GNGTS 2018 - 37° Convegno Nazionale

440 GNGTS 2018 S essione 2.2 geophones. The topographic surveys for source and receiver locations were performed using real-time differential GNSS positioning techniques called VRS-Virtual Reference Station. A MiniVib in SH-wave mode was used to acquire SH-wave data. At the end of the seismic line deployment a test was performed to evaluate the best seismic sweep to be adopted. The SH-wave acquisition parameters are reported in Table 1. Tab. 1 - SH-wave acquisition parameters. Source: IVI MiniVib T-2500 (SH-wave mode), upsweep 10 – 120 Hz, 12 s, 1800 - 2000 Lb Sensors: horizontal geophone (40 Hz) Line: 475 m Channel interval: 2 m Active channels: 240 (fixed spread) Pilot signals: Ground Force, Filtered Ground Force, Baseplate, Reaction mass, Synthetic Seismic source interval: 4 m N. energizations/point: 2 – 4 Sampling rate: 2 ms Data length: 15360 ms Correlated data length: 3000 ms (Ground Force) Horizontal geophones (oscillation axis perpendicular to the line) were deployed to detect SH-wave. Data acquisition was performed by DMT- Summit II seismograph with a shot by shot QC. The acquired reflection seismic data underwent a standard processing sequence developed for shear-wave data (Table 2). Static corrections were not applied due to a flat topography and the absence of surface lateral variations. A reliable interactive velocity analysis was performed owing to the inter-receiver spacing of 2 m and the presence of near horizontal seismic interfaces (i.e., seismic line oriented along the strike direction). Tab. 2 - Data processing sequence. SEG2 > SEGY data conversion Data editing Vertical stack Geometry Top muting (direct, refracted wave, spurious P-wave)) Spherical divergence compensation CDP sorting Velocity analysis NMO correction Stack Time to depth conversion (smoothed version of NMO correction) Discussion. Seismic data penetration and resolution depend on the seismic source and receiver, acquisition geometry and on the geological conditions of the study area. The SH-wave reflection surveys are not frequently executed and, before this pilot study, no data were available in this area. Data acquired are of good quality and several clear reflections can be recognized