GNGTS 2014 - Atti del 33° Convegno Nazionale

surveys (such as magnetic measurements) and to support the Sarmiento de Gamboa vessel during the complex survey with a 3 km long multi-channel seismic streamer. Field operations started one week before the beginning of the experiment, with deployment of 98 seismic stations (80 short period and 18 BB stations provided by the GFZ-Potsdam) on land, supplementing the 70 INGV permanent stations, leading to a total of 168 seismic stations (Fig. 1). Moreover, 27 OBSs (Ocean Bottom Seismometers) were deployed in the Ionian and Tyrrhenian seas (22 in the Ionian Sea and 5 in the Tyrrhenian Sea; Figs. 2,3). 15 OBS/H (OBS with hydrophone) were positioned by the Sarmiento de Gamboa vessel, while 10 OBS-SP (Short Period OBS) and 2 OBS-BB/H (broadband seismometer with hydrophone) by Galatea vessel. The network of OBSs was recovered at the end of the experiment with the exception of the two OBS-BB/H, which will be recovered in November 2014 by the Aegea vessel. These two OBSs will record natural seismicity for a longer period of time. For the active-source imaging experiment two seismic exploration techniques have been used during the cruise of the Sarmiento de Gamboa vessel, in order to carrier out Wide Angle Seismic (WAS) and Multi-Channel Seismic (MCS) surveys along the programmed lines. Wide Angle Seismic (WAS): during this phase of the experiment about 9,500 shots were carried out along a total route of approximately 2560 km. The source of the acoustic pulses consisted of an array of two batteries of airguns, each having 8 guns (Sercel, G-GUN II) for a total volume of 5,200 cubic inches generated at about 2000 p.s.i. (1.382 × 10 7 Pa) pressure. The guns were kept at a depth of about 15 m below the sea level and the shots were carried every 90 seconds. This shot interval was used in order to avoid records contamination by previous-shot induced noise. The configuration of airguns has been opportunely chosen, through previous simulations (performed by UTM-CSIC in Barcelona) to obtain lower frequencies and to allow a greater penetration. The 3-D velocity structure of Mt Etna and surrounding areas will be performed on the base 220 GNGTS 2014 S essione 1.3 Fig. 1 – Lines of shots: a) Wide Angle Seismic (WAS); b) Multi-Channel Seismic (MCS). In b) the processed transect of Fig. 3 is highlighted.