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 engineering activities at high latitudes and for geological mapping of sediment thicknesses and architecture. The freezing of sediments is known to greatly affect their seismic velocities. In polar regions the actual velocities of the upper sediments may therefore potentially reveal water saturation and extent of freezing. We apply various strategies for modeling seismic velocities and reflectivity 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 transition 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 velocities 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 different porosity may depend more strongly on the actual saturation rather than changes in granular characteristics. For fully water-saturated sediments, the P-wave reflectivity decreases sharply with freezing, while the reflectivity becomes less affected as the water saturation is lowered. Thus, a combination of velocity and reflectivity information may reveal saturation and freezing conditions.