Umberta Tinivella

Seismic tomography study of a bottom simulating reflector off the South Shetland Islands (Antarctica)

Abstract Reflection tomography techniques have been applied to two multi-channel seismic profiles, acquired across the accretionary prism of the South Sheltand margin, in order to reconstruct the velocity field associated with gas hydrate and free gas layers in the sedimentary sequence. Data show the presence of a strong bottom simulating reflector (BSR), running along the slope in water depths ranging from 1000 to 4600 m, locally underlain by a weak normal polarity reflector about 80 ms deeper in the section. The analysis indicates a velocity trend from the sea floor to the BSR generally consistent with that of normally compacted marine sediments, with an abrupt decrement between the BSR and the underlying reflector, indicating the presence of free gas in the sediment pore spaces. The calculated thickness of this gas-bearing layer is approximately 50 m. Local increments of tomographic velocity above the BSR can be related either to gas hydrate abundances in normally compacted slope basin sediments or to overcompaction in accreted sediments, as imaged by the pre-stack depth migrated sections. We conclude that clathrates and free gas distribution on the South Shetland continental slope are strongly controlled by the structural setting of the accretionary prism, where faults act as conduits for migration of natural gas towards the surface. A brief description of the adopted tomography method is also presented.

Gas hydrate stability zone in shallow Arctic Ocean in presence of sub-sea permafrost

Vast quantities of gas hydrate could be trapped in the Arctic Ocean, and there is concern that a rise in the ocean temperatures could induce dissociation of these hydrate accumulations, potentially releasing large amounts of methane. In first approximation, the hydrate is not stable in the Arctic shelf because of the low pressure (shallow water), but if we consider presence of sub-sea permafrost in this area the hydrate can be stable. In this context, we define the areas where conditions prevail for gas hydrates stability in the Arctic shelf versus sub-sea permafrost thickness, temperature and geothermal gradient.

High Gas Hydrate and Free Gas Concentrations: An Explanation for Seeps Offshore South Mocha Island

Recent studies have reported cold seeps offshore of Mocha Island. Gas hydrate occurrences along the Chilean margin could explain seeps presence. Gas-phase (gas hydrate and free gas) and geothermal gradients were estimated analysing two seismic sections. Close to Mocha Island (up to 20 km) were detected high (up to 1900 m/s) and low (1260 m/s) velocities associated with high gas hydrate (up to 20% of total volume) and free gas (up to 1.1% of total volume) concentrations, respectively. A variable and high geothermal gradient (65–110 ◦C/km) was obtained. These results are related to high supply of deep fluids canalised by faults and fractures. Faraway from Mocha Island (>60 km), free gas concentrations decrease to 0.3% of total volume and low geothermal gradient (from 35 to 60 ◦C/km) are associated with low fluids supply. Finally, we propose gas hydrate dissociation processes as the main supply source for seeps in the vicinity of Mocha Island. These processes can be caused by: (a) active faults and seismic activity; and (b) warm fluid expulsion from deeper zones altering hydrate stability conditions. In both cases, gas hydrate dissociation could generate slope instability and landslides, as occurred in the past in this region and reported in the literature.

Tomographic and AVO analyses of Bottom Simulating Reflectors (BSR) on real data

In this paper we describe the application of different tools, as AVO, reflection tomography and pre-stack depth migration, in order to analyze the physical properties of BSR observed in two different real data sets. The tint dataset pertains to a marine line acquired in the South Shetland margin and the second one is relative to offshore South Chile. In both cases the results of the seismic study are consistent with the presence of a consolidated gashydrate layer which overlies a porous level saturated with free gas.