GNGTS 2022 - Atti del 40° Convegno Nazionale

408 GNGTS 2022 Sessione 3.2 scans – 3 external and 8 internal-in a Traverse mode (Fig. 3a), aligned through survey prism in order to optimize the data acquisition of the whole building. This method allowed the strict placement of the external and the internal data of the building, avoiding cloud alignment operation in the post-processing phase. Ground Penetrating Radar. The GPR survey inside the church was aimed at collecting data about the presence of unknown structures, chambers, and tombs under the floors of the church and the crypts. For this research, the GPR survey was performed using a RIS MF HI- MOD system (IDS GeoRadar s.r.l.), equipped with an array of multi-frequency of two antennas of 600 MHz and 200 MHz. The antennas have been selected according to the resolution and depth of investigation, therefore we used both antennas simultaneously, one with 600 MHz to get a good superficial resolution and another one with 200 MHz to get a suitable depth. The measurements were carried out along parallel profiles through the nave of the church; in all, 15 profiles have been acquired and spaced 0.5 m, the investigated area measuring 7.5x25.5 m. Data were collected using the reflection technique. Data elaboration was performed with Reflex software. To get a 3D model of reflectivity of subsoil, we used a time-slice technique, which consists in the construction of horizontal sections at certain reflection times (time-slices) or depths (depth-slices). The first two meters of the 3D model were obtained using the data acquired with the 600 MHz antenna, while up to four meters deep, the data acquired with the 200 MHz antenna were considered. Fig. 3d shows a view from the top of the reflectivity model superimposed on the plan. It can be noted that the locations of the high reflectivity anomalies often coincide with those of the tombstones, indicating the presence of numerous burials immediately below the paved floor of the church. Infrared Thermography. The thermographic survey of the Mother Church was carried out by using a Flir T 530 thermal imaging camera. The measurement scale used is between -20 ° C and 120 ° C. The thermographic analysis was carried out on the wall surface as it appeared, in conditions of normal thermal operation. The investigated walls of the church are the southern one, located between the bell tower and the Crocifisso altar (Fig.3 a), and the western one corresponding to the presbytery (Fig.3a-c). In the Mother Church, the thermographic images captured the most common lithotypes used in the area (lime, mortar and volcanic rock). The thermographic signal detected a significant difference of the temperature among materials. For this reason, it was not only possible to analyze wall textures covered by the plaster, but also to discriminate among the different lithotypes used in the masonry. Furthermore, the images highlighted the presence of a probable wall structure under the plaster. This structure was identified due to the different thermal response between the filling and the surrounding masonry. Conclusions. By analysing the lava field structure, it can be presumed that the church suffered twomain phases of lava flow impact. The first onewhen the basal lava flow surrounded the northern and western part of the edifice with a lava front some dozens meters thick; inhabitants, aware of the risk, closed the southern door of the presbytery with stones. At a first moment, the northern and western walls were weakened by the lava thrust, and shortly after they collapsed. The impact of the second phase is related to the evolution of the lava flow field that gradually covered the site (Fig. 2c). Lava gradually filled the church’s nave destroying the wooden remains but preserving the paved floor, and cooked the lime of the altars. Looking at the internal part of the church through the results of GPR, IRT and TLS surveys, it was possible to identify some modifications suffered by the edifice before its destruction.