GNGTS 2014 - Atti del 33° Convegno Nazionale

of the above described large data base. It is expected to have more than 2·10 6 first arrivals recorded both offshore and on-land. The algorithm used for the inversion is a modification of the well known ATOM-3D program in which natural seismicity and active signals will be used simultaneously. The first step is to apply an automatic first arrival picking procedure using spectral and temporal characteristics of the signals. The 3-D inversion will be performed in three phases: a) general image of the region under study using large lattices and only with the inversion of the active signals; b) inclusion on passive seismicity to constrain better the deeper portion of the region; c) a high definition seismic tomography of the Etna area using both active and passive seismicity and small cells. Multi-Channel Seismic (MCS): during this phase of the experiment has been used a hydrophone cable (streamer, SENTINEL Sercel) 3 km long with 240 channels spaced at 12.5 m (each channel contained a group of 8 hydrophone) for record signals produced by airgun shots. The streamer was maintained at a depth of about 10 m from the sea level for an optimal signals acquisition. Also in this case, the source of the acoustic pulses was an array of two batteries of airguns, each having 8 guns (Sercel, G-GUN II) for a total volume of 4,340 cubic inches (cubic inches - us) generated at about 2000 p.s.i. pressure; the guns were kept at a depth of about 7.5 m below the sea level. More than 16,500 shots were generated in this phase. The source frequency bandwidth was designed to ensure a better penetration in the subsoil. During the MCS phase, the airgun array was fired each 37.5 m, i.e., ~ each 20 seconds (considering an average vessel speed of 3.5 knots during the acquisition). This field setup allowed a theoretical maximum fold of 40 traces to each CDP location. Overall, 18 MCS profiles have been acquired for a total length of ~ 620 km. We show in Fig. 3 about 4 km of profile T-11 (see location in Fig. 1) after a preliminary CDP stack and post-stack depth migration, with the aim of providing the reader with an idea of the overall data quality. CDP data processing included geometry installation, and spiking/ predictive deconvolution followed by band-pass filtering. Semblance based velocity analysis methods were used were used to define a 2D background stacking velocity model for CDP ensemble stack. The stacking velocity model was then smoothed and converted in interval velocity to provide an interval velocity model for post-stack Kirchhoff depth migration. Fig. 2 – a) The vessel Sarmiento de Gamboa; b) activity of discharge of one OBS; c) source of the acoustic pulses generated by the array of two batteries of airguns. GNGTS 2014 S essione 1.3 221