GNGTS 2018 - 37° Convegno Nazionale

GNGTS 2018 S essione 3.2 649 320 l/min at 140 bar each, and was activated in “Harmonic mode” every 18.75m or 15.625m when possible. The data were collected by a 96 channel 300 m long Geometrics GeoEel digital streamer, with a trace distance of 3.125m. Both the source and the seismic cable were towed at a depth of 1.5m, to keep the spectra as broad as possible. During the Multi-Channel Seismic acquisition, a realtime QC was performed onboard. After survey completion the dataset was processed in OGS facilities. The processing sequence was briefly as follow: editing; geometry assignment; water bottom picking; surface-related multiple elimination (SRME); amplitude recovery; predictive deconvolution; velocity analysis; normal moveout correction (NMO), manual stretch mute and Stack. The “Post-stack” processing consisted in the following steps: F-X deconvolution; time varying bandpass filtering; Kirchhoff time migration followed by amplitude balancing. The second phase of the survey took place in winter 2017-2018 after borehole drilling, completing the dataset with onshore seismic data, and borehole seismic (VSP) for validation, calibration and time to depth conversion. In two boreholes located in the north coast of Malta island and in the south coast of Gozo island (Fig. 1, indicated as BH3 and BH6) a total of five VSP, three for P-waves and two for SH-waves has been acquired. For P-waves an hydrophone streamer down the hole, and a PWD (accelerated Weight Drop, about 74kg) mounted on a tractor as impulsive source were used. For SH-waves a 3C geophone down the hole, and a MiniVib source (11kN peak force) as vibrational source were used. Two refraction seismic lines has been acquired close to BH8 and BH9, to determine the P and SH velocity nearby the planned tunnel portals. In November 2017 an extensive airborne gas concentration survey has been carried out in the area between Gozo and Malta islands. The mission was focused to map CH4 and CO2 concentrations. The acquisition has been carried out with a Los Gatos Research (LGR) Ultraportable Greenhouse Gas Analyzer (UGGA) carried on-board of a Cessna 172 aircraft. The flight pattern consisted in 22 parallel lines (Fig. 1, dotted white lines) over an area of 30 km 2 with a set flight altitude of 500 ft at a speed of 70 Kts IAS in VMC and low wind intensity. The LGR inlet pipe has been placed on the wing leading hedge to sample undisturbed and uncontaminated samples of air. OGS technicians implemented a specific LGR data logger based on Arduino platform. The LGR’s UGGA reports and store all measured fully resolved absorption spectra which allows the instrument to accurately correct for water vapor dilution and absorption line broadening the effects and thus to report CH4 and CO2 on a dry (and wet) mole fraction basis directly without drying or post processing. CO2 and CH4 data have been plotted using simple moving average filter, over a period of 10 seconds, in order to remove short-term fluctuations. Also, data from LGR’s UGGA have been interpolated over a 100x100 m grid, in order to produce a map concentration stackable with other significant data, e.g. trajectory information, or pockmarks detected at seafloor. The data shows some local anomalies where a major increase in concentration of CO1 (up to 403 ppm) and CH4 (up to 1890 ppm) are observed. Results. A complete dataset including offshore multichannel seismic, HR seismic, VHR seismic, bathymetry, aerial CH4 and CO2 concentrations, land seismic, borehole stratigraphy, and VSP has been acquired over two years. The overall data quality is good. The objective “target depth” has been successfully reached with seismic penetration up to 600 m below seafloor. The seismic velocities have been determined through the VSP and refraction seismic acquisition. Conclusions. The dataset will allow to build a 3D geological model to fully characterize and better understand the structures in the area and to determine the Blue Clay formation limits. The integration of seismic sections with VSP data will give useful depth information for the correct tunnel planning. Aerial gas measurement will provide a tool to better understand the features revealed from seismic and bathymetric data.