GNGTS 2017 - 36° Convegno Nazionale

592 GNGTS 2017 S essione 3.1 With the aim of studying gas hydrate along the Chilean continental margin, two seismic lines were analysed, the one located in the northern sector, offshore Itata, the other in the southern sector offshore Valdivia (Fig. 1). Our procedure include: 1) determination of the velocityfield by using pre-stack depth migration (PSDM), 2) geological interpretation of velocity anomalies and 3) determination of gas hydrate and free gas concentrations by using reference velocity in the absence of gas hydrates and free gas. Then, from sea floor, BSR depths, and sea bottom temperature, the geothermal gradient is estimated. Data and methods. Seismic data acquired by the RV Sonne (January– February 2001) within the framework of the project “Subduction Processes off Chile (SPOC)” and by the RV Conrad (January–February 1988) within the framework of the project “Mid-Ocean Spreading Ridge (Chile Ridge)” of the Ocean Drilling Program (ODP) were analysed. Two seismic lines were selected, RC2901-727 and SO161-29 (Fig. 1). Seismic processing was performed by using Seismic Unix (SU) software. To detect and quantify the gas hydrate and free gas presence a procedure already tested (Tinivella et al., 2009) was used. The RC2901-727 seismic line was acquired with a 3000 m long digital streamer, with 240 channels and an intertrace of 12.5 m. The seismic source was a tuned array of ten airguns with a total volume of 61.3 1, and a shot spacing of 50 m. The SO161-29 seismic line was acquired using a 3000 m long digital streamer with 132 channels; from channel 1 to 24 the intertrace was 12.5 m, while from channel 25 to 132 it was 25 m. The seismic source was a tuned array of 20 airguns, providing a total volume of 54.1 l, with a shot spacing of 50 m. Results. Velocity models and PreSDM sections. In the northern sector (RC2901-727), the final velocity model shows a high velocity layer (1750-1950 m/s) above the BSR, which can be associated with the presence of gas hydrate. Below the BSR, local low velocities of about 1,400 m/s were detected, associated with the presence of free gas (Fig. 2). In the southern sector (SO161-29), the high velocity layer associated with the presence of gas hydrate reaches a maximum value of 2500 m/s. A strong lateral velocity variation is evident along the sections. In particular, in the southern sector the hydrate layer shows a minimum value of about 2000 m/s, which corresponds to a structural high, whereas the maximum value of about 2500 m/s is present in the accretionary prism (Fig. 2). In the northern sector, the PSDM section allowed us to recognise a continuous BSR with a variable depth ranging between 200 to 470 mbsf (meter below sea floor), while in the southern sector a strong, discontinuous and deeper BSR (about 600 mbsf) was recognised. The BGR was only identified in the northern sector. Here, we determined a free gas thickness of about 80 m (Fig. 2). Fig. 1 - Location map of the RC2901-727 and SO161-29 seismic lines (solid lines). Bathymetric map based on GEBCO_08 Grid (version 20091120,  http://www.geb co.net  (last visit 19-08-2016)).