GNGTS 2014 - Atti del 33° Convegno Nazionale

90 GNGTS 2014 S essione 3.1 Application of seismic refraction tomography to detect anthropogenic buried cavities in province of Naples (Campanian Plain, Italy) S. Maraio 1 , P.P.G. Bruno 2 , G. Testa 3 , P. Tedesco 3 , G. Izzo 4 1 Dipartimento di Scienze della Terra e Geologico-Ambientali, Università di Bologna, Italy 2 INGV, Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma 2, Italy 3 Dipartimento di Scienze e Tecnologie, Università degli studi del Sannio, Benevento, Italy 4 Studioizzo Geologia, GIS & Ambiente, Airola (BN), Italy Introduction. The presence of near-surface buried cavities often ����� ������� ������� poses serious hazards for human safety and infrastructures. Underground voids develop naturally in karst territories while are produced by human activities the cavities such as underground mines, tunnels, buried passage-ways, etcetera (Del Prete, 2008). The sudden collapse of the vault of these cavities is a recurring problem in landscape management and it is a main origin of sinkholes. If the collapses originate from man-made cavities, these phenomena can be classified as “anthropogenic sinkholes” (Guarino et al. , 2012). ���������� �������������� �� ����� ����������� ����� �� �� Therefore, identification of these underground voids is an essential prerequisite for both minimization of risks arising from their presence, and for optimal planning of land use. The geophysics technologies are often the most convenient way to gather information aimed at the characterization of the subsurface and the search for underground voids. Up-to-date different geophysical prospecting methods have been tested for underground voids detection and their success is strongly influenced by local conditions and their ability in terms of penetration and resolution. ��� ���� ������������ �� ������� �������� ����� ����������� The main difficulties in finding cavities using geophysical methods are generally related to the size and shape of the voids, and the subsurface lateral velocity variations that can create responses similar to those of underground voids (Riddle et al. , 2010). ��� ������������ �� ��������� ������ ��������� ������ ������� The applications on synthetic models (Mandell, 2005a; Sheehan et al ., 2005a, ������� �� 2005b), in laboratory experiments on physical models (Grandjean and Leparoux, 2004; Grandjean, 2006) and the application on real data (Sheehan et al. , 2005c; Cardarelli et al., 2010), have shown that the seismic refraction tomography represents a powerful tool for the detection of subsurface cavities. It has the advantages of being a non-invasive technique, of having a good penetrating power while maintaining a good resolution, and of allowing a three-dimensional reconstruction of the underground cavities. In this work we used a densely spaced grid of 2D seismic refraction profiles processed with first-arrival traveltime tomography to detect man-made cavities and to create a three-dimensional model of the subsurface in Casamarciano, in province of Naples. After modelling, some core-drillings have been made to compare and validate the seismic results. Geologic contest and underground cavities. The area of present work is located in the central sector of the Campanian Plain. This latter occupies the bottom of a large structural depression (graben) oriented in NW-SE direction and bounded by carbonate ridges of Mt Massico to NW, of M.ti Lattari to SE and M.ti of Caserta and Partenio to NE and to E. The Campanian Plain graben ����� �� ���� �� ��� ���� �������� �� � ������ �� �������� �������� ���� began to form in the Late Pliocene as a result of tectonic activity that characterized the Apennines orogenic uplift (Bartole et al. , 1984). The area was affected by intense volcanic activity and the graben was filled by the products of the nearby volcanism, i.e. pyroclastic deposits and lava, and by alluvial and marine deposits. Generally ��� ������������� the stratigraphic series of the study area consist of volcanic deposits dating back to Upper Pleistocene - Holocene, such as the Neapolitan Yellow Tuff fm. (NYT: 12 ky b.p.) and the Campanian Ignimbrite fm. (CI; 37 ky b.p.) (Orsi et al. , 1992, 1996). The stratigraphic series is completed to the top by loose pyroclastic deposits related to the subsequent post-Yellow Tuff activity (<10.5 ky b.p.) (Guarino et al., 2012; Di Vito et al., 2008; De Vivo et al., 2001; Bellucci, 1994). Part of the survey area was investigated by four core drillings that reveal the stratigraphy down to depth of about 19 m. The stratigraphic series of the area consists of the following lithotypes (from top to bottom): pyroclastic deposits rich in yellow pumices down to depth of 6.5 m; between 6.5 and 11.5 m in depth there are alluvial gravel deposits in a grey pyroclastic