GNGTS 2015 - Atti del 34° Convegno Nazionale

38 GNGTS 2015 S essione 3.1 Similarly, the program creates a number of other sections, including a slope section (the slope of the fitted line), a product section (slope multiplied by the zero-offset amplitude), a Poisson’s ratio section, and S-wave amplitude section. The latter two are formed from the zero- offset amplitude and the slope sections, using assumptions of small change in Poisson’s ratio and an average Vp/Vs ratio of 2.0 (Poisson’s ratio = 0.333). For this analysis we prepared the original CDP by applying a band-pass filtering (cut frequencies 4-8/50-120 Hz) and in particular by the recovering the true amplitude of the reflected signal by spherical divergence correction. Here we reproduce the results of the P-wave reflectivity and S-wave reflectivity in order to find their correlation. We observe (Fig. 3a) that across the BSR the P wave intercept is mainly characterized by positive values (blue color) and in particular by no relevant polarity inversion. On the other hand, the high amplitude reflection values could be significant of a change of fluids matrix properties of the material. In Fig. 3b the S wave amplitude section has been obtained: it shows weak reflections in correspondence of the BSR. In association of what we observed on P wave reflectivity along the interface, even a change of density can be assumed. Conclusions. The analysis of the portion of the seismic profile IT95-167 crossing the oceanic Dove Basin, located in the south Scotia Sea, has allowed to identify a high amplitude anomalous reflector that cuts the primary reflections assigned to the sedimentary infilling of the basin and mimic the shape of the seafloor. This type of signal could be interpreted as a BSR due to the gas hydrate layer or to an Opal-A/Opal-CT transformation zone. Using different approaches to analyze this seismic BSR, we can synthesize our results as following: - Velocity analysis: the velocity spectrum is ambiguous; a velocity inversion is not clear, but possible. Both the considered causes to explain the presence of the BSR could be realistic. - Polarity attribute: not negative polarity has been highlighted by the polarity attribute analysis; this suggests an Opal -A/Opal-CT transformation. - The measured thickness of the sediments above of the BSR seems to be higher than the common values of GHSZ; this turns away the hypothesis that the reflector could be related to gas hydrate. - AVO: P and S-waves reflectivity only support density change across the BSR. The comparison and integration of the different obtained results seem to be not sufficient to support the hypothesis of a gas hydrate BSR potentially associated to the presence of abiogenic gas in the sedimentary succession ��������� ��� ������� �������� �� ��� ���� ������ �� ��� overlying the oceanic basement �� ��� ���� ������ �� ��� of the Dove Basin. On the contrary, an Opal-A/Opal-CT transformation seems to be a more plausible hypothesis. We intend to apply further analysis to better clarify the origin of the identified BSR. Acknowledgements. The authors gratefully acknowledge Paradigm though the OGS Focus and Geodepth processing software, and Schlumberger through the University of Trieste Petrel academic grant for interpretation software. References Barker P.F., Lawler L.A. and Larter R.D.; 2013: Heat-flow determinations of basement age in small oceanic basins of the Southern central Scotia Sea . Geological Society, London, Special Publications 381, 139-150. Carcione J. M. and Tinivella U.; 2000: Bottom-simulating reflectors: seismic velocities and AVO effects . Geophysics, 65 , 54-67. Castagna J.P. and Swan H.V.; 1997: Principles of AVO crossplotting . The Leading Edge, 16 , 337-342. Chopra S. and Marfurt K.J.; 2005: Seismic Attributes-A historical perspective . Geophysics, 70(5) , 3-28. Del Ben A. and Mallardi A.; 2004: Interpretation and chronostratigraphic mapping of multichannel seismic reflection profile I95167, Eastern Falkland Plateau (South Atlantic) . Marine Geology, 209 , 347-361. Eagles G., Livermore R.A., Fairhead J.D. and Morris P.; 2005: Tectonic evolution of the est Scotia Sea . Journal of Geophysical Research B: Solid Earth, 110(2) , 1-19. Etiope G. and Sherwood Lollar, B.; 2013: Abiotic Methane on Earth . Reviews of Geophysics, 51 , 276-299.