GNGTS 2017 - 36° Convegno Nazionale

648 GNGTS 2017 S essione 3.2 This represents a potential risk for the tunnel excavation in areas where the faults positions are either unknown or poorly mapped; 2) The presence of the Blue Clay formation, that it would be preferable to avoid during the tunneling because of its softness and its generally poor mechanic characteristics. Where possible, in fact, the tunnel alignment should be developed below the impermeable Blue Clay layer, and extend as far as possible within the Globigerina and Lower Coralline Limestone formations, that are harder and where karsts formations are understood to be scarce Plan of work and methodologies. The first phase of the geophysical investigation took place in autumn 2016, and comprised the following methodologies: 1. multibeam echosounding, to generate a detailed map of seafloor morphology and composition, and to identify features of geologic and geomorphic interest; a 500 kHz Reson Seabat multibeam was used to produce a centimeter scale 3D map of the seafloor in the channel between Malta and Gozo and surrounding the island of Comino; 2. high-resolution 2D sub-bottom profiling, to reveal shallow geology beneath the seabed, with particular emphasis on the illumination of any faults displacing most recent sediments; a Ultra High Resolution boomer seismic source was used together with a single channel ministramer allowing a fine scale (between 20 and 40 cm) imaging of the sub-seafloor up to a few tens of meters; 3. multichannel reflection survey, to characterize the nature and stratigraphy of the bedrock geology and to identify any faults that intersect the area. The main concern was in this case to identify the interface between the Blue Clay and the underlying rock, at a depth ranging from 300 m to 400 m below the seafloor. The seismic source was a 60 cu.in Sercel Mini GI-gun operating in Harmonic Mode (30 G + 30 I). With this source, feeded by three 320 l/min Coltri compressors operating at 140 atm., and activated every 18.75 m, a penetration of some hundreds of meters was achieved. The data were collected by means of a 96 channels, 300 m long Geometrics GeoEel liquid state digital streamer, with a trace distance of 3.125 m. Both the gun and the streamer were towed at a depth of 1.5 m to limit the ghost effect and keep the spectra as broad as possible. A second phase, that will take place in the winter 2017/2018, will involve offshore drilling to calibrate the geophysical data and obtain samples for geologic, geotechnic and hydraulic characterization over four sites, that will be located on analyses of the geophysical data. During this phase, borehole data as well as onshore seismic refraction/reflection will be carried out. Ongoing activities and preliminary results. At the time being, the geophysical data are still under processing and interpretation; in particular, an integrated data analyses is being performed on the three different scale and resolution datasets. This is allowing to correlate the geological structures directly observed onshore to those imaged by echosounding and seismic methods. Both the top and the bottom of the Blue Clay formation has been well imaged by means of the multichannel seismic, even if the data seem to suggest a geological setting more complex than expected. Conclusions. Because the data are still under processing and integration, no conclusion have or can be drawn from what has been so far analysed. Furthermore, seismic data need borehole data for validation, calibration and time to depth conversion. Nevertheless, from the preliminary results it can be stated that the complete geophysical characterization of the site has been performed and this will constitute a valuable tool for the support to the assessment of the risks associated to the realization of this sub-sea infrastructure.