GNGTS 2018 - 37° Convegno Nazionale

678 GNGTS 2018 S essione 3.2 Multibeam echosounder, Single-channel boomer seismic reflection and multi-channel seismic reflection surveys across study area were carried out between October and December 2016. The main geophysical product was the offshore reflection seismic sections, natively represents the underground reflectors (layers) depth position in time (two-way time). Obviously the position of the reflectors to be useful for geotechnical application should be expressed in depth. The conversion from time to depth requires velocity measurement of the sediments and rock that are present in the study area. Surface and borehole seismic data acquisition allow to measurement on-field rock/sediment velocities. Seismic refraction is the main method to obtain velocities information from the surface. This approach is widely adopted for near-surface/medium depth investigations (i.e., portals rock characterization, low coverage tunnel sectors, etc.). In the study area, the presence of the Blue Clay formation between two more competent formations (i.e., the Upper Coralline Limestone Formation at the top and the Globigerina Limestone formation at the bottom) limits the applicability of the refraction method because the velocity inversion. To overcome this limitation borehole measurements were planned. Seismic borehole technique consists of sending a seismic signal from a surface source down to sensors (geophone and/or hydrophone) located into a borehole. The one-way traveltime from surface to depth is obtained by picking the times of first arrival on the observed records. Repeated measurement are taken at different depth along the borehole in order reconstruct the velocity function along the borehole itself. The borehole and surface seismic survey took place in February 2018 after the borehole drilling completion. P- and S-wave vertical seismic profiles (VSP) were acquired in two boreholes located close the sea in the north cost of Malta (BH3 - Marfa Road, Ċirkewwa) and in the south coast of Gozo (BH6 - Zewwieqa Road, Għajnsielem). These boreholes were cased and well-grouted to ensure good seismic coupling between the downhole geophones and the surrounding rock. P-wave VSP data were acquired by an accelerated weight drop (PWD, about 74 Kg) and hydrophone streamer (24 channels with 5 m inter-trace distance) or 3C borehole geophone, while SH-wave borehole data by a 3C geophone (15 Hz) and a MiniVib IVI T-2500 seismic source (sweep: 12s, 10 – 120 Hz, force 1200 Lb). In addition to the borehole data two refraction seismic lines has been acquired. The aim of this survey was the measurement the P- and SH- velocity nearby the planned tunnel portals for geotechnical purposes. Two sites were selected: the first one (MA_01) on the Malta Island nearby the borehole BH9, the second one (GO_01) on the Gozo Island nearby the borehole BH8. In both lines was utilized 24 channels spaced 10 m and connected with 10 Hz vertical geophone and 10 Hz horizontal geophone for P- and SH-wave acquisition, respectively. As for borehole data acquisition an accelerated weight drop as P-wave seismic source and Minivib in SH mode for the shear wave acquisition were adopted. Borehole data allow to compute the seismic velocities of the geological formations present in the study area. Velocity versus depth information were used to convert time to depth in order to assign reliable depths to the events seen in the offshore seismic reflection profiles. Further borehole data give also insight into seismic wave propagation/reflectivity and provide a guide for interpretation of the offshore seismic data. References Galea P. (2007). Seismic history of the Maltese islands and considerations on seismic risk. Annals of Geophysics. 50. pp. 725–740. Petronio L., Facchin L., Zgur F., Gordini E., Baradello L., Cotterle D., Romeo R., Forlin E., Visnovic G., Deponte M., Affatato A., Coren F., Galea P., Pace S., Micaleff A., Spatola D., (2017). The Malta-Gozo tunnel project: preliminary geophysical offshore investigations. GNGTS, Trieste, S 3.2, pag. 647 – 648.