GNGTS 2014 - Atti del 33° Convegno Nazionale

GNGTS 2014 S essione 3.1 91 matrix and from 11.5 m to the bottom (19 m) the well shows the Campanian Ignimbrite formation. The NYT and CI fm. outcrops are associated to a high number of underground excavations (ancient aqueducts, tunnels, tombs, etc.). Good physical and mechanical properties of these tufa formations, wide distribution of these deposits, at depths and easy access, together with limited means of transportation available in the past centuries represented additional reasons that led ancient the communities not only to use the material quarried but also to take advantage of the excavations carried out accordingly (Vallario, 1992). ��� ���������� ����� ���� ��� ���� ��� The artificial caves were dug into the layers of pumice and tuff to reduce the cost of construction of the buildings, until the beginning of 1900. The building material was extracted widely ​from underground vertical shafts that reach the layers of loose pumice and the underlying tuff. Sub-horizontal tunnels were instead excavated within the layers of pumice. Once reached the tufa through vertical wells, sub- horizontal cavities of varying height and width (~ 4-8 m) were dug. After the extraction of the building material, voids were usually used as foodstuffs deposits and wine cellars. The vertical wells of the cavities, located outside of the towns such as the study area, usually were closed; therefore the knowledge about their presence in the subsoil was lost. The loss of maintenance of the cavities and water infiltration within them can result in instantaneous or progressive collapse. Data acquisition and method. The investigated area, of ~10,000 m 2 , is a private property and its aerial photography is shown in Figs. 1a and 1b. Some surface evidences of sinkholes are clearly visible from aerial photography (red arrows in Fig. 1b and Fig.1c). For instance one the larger sinkholes in the area (red dashed line in Fig. 1b) has about 50 m in diameter and 20 m in depth. The analysis of visible evidences of sinkholes, together with the measurement of an accessible cavity, has allowed us to define the target of the investigation, namely the search of voids with lateral dimensions of about 10 m and heights of about 8-9 m located within 20 m of depth. In order to determine the optimal acquisition parameters and to test the accuracy of tomographic technique for the identification of these cavities into geologic contest investigated, we used the results of seismic refraction Fig. 1 – a) Aerial photography with location of the survey area by Google Earth (yellow dotted line); b) survey area on Google Earth aerial photography (white dashed line) with some surface evidences of sinkhole (red arrows) and larger sinkhole in the area (red dashed line); c) photo of some surface evidences of sink-hole.