Seismic mapping and modeling of near‐surface sediments in polar areas

Abstract

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.

Minimum distribution of subsea ice‐bearing permafrost on the U.S. Beaufort Sea continental shelf

“…Laboratory and field studies indicate that the P‐wave velocity ( V p ) of ice‐bearing coarse‐grained sediments is strongly dependent on the saturation of ice in pore space. Using effective media theory and mixture models, Johansen et al [2003] determine theoretical seismic velocities ranging from ∼2.5 to 2.8 km s −1 for saturated sands with 0 to 40% ice saturation and from 3.4 km s −1 to as high as 4.35 km s −1 for ice filling 40 to 100% of pore space. The abrupt increase in seismic velocity at 40% saturation of the frozen pore‐filling phase has also been observed in hydrate‐bearing sediments [ Yun et al , 2007] and reflects the onset of cementation of sediment grains at this threshold saturation value.…”

Faint traces of high Arctic glaciations: an early Holocene ice‐front fluctuation in Bolterdalen, Svalbard

“…The fjord floor has an average sediment thickness of 12 m (Elverhøi et al 1995) and rises evenly towards the east with no evidence of distinct ice-front stillstand positions (unpublished seismic data, UNIS). The deglaciation succession along the valley floor is covered by prograding delta facies that reach c. 80 m thickness at the fjord head (Johansen et al 2003). The withdrawal of ice in the Adventdalen palaeofjord, east to Bolterdalen, appears to have been relatively uniform and spanned about a thousand years in time, with an average glacioisostatic uplift rate of c. 0.8 m/100 years, rising to 1.6 m/100 years from 10 to 9.8 kyr BP.…”

CO2 storage in the high Arctic: efficient modelling of pre‐stack depth‐migrated seismic sections for survey planning

“…The ice content in the permafrost will largely influence the elastic properties (Johansen et al . ) and thus significantly affect the velocity model. The impact of geological heterogeneities in the target aquifer, including igneous intrusions, faults and a pervasive fracture network, will not only influence the CO 2 flow paths and distribution, but also the seismic imaging.…”