Seismic modelling of gas hydrate and free gas in sediments, from ocean-bottom seismometer data along the continental margin of Western Svalbard
Over the next decades, the shallow parts of continental margins in the Arctic are likely to experience warming of bottom-water. It is, therefore, important to evaluate how methane hydrate beneath the seabed in these margins will react to future increases in bottom-water temperature and whether release of methane from hydrate will have an impact on climate. As part of the International Polar Year initiative, a multidisciplinary marine expedition was carried out in August-September 2008 along the continental margin west of Svalbard in the Arctic Ocean. One of its objectives was to determine the extent of the gas hydrate stability zone (GHSZ) along the continental slope and to quantify the amount of methane present as hydrate or gas beneath the seabed, using seismic techniques. Thirteen ocean-bottom seismometers (OBS) were deployed at 5 representative sites along and across the continental margin. High frequency airguns (GI guns) were fired at 5-s intervals and the data were recorded at a high sampling rate (1 kHz) in the OBS. The records show clear P-wave reflections at short offsets, as well as refracted arrivals at larger offsets, from depths up to 2 km below the seabed. The sub-seabed variation of P-wave velocity was modelled for three sites located above and below the upper limit of GHSZ, using ray-traced forward modelling. The velocity model for the deepest site (~1250 m deep) below the upper limit of the GHSZ shows a zone about 120 metres below the seabed with a greater velocity (1.8 km/s) than expected for terrigenous sediment. This high velocity zone lies above a lower velocity zone (1.55 km/s) and the acoustic contrast between the two zones forms a bottom simulator reflector (BSR) at approximately 170 m below the seabed. The BSR marks the boundary between sediments containing gas hydrate above and free gas below. The velocity model from the shallow site (~480 m deep), below the upper limit of the GHSZ, indicates the presence of a low velocity zone (1.60 km/s) about 160 m below the seabed, which is interpreted as a free-gas reservoir. However, no clear bottom simulator reflector (BSR) has been recognised at this site. The velocity model for the third site (~350 m deep), above the upper limit of the GHSZ indicates high velocity, interpreted as glacial over-compacted sediment, with no clear evidence for free gas. Our preliminary results indicate the presence of gas hydrates at the deepest site of the experiment, but have so far failed to produce unequivocal evidence for gas hydrate at the shallower site where hydrate would be most sensitive to bottom water changes. However, the fact that the presence of free gas can be inferred at two different sites nearly 100 km apart, suggests that free gas is widespread along the Svalbard continental margin and that hydrate may exist close to the upper limit of the GHSZ. Ongoing modelling of the P- and S-wave velocities should provide a better understanding of the sub-seabed distribution of the seismic properties from which the amount of hydrate present in the sediment can be estimated.
AGU Fall Meeting Abstracts
- Pub Date:
- December 2009
- 3004 MARINE GEOLOGY AND GEOPHYSICS / Gas and hydrate systems;
- 3025 MARINE GEOLOGY AND GEOPHYSICS / Marine seismics