GNGTS 2017 - 36° Convegno Nazionale

550 GNGTS 2017 S essione 3.1 RSS-2C is seaward thickening and is made by continuous, low amplitude seismic reflectors, onlapping on the lower unit. Mounded features with internal chaotic acoustic facies are observed along the flank of the Central High. The combination of the seismic facies with the stratigraphic information at DSDP site 270 suggest the evolution in the early Miocene from shallow marine and ice proximal sediments (Kraus et al. , 2015) (in sub-sequence 1), to ice distal sediments (in sub-sequence 2 and 3) (Fig. 2). We observe three sub-sequences of similar geometry also in RSS3, suggesting that seismic facies assemblage indicate ice sheet advance and retreat. However, further modeling including glacial isostatic adjustment is needed to reconstruct ice volume and sea level change during early Miocene site. Tomography. Seismic tomography is a geophysical technique that help to estimate the seismic velocity in depth for any sources and receivers position and combination. We applied travel time reflection tomography on three multichannel seismic reflection 2D sections to obtain a detailed model, defined by the geometry of 6 surfaces and by the velocity field. We selected a portion of 130 km and 7 horizons on three prestack seismic lines (BGR80-007A, IT89AR33 and IT89AR34), that intersects the DSDP site 270. The Tomographic inversion has performed by CAT3D software (Cat3D User Manual), based on the SIRT (Simultaneous Iterative Reconstruction Tecnique) method and the minimum-time ray tracing. It uses the picked reflected travel times and the estimates the velocity field and the unit structure in sequence, from the shallowest to the deepest horizon (Bӧhm and Rossi, 2005). For each horizon we use an iterative procedure which starts from a constant velocity within the unit and horizontally. We first invert the picked travel times and update the velocity model. The interface is estimated following the principle of minimum dispersion of the reflected point. The travel time residuals associated to each reflected event are converted in depth by using the velocity field updated in the first step of any iteration. We use staggered grid method (Vesnaver and Bӧhm, 2000) to improve the tomographic solution. This technique averages the velocity values obtained from different inversions of a coarse grid slightly shifted in the space. From the tomography of the reflected arrivals we reconstructed the P-velocity volume of the sediment sequences below the sea bottom and the depth structures of five horizons in the pre-stack data. The velocity field was used for Fig. 2 - Multichannel seismic section BGR80-007A; single channel seismic section PD-30 in the details.