NS14A-01
The Potential for use of 3D-Seismic Data in Glacial Geological Studies With Examples From the Norwegian Shelf
The Norwegian shelf is probably the best investigated glaciated margin in the world. This is mainly due to an extensive exploration by the hydrocarbon industry, which has generated a huge data base comprising 2D and 3D seismic lines, detailed bathymetry, exploration wells and geotechnical borings. A large part of the Norwegian shelf is covered by 3D seismics, and these data have a great potential for use in glacial geological studies. The spacing between the seismic lines in 3D-seismic cubes is generally 12.5 m, and the vertical resolution is 4 ms (c. 3 m) providing excellent data coverage. All types of cross-sections and time-slices can be performed and visualised, giving a good overview of the whole seismic cube in three dimensions. The mapping of deeply buried surfaces several hundred metres below the present sea floor has given us evidence of glacial influence such as glacial lineations and iceberg ploughmarks far back in time. Late Tertiary uplift of Scandinavia and several tens of glaciations have denuded the inner shelf and corresponding land areas (the western part of the mountain range) an average of 450 m. The Naust Formation (< 2.8 million years) comprising all the glacial sediments of the mid-Norwegian shelf is more than 1000 m thick over large areas of the outer shelf. The large volume of the Naust Formation has raised the question of how these sediments were eroded and transported onto the shelf during such a short time. Mapping by 3D seismics has revealed glacial influence up to 2 million years back in time and has given some clues to these processes.
NS14A-02 INVITED
Monitoring and Quantifying Subsurface Ice and Water Content in Permafrost Regions Based on Geophysical Data Sets
Based on recent observational evidence of climate change in permafrost regions, it is now recognised that a detailed knowledge of the material composition of the subsurface in permafrost regions is required for modelling of the future evolution of the ground thermal regime and an assessment of the hazard potential due to degrading permafrost. However, due to the remote location of permafrost areas and the corresponding difficulties in obtaining high-quality data sets of the subsurface, knowledge about the material composition in permafrost areas is scarce. In frozen ground subsurface material may consist of four different phases: rock/soil matrix, unfrozen pore water, ice and air-filled pore space. Applications of geophysical techniques for determining the subsurface composition are comparatively cheap and logistically feasible alternatives to the single point information from boreholes. Due to the complexity of the subsurface a combination of complementary geophysical methods (e.g. electrical resistivity tomography (ERT) and refraction seismic tomography) is often favoured to avoid ambiguities in the interpretation of the results. The indirect nature of geophysical soundings requires a relation between the measured variable (electrical resistivity, seismic velocity) and the rock-, water-, ice- and air content. In this contribution we will present a model which determines the volumetric fractions of these four phases from tomographic electrical and seismic data sets. The so-called 4-phase model is based on two well-known geophysical mixing rules using observed resistivity and velocity data as input data on a 2-dimensional grid. Material properties such as resistivity and P- wave velocity of the host rock material and the pore water have to be known beforehand. The remaining free model parameters can be determined by a Monte-Carlo approach, the results of which are used additionally as indicator for the reliability of the model results. First results confirm the good model performance for various field cases in permafrost research. Especially the 2- dimensional monitoring and detection of ground ice and air cavities in the blocky surface layer was substantially improved. Validation of the model results was obtained using borehole and energy balance data from different permafrost sites.
NS14A-03 INVITED
Advancing Active Source Seismic Methods for Exploration of the Cryosphere
Active source seismic methods have been providing critical information to the study of the cryosphere by probing kilometers below surface to the ice-bed interface and imaging geologic formations beneath ice sheets. Considering the large expanses of rapidly changing ice masses around the globe, there is a need for improving the efficiency of active source seismic methods. Researchers at the Center for Remote Sensing of Ice Sheets (CReSIS) developed a new seismic streamer for reflection imaging through polar ice and utilized active source seismic surface wave methods to determine polar firn properties. These methods were tested on the Jakobshavn Glacier, Greenland, in summer 2007. Seismic streamers deployed in polar environments have been plagued by poor coupling to the snow surface and by wind noise contamination compared to manually buried geophones below the surface. A 24-channel seismic streamer prototype was constructed at The University of Kansas consisting of geophones mounted on metallic plates and towed behind a sled. The streamer was deployed on Jakobshavn Glacier alongside manually buried "control" geophones recording simultaneously the same explosive source signals. The streamer imaged seismic reflections from the bed at approximately 1.7 km depth and internal ice layers. In wind conditions up to 5 knots, streamer data were identical to control seismic data exhibiting bed reflections with frequency content in excess of 200 Hz. In 5-10 knot wind conditions, bed reflections and internal ice layers were clearly imaged by the streamer, although some wind noise was present compared to control data. In excess of 10 knot winds, streamer and control data showed increased noise content. Bed reflections were clearly recorded by the streamer, but internal layers were not discernible in single trace recordings. Multi-fold processing of streamer data enhanced signal-to-noise and improved imaging in windy conditions. It is estimated that in field conditions encountered at the Jakobshavn Glacier, streamer technology can yield a five- to ten-fold increase in seismic surveying efficiency without considerable loss of data quality. Seismic wavetrains contain surface waves propagating within one wavelength from surface. Their dispersive characteristics can be exploited to construct shear wave velocity profiles of the near-surface. We analyzed the phase velocity of Rayleigh waves by employing the Multichannel Analysis of Surface Waves (MASW) method and obtained shear wave velocity profiles of polar firn and glacial ice to approximately 78 meters depth. Shear wave velocities progressively increase from 900 m/s at the surface to 1800 m/s at 45 m depth. Between 45 and 78 m depth, shear wave velocity is predominantly 1800 m/s, which indicates that the firn-ice transition is at 45 m below surface. Surface wave methods do not require the generation and recording of shear waves and can map velocity inversions that cannot be detected by seismic refraction methods. Surface wave methods can provide continuous shear wave velocity mapping of polar firn which can help understand better firn mechanical properties and mechanisms of crevasse formation.
