Compressional velocity structure and Poisson's ratio in marine sediments with gas hydrate and free gas by inversion of reflected and refracted seismic data (South Shetland Islands, Antarctica)

A Bottom Simulating Reflector (BSR) on the South Shetland Margin (Antarctic Peninsula) was surveyed, within the Progetto Nazionale di Ricerche in Antartide (PNRA). High resolution multichannel (MCS) reflection profiles were acquired, and a three- component Ocean Bottom Seismograph (OBS) was deployed where the BSR was particularly strong. The compressional velocity field associated to the BSR was determined by the inversion of travel times of reflected (MCS data) and refracted (OBS data) events. Information on shear wave velocities was extracted from the horizontal components of the OBS records. The analysis indicates the presence of velocity anomalies both above and below the BSR that are not related to the lithologic characteristics of the sedimentary column, but can be associated to gas hydrate and free gas presence in pore space. Amplitude Versus Offset (AVO) analysis was performed on Common Depth Point gathers, which correspond to the OBS position. The study determined reflection coefficient trends that provided qualitative information about gas hydrate and free gas abundance. A theoretical model for elastic porous media was applied to quantify amounts of gas hydrate and free gas. The resulting theoretical velocity curves were compared to interval velocity obtained by seismic inversion. A quantitative estimation of free gas and gas hydrates trapped in the sediment pore spaces was made by interpreting velocity anomalies with respect to reference velocity curves (normally consolidated marine terrigenous sediments). The comparison of the results obtained by these independent methods (seismic inversion, AVO analysis, and theoretical model) gives consistent values of Poisson's ratio and gas hydrate and free gas estimation. 2000 Elsevier Science B.V. All rights reserved.