italy)

Abstract

The “Chiesa Matrice” is the main Church of Botrugno village, located about 70 km south-east of Lecce (Italy). The church was built between the end of 1500 BC and the beginning of 1600 BC. In the June 2002 a restoration work has been started to secure the church which suffers from weakened subsoil. Therefore, the structure of the subsoil of the church had to be outlined. Ground Penetrating Radar (GPR) and 3D Electrical Resistivity Tomography (ERT) geophysical surveys have been integrated to identify the possible presence of voids and fractures in the subsoil. At accessible spaces around the church only GPR profiles were obtained. GPR analysis has revealed the high electromagnetic (EM) energy attenuation (almost surely due to the presence of water filling the subsoil and the pavement). Despite the EM energy attenuation the processing and the interpretation of the GPR data point out the presence of voids and fractures. 3D ERT surveys, performed using an area of 10 × 15 m, confirmed the presence of water in the subsoil and allowed to locate a cavity at about 2 m in depth. Even if the presence of several scaffolds within the building does not allowed to collect the GPR data in order to get a 3D visualization, the comparison between the map of the anomalies obtained with GPR measurements and the 3D ERT, confirmed the effectiveness of the geophysical application presented in this paper and allowed to assess the subsoil conditions of the sacred building.