NS14A-04 INVITED
Laboratory and numerical experiments of radar back scattering: towards interpreting the shape of subsurface targets in cryospheric applications
Subsurface radar remote sensing is a powerful tool for examining the top 100-101 m of soil and seasonal snow, and 102-103 m of glacier ice. High frequency radar has high resolution, but poor penetration, while the opposite is true for low frequency radar. If targets are in lossy media (e.g. temperate ice, wet snow, and wet soil), only low frequency radar is usable but visualization of individual targets is unfeasible. We have tested an idea that echo-intensity variations in terms of radar polarization and frequency can constrain the shape and dimensions of targets. Scattering amplitude and its directional pattern are a function of refraction index of the target, relative to the medium, and dimensions of the target, relative to the wavelength in the medium. By changing the relative wavelength and refraction index, we can perform experiments that are practical in the laboratory, and use the findings to make inferences at larger scales and in different media. We measured microwave (2-18 GHz) radar backscatter of small targets in open-air in the laboratory, giving us accurate control and knowledge of target orientation, size and shape, as well as refractive index. Computer simulations were made with the Discrete Dipole Approximation model. This approximation allows larger target dimensions and higher refraction indecies than other computing methods. In this talk, we first review a range of targets of interest in the cryosphere, and then present several examples of laboratory experiments and computer simulations designed to simulate these conditions.
NS14A-05
Review of Electromagnetic Methods to Investigate Arctic and Antarctic Sea Ice and Snow
During the last 5 years we have applied a variety of near-surface electric (ie, resistivity) and electromagnetic methods to investigate sea ice and snow on sea ice in the Antarctic and Arctic. Here we present field cases and lessons learned on the applicability for resolving distinct target parameters. The geophysical challenges of sea ice include its composition of (a) homogeneous, vertically anisotropic, one-dimensional (level) ice 0.5 to 4 m thick, and (b) highly heterogeneous, partly water impregnated three-dimensional pressure ridge features 2 to 10 m thick. Snow on sea ice is generally dry (until melt onset) and spans a thickness range of some centimetres up to a few meters. We applied several different types of equipment covering the frequency range from DC to radar for different tasks and targets. Ground Penetrating Radar (GPR) proved to be fast and portable for snow thickness profiling with the limitation of a minimum snow thickness around 10 cm. Electromagnetic induction (EMI) is a classic sea ice thickness profiling method used hand held on the ice, ship-borne suspended from outrigger-like constructions as well as airborne as helicopter towed sensors. Mostly regional ice plus snow thickness is derived from EMI measurements. Attempts have been made to retrieve internal ice properties such as porosity or age (conductivity) from EM soundings. DC-resistivity sounding clearly shows the vertical conductivity anisotropy of level sea ice, due to its crystalline structure and aging processes. Electrical Resistivity Tomography was conducted on Baltic and Arctic sea ice to determine the porosity of pressure ridge keels. Our results show the potentials and limitations of the different methods for climate related and engineering sea ice studies. http://www.pfaffling- geophysics.com/projects
NS14A-06
Local-scale Snow Accumulation Variability On The Greenland Ice Sheet From Ground- penetrating Radar (GPR)
Measurements of snow accumulation are critical to studies of mass balance. Traditional point measurement techniques (snow pits, manual probes, firn and ice cores) are limited in space and often do not represent the region surrounding them due to spatial variability that is caused by a variety of factors, including surface slope and deposition and erosion by wind. Current accumulation maps of Greenland are based on point measurements and have estimated errors of 20-25%. Ground-penetrating radar (GPR) