Impact of Climate Change on Glacial Materials and Frozen Ground: Geophysical Assessment and Monitoring
Presiding: B Kulessa, Queen's University Belfast; H French, Norwegian Centre for Soil and Environmental Research; J West, University of Leeds
NS43A-01 INVITED 13:30h
Geophysical Techniques for Detecting, Analysing and Monitoring Frozen Ground
Permafrost degradation due to contemporary climatic change has a significant impact on the stability of mountain slopes, buildings and infrastructure in permafrost regions. In order to assess the potential risks of accelerated permafrost thawing and develop suitable mitigation strategies, ground ice occurrences have to be detected, mapped and monitored on various temporal and spatial scales. Applications of geophysical techniques for permafrost detection present comparatively cheap and logistically feasible alternatives to the single point information from boreholes. Recently applied methods include electrical resistivity tomography (ERT), refraction seismic tomography, electromagnetic induction methods and ground penetrating radar (GPR). In this contribution the different geophysical methods are evaluated concerning their applicability on frozen ground based on numerous field and laboratory studies. The advantages and disadvantages of each method concerning sensor coupling in heterogeneous and blocky terrain, steep topography, measurement accuracy and interpretational ambiguities are discussed. In addition, the indirect nature of geophysical soundings requires a relation between the measured variable (e.g. electrical resistivity, seismic velocity) and the respective parts of the material composition (rock, water, air, ice) - in our case the ice content. With combined resistivity and seismic data sets as input variables the ice and unfrozen water contents of the subsurface can be calculated using a 4-phase model. The model is based on two well-known geophysical mixing rules for electrical resistivity and seismic P-wave velocity, Archie's law and Timur's equation. In addition to prescribing the material dependent free parameters in Archie's law, the resistivity and P-wave velocity of the rock material and the pore water have to be known in advance. Besides, one of the volume fractions has to be explicitly prescribed (usually the porosity). First results confirm the good model performance for various field cases in permafrost research. Extensive validation using a series of shallow boreholes is needed to further analyse the quantitative performance of the model for different glacial and periglacial environments.
NS43A-02 13:45h
Hydrogeophysics in Areas with Seasonally Frozen Soils and Permafrost
Hydrology in the northern hemisphere is greatly affected by permafrost and seasonally frozen soils. Frozen ground near the surface will influence the distribution of melt-water and rain to run-off or infiltration. In areas of permafrost only the top layer of the ground, is active hydrologically in the summer season. Global warming can have large mechanical and hydrological effects in these areas. The average increase in temperature is for seasonally frozen soils expected to have lower impact than the frequency of freeze-thaw cycles, arrival time of snow compared to start of freezing temperatures etc. It is important to have methods for studying the dynamic hydrogeological processes under such conditions in order to improve our understanding of how processes are coupled and what the consequences of a changed climate may be on these processes. In this paper examples of different applications of geophysical methods in Norway, Svalbard and Russia will be presented. The examples relate to: Infiltration and solute transport in a partially frozen soil monitored with conventional lysimeter techniques and electrical resistivity, both surface arrays and tomography; Leakages below frozen dams using geometrical EM soundings and finally an example showing EM soundings of saline permafrost and abnormal polarization.
NS43A-03 INVITED 14:00h
Application of Near-Surface Geophysics to Problems in Glacier Dynamics, Pitted Outwash Plain Formation, and Glaciotectonics, Matanuska Glacier, Alaska
From 2000 to 2004, near-surface geophysics data in various forms was collected near the active terminus of Matanuska Glacier, Alaska, to address several specific hypotheses and also provide general subsurface information in several relatively unsampled zones of the subsurface. (1) Seismic reflection data was collected on the glacier to test the predicted thickening of debris-rich basal ice in response to the motion of the glacier out of a localized overdeepening. The seismic data imaged the 5-to-10 meter thick basal ice at depths of 50-150 m, and clearly showed a 50 percent thicking--supporting the glaciohydraulic supercooling mechanism for basal ice formation. An interesting result of this basal-freeze-on mechanism of forced equilibrium is the concept of the "graded glacier" that has implications of sediment-pumping effects in response to changes in the surface slope of the glacier in response to climate change. (2) Seismic reflection, GPR, and electrical resistivity data were collected in a proximal pitted outwash plain abutting the active margin of Matanuska Glacier. These data indicate the presence of laterally continuous "slabs" of buried ice, in places possibly in duplex-like structures formed during readvances. The formation of thermokarst resulting in pits (sinkholes) in the surface of the outwash plain are therefore interpreted to result from preferential pathways of melting (relict moulins?) in the buried ice slabs, rather than from discreet blocks of ice. (3) 4-D GPR data (3-D through time) were collected at the terminus of Matanuska Glacier in an attempt to examine the formation of debris-flow-generated stratigraphy during the collapse of a portion of an ice-cored moraine complex. During a 30-m readvance of the glacier (2002-2003, between "snapshots" of data collection) glaciotectonic deformation of the moraine stratigraphy was observed. The GPR data indicate that the readvancing glacier impacted the rigid buried ice within the moraine, passively translating and deforming the overlying sediment in response to occasional brittle faulting and thrusting of the buried ice.
