On seismic mapping and modeling of near-surface sediments in polar areas
A knowledge of permafrost conditions is important for planning the foundation of buildings and engineer-ing activities at high latitudes and for geological map-ping of sediment thicknesses and architecture. The freez-ing of sediments is known to greatly affect their seismic velocities. In polar regi...
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| Format: | Text |
| Language: | English |
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| Online Access: | http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.1026.4032 http://geophysics.geoscienceworld.org/content/gsgpy/68/2/566.full.pdf |
| Summary: | A knowledge of permafrost conditions is important for planning the foundation of buildings and engineer-ing activities at high latitudes and for geological map-ping of sediment thicknesses and architecture. The freez-ing of sediments is known to greatly affect their seismic velocities. In polar regions the actual velocities of the upper sediments may therefore potentially reveal wa-ter saturation and extent of freezing. We apply various strategies for modeling seismic velocities and reflectiv-ity properties of unconsolidated granular materials as a function of water saturation and freezing conditions. The modeling results are used to interpret a set of high-resolution seismic data collected from a glaciomarine delta at Spitsbergen, the Norwegian Arctic, where the upper subsurface sediments are assumed to be in transi-tion from unfrozen to frozen along a transect landward from the delta front. To our knowledge, this is the first attempt to study pore-fluid freezing from such data. Our study indicates that the P- and S-wave veloci-ties may increase as much as 80–90 % when fully, or almost fully, water-saturated unconsolidated sediments freeze. Since a small amount of frozen water in the voids of a porous rock can lead to large velocity increases, the freezing of sediments reduces seismic resolution; thus, the optimum resolution is obtained at locations where the sediments appear unfrozen. The reflectivity from boundaries separating sediments of slightly differ-ent porosity may depend more strongly on the actual sat-uration rather than changes in granular characteristics. For fully water-saturated sediments, the P-wave reflectiv-ity decreases sharply with freezing, while the reflectivity becomes less affected as the water saturation is lowered. Thus, a combination of velocity and reflectivity informa-tion may reveal saturation and freezing conditions. |
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