Barents Sea Margin and Svalbard Margin

The largest contiguous sedimentary prism in the North Atlantic is along the western Barents Sea margin (Myhre and Eldholm, 1988), especially in the Bear Island Fan, where sediments are up to 7 km thick (Vorren et al., 1998; Hjelstuen, et al., 1999). Because rapidly deposited sediments derived primarily from a continental shelf area constitute good source beds, the west Svalbard sediment prism is likely to contain significant amounts of organic material. Moreover, the ocean continent boundary of the western Barents Sea is characterized by high heat flow (Eldholm et al., 1999) facilitating rapid generation of hydrocarbons. Hydrate has been identified in the Hakon Mosby Mud Volcano (HMMV) in the Bear Island Fan (Vogt et al., 1999; Ginsburg et al., 1999), in the Vestnesa Ridge (Vogt et al., 1994) and nearby (Posewang & Mienert, 1999) to the SW of the Yermak Plateau, and by inference within the Malene Bukta embayment (MB) in the Arctic margin immediately to the north of Svalbard (Cherkis et al., 1999) (Figs. 1) Both shallow gas and hydrate have been identified in sediments of the western Barents Sea slope (Eiken and Austegard, 1987; Eiken and Hinz, 1993). Seismic velocity profiles indicate trapped gas below hydrate on the upper continental slope (Austegard, 1982).

The Barents Sea gas hydrate site is at about 350 m water depth. A single shallow seismic reflector of anomalous high amplitudes cuts through dipping layers, interpreted as the base of a gas hydrate cemented layer (Fig. 3) (Andreassen et al., 1990). The BSR that here has the appearance of a "bright spot" occurs in a depth of 0.17s TWT, corresponding to approximately 180 mbsf, one of the shallowest known BSRs observed in north Polar Regions. Velocity analysis from multi-channel seismic data show a high velocity layer above the BSR (>2400 m/s) interpreted as a gas hydrate layer (Andreassen et al., 1990). The strong velocity decrease to values of about 1625 m/s below the BSR, is an indicator of free gas in the sediments.

The Svalbard gas hydrate site ranges in water depth from 860 m to 2350 m. A well-developed BSR exhibits strong amplitude variations, and parts of it show high amplitude reflections below (Fig. 4) (Posewang and Mienert, 1999b). Furthermore, frequency analysis revealed that sedimentary layers below the BSR

act as a low-pass filter on seismic signals. Above the BSR, frequencies up to 170 Hz predominate; below the BSR, frequencies of

Permafrost Svalbard

Figure 3. Section of a multi-channel reflection seismic profile located in the Barents Sea. In a depth of 0.17s TWT bsf a seismic anomaly appears interpreted as a BSR. The BSR crosses the dipping sedimentary strata, is characterized by high amplitudes and shows a limited lateral extension above a fault complex (from Andreassen et al. 1990).

Figure 3. Section of a multi-channel reflection seismic profile located in the Barents Sea. In a depth of 0.17s TWT bsf a seismic anomaly appears interpreted as a BSR. The BSR crosses the dipping sedimentary strata, is characterized by high amplitudes and shows a limited lateral extension above a fault complex (from Andreassen et al. 1990).

less than 80 Hz prevail. Free gas, which is trapped below the BSR, might explain the high reflection amplitudes in this subbottom depth and the existence of a low-pass filter on seismic signals. The sealed free gas migrates into the overlaying strata, and therefore, the thickness of the free-gas layer below the BSR varies (Posewang and Mienert, 1999b). Due to the high-resolution character of the data, the so-called 'Base of the Gas Reflection' (BGR) (Camerlenghi and Lodolo, 1994) could be detected and the thickness variation could be calculated (Mienert et al., in press). The reflection amplitude of the BSR also varies according to the thickness of the free gas layer. Due to the short distance to the Vestnesa Ridge, which acts as a heat source (Vogt et al., 1994), the sub sea floor temperature increases downslope. Therefore, the subbottom depth of the BSR decreases with increasing water depth from 0.26 s TWT bsf upslope to 0.22 s TWT bsf downslope (Posewang and Mienert, 1999b). Velocity analysis of multi-channel seismic data shows three distinct layers with different velocities (Andreassen and Hansen, 1995). A nearly 200 m thick layer between the sea floor and the BSR has an average velocity of 1630 m/s followed by a low-velocity layer of 100 m thickness (1450 m/s) and a zone of higher velocities. The high-velocity layer above the BSR is interpreted to contain gashydrated sediments, whereas the low-velocity layer indicates free gas beneath the BSR. The interpretation of a gas layer below the BSR was confirmed by velocity-depth models calculated from High-frequency Ocean Bottom Hydrophone (Hf-OBH) data (Posewang and Mienert, 1999b).

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