GNGTS 2018 - 37° Convegno Nazionale

648 GNGTS 2018 S essione 3.2 The first phase comprised morphological and seismic data acquisition. The project area (Fig. 1) was fully covered with a Reson Seabat 7125 Multibeam (500kHz). The bathymetric data was acquired and processed with Teledyne PDS software that provide, during acquisition, some automatic tools to detect and delete data with a bad quality level. All the data set was processed with the following steps: sensor calibration and geometry validation; sound velocity profiles control; positioning and tidal control, application of Statistic, Slope, Flying Object, and Beam Quality automatic filters; data replay, with different point of view, to detect residual spikes and anomalies. Finally Digital Terrain Models, with square cell from 2 to 20 meters, were created. A centimetric scale 3D map of the seafloor was achieved. To image the subseafloor with a fine scale up to few tens of meters, a High Resolution Seismic survey was performed using a boomer source with a single-channel streamer (8 piezoelectric elements connected in series with a 2.8 m active array). The boomer plate (AA301) produces a theoretical minimum phase wavelet with amplitude spectrum between 400 and 4000 Hz. With this frequency range, under favourable conditions, we can obtain data with decimetric resolution. Boomer data was processed with conventional mono-channel processing sequence: after DC removal (continuous noise from electrical devices) and filtering (time variant band- pass in a range 100-6000 with slope in dB/octave), we applied a gain (spherical divergence and balance gain obtained by inverse of amplitude decay curve) to recover the seismic energy. The weather condition affected the HRS data: the sea waves lifting and lowering the streamer and the plate, creating a wavelet effect on data. These undulations modified and destroyed the reflectors. To attenuate this effect in several profiles we applied a static correction calculated on a plot trace within a predetermined window. Amultichannel Reflection survey was conducted to characterize the nature and stratigraphy of the bedrock, the main issue was to identify the main faults and the interface between the Blue Clay and the underlying rock. As a whole 108 seismic lines were acquired (236 km). The grid was constituted by lines spaced 75m (Fig. 1, Red Lines), along 70°N and 160°N directions (Fig.1). To reach the “target depth” of 300m below the seafloor, a 60 cu.in. Sercel Mini GI-Gun was used as source. The gun was feeded by three high pressure Coltri compressors, capable of Fig. 1 - Acquisition Map. Bathymetric data, Seismic Lines (Red), Airborne Survey (dotted White), and Borehole location.