Song, Sha (2018) Geophysical characterization of gas hydrate systems of the South Shetland margin (Antarctica). [Ph.D. thesis]
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Abstract (italian or english)
During the last few decades, interest in gas hydrates has been increasing significantly because of their economic potential as future energy source and their potential role in geohazards and global climate change. The global climate change is a particularly sensitive issue for the Polar Regions, such as Antarctica. In the South Shetland margin (Antarctic Peninsula), the occurrence of a potential gas hydrate reservoir has been demonstrated from the analysis of geophysical data acquired during three Italian Antarctic cruises. In order to enhance the knowledge of gas hydrate systems, I analyzed Ocean Bottom Seismometer (OBS) and coincident multi-channel seismic (MCS) data acquired in 2004. The main objectives of this thesis are to estimate the distributions and concentrations of gas hydrate and free gas within the sediments, and to investigate the system’s petrophysical properties.
Travel time inversion and forward modeling of OBS data were performed to estimate detailed P- and S-wave velocity fields. The P-wave velocity field was determined by the inversion of refractions and reflections in OBS data, while the S-wave velocity field was obtained by ray-tracing forward modeling of the converted S-waves from the horizontal components of OBS data. Several velocity models were tested in order to reduce the errors caused by the spatial drift of the OBS from the MCS line during sinking, and the errors from inversion. The final velocity model shows that P-wave velocity increases gradually with depth down to the bottom simulating reflection (BSR) at approximately 510-650 m below the seafloor. The layer with high P-wave velocity of 2.0-2.1 km/s just above the BSR can be associated with the presence of gas hydrates. Below the BSR, a low velocity layer of 1.4-1.6 km/s is observed, which indicates the presence of free gas. From the analysis of critical refractions in OBS data, the base of free gas layer (BGR) occurs at a depth varying between 80-160 m below the BSR. Forward modeling of converted S-waves in OBS data allows us to obtain Poisson’s ratio estimates. We observe that Poisson’s ratios are fairly uniform within each layer and they show good agreement with previous study performed in this area. The comparison of Poisson’s ratio indicates that the gas hydrate reservoir shows no significant regional variations.
The resulting velocity fields were translated in terms of gas hydrate and free gas concentrations, using a modified Biot-Geerstma-Smit theory. The results show that hydrate concentration in the layer just above the BSR ranges from 10% to 15% of total volume, and free gas concentration is approximately 0.3% to 0.8% of total volume assuming a uniform gas distribution.
Part of this research related to the OBS analysis and gas-phase estimation, has been published in the international journal Energies (Song et al., 2018) and included in the Appendix 1.
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