NS43A-04 14:15h
Grounding line position of Whillans Ice Stream, West Antarctica .
The grounding lines of ice streams are zones of particular interest because of the sensitivity of their position to the size of the ice shelf beyond. Grounding line retreat rates along the Siple Coast of Antarctica are poorly known, but crucial to a fuller understanding of the history and stability of the ice sheet. We describe the results of a joint GPS and ice-penetrating radar study at the grounding line of Whillans Ice Stream, West Antarctica. We compare our current grounding line location with the results from the IceSAT laser altimeter and the Radarsat-derived grounding line from the 1997 RAMP (Radarsat Antarctic Mapping Project) mission. We conducted a kinematic GPS survey of a deeply-embayed portion of the grounding line as well as a more-concave stretch of the grounding line to determine the current grounding line and the height above floatation of the grounded portion. In addition, we collected ice-penetrating radar data along those lines to determine ice thickness and map internal layers across the grounding line. Finally, we deployed a strain grid across the grounding line to measure the response of the ice to tidal forcing.
NS43A-05 14:30h
Electroseismic Exploration of Glaciers
We report the first electroseismic soundings on a glacier. More than 80 repeat soundings were conducted in four locations using antennas aligned both transverse and parallel to the orientation of a baseline. Strong electrical fields were generated in all cases, peaking early in time, and decaying approximately exponentially over some tens of milliseconds. In some cases much shorter-term electrical fluctuations are superimposed on this long-term decay. Comparison with GPR data reveals that the fluctuations are consistent with the arrival of independently propagating electromagnetic fields, generated by electrokinetic conversion as seismic waves pass (i) the interface between two layers of relatively low and higher water content; and (ii) the interface between relatively clean ice and debris-richer ice, or the interface between ice and bedrock, or potentially both. Electroseismic signatures depended on antenna orientation in all cases. Future work will focus on identifying the exact origin of the electroseismic signatures, and on linking these signatures to physical properties of glaciological interest. Electroseismic methods potentially allow the hydraulic conductivity of snow, ice and subglacial substrate to be estimated, even where substrate layers are thin. This is more challenging to achieve using other surface-based geophysical methods.
NS43A-06 14:45h
Dielectric Properties of Ice-Water Systems: Laboratory Characterization and Modeling
Glacier mechanical properties, and hence their response to climatic change, depend strongly on the proportion and distribution of unfrozen water at ice grain boundaries. Glaciologists have characterized unfrozen water content in several ways, notably via thin section microscopic analysis of ice cores to measure porewater contents, and field surveys of electromagnetic properties using radar. Water content has a very strong influence on the velocity of electromagnetic (radar) waves in ice, because of the high dielectric constant of water (~80) in comparison with ice (~3). However, there is a strong discrepancy between the two methods of measurement, with field radar surveys on glaciers giving unfrozen water contents of several volumetric percent, whereas ice-core microscopy gives values of less than one percent. This discrepancy has called into question the approach used to obtain the unfrozen water content from radar wave velocity. This approach assumes that the ice-water mixture is a lossless medium. Here, we report a laboratory and modeling based investigation of the relationship between dielectric properties and unfrozen water content of ice cores from the Glacier de Tsanfleuron, Switzerland, aimed at resolving the discrepancy. The laboratory study uses the technique of Time Domain Reflectometry to characterize the dielectric properties of ice cores from a range of ice facies. `Press on' TDR waveguides have been developed specifically for use on ice cores. Several press-on probe designs have been developed and aspects of their performance are reported. An independent estimate of unfrozen water content is determined from temperature and total pore fluid ionic strength. The results allow the establishment of relationships between the high frequency (~500MHz) dielectric properties and water content for various ice-crystal geometries. Mathematical modeling of the dependence of dielectric constant on frequency and water phase conductivity has been undertaken using Effective Medium Theory (EMT). The water phase in glacial ice has very high conductivity, despite the overall low ionic strength of glacier water, because dissolved ions preferentially remain in the liquid water phase during freezing. The EMT approach shows that the high conductivity of the water phase increases the apparent dielectric constant of the mixture at radar frequencies (~100MHz). It is proposed that this conductivity effect leads to the erroneously high water contents reported from field radar surface using conventional dielectric mixture relationships.