GNGTS 2022 - Atti del 40° Convegno Nazionale

GNGTS 2022 Sessione 3.2 443 we realized that many of the cavities had been made inaccessible, the cause was very often linked to past earthquakes (1997-2016). The geophysical surveys consisting of non-invasive ERT and GPR data were combined with geological and stratigraphical analysis of underground sediments using well logs. For the GPR, two different antennas (200MHz, 400MHz) integrated to the SIR 4000 system (GSSI) were used. The GPR techniques was used to check and verify if the ERT surveys was capable into cavities detection. In the case of the electrical tomography (Wenner-Schlumberger array, 64m long, 32 electrodes, 2m electrode spacing), unexpensive flat-base electrodes in combination of an electroconductive gel (Vasconez-Maza et al. , 2020) were implemented to avoid pavement damages and, at the same time, reducing its electrical resistance. In addition to the previous information, the historical records and oral communications were combined into a GIS database and contrasted with historical maps of the village. Further analysis consisted in a comparison between the resulted map of the underground cavities and the distribution of buildings damaged by the 2016/17 seismic sequence of Central Italy. This method allowed us to detect a complex cavity network under the town centre (Fig. 1). Fig. 1 - Map of historical cavities distribution, verified and no longer accessible in relation with the survey line and some detailed photo of the C1 cavity. Results. For the interpretation phase, we combined both geophysical surveys and geological data from wells with cavities map (Fig. 2). The cavities were divided into historical, inaccessible, and verified. These hypogea were excavated between 3 to 6 meters depth with the main purpose of storing water and food. In the case of the ERT, it was not possible to completely define the geometry of the cavities. In the ERT section, there is a highly resistivity area in coincidence of the cavity C3 location, whereas the cavity C1 is not so evident. However, the presence of these structures is related to an increment of the resistivity in the adjacent area hosting the cavities and a thinning of the low resistivity layers above (Fig. 2a). These results can be easily misinterpreted as a thinning of soil or weathered sediments. In figure 2b we can observe the GPR survey, carried out whit a 200MHz antenna, 23m long and parallel to the ERT one. In the case of GPR, we found that the upper part first 2-3 meters is the underground services (water, electricity) may create significant disturbance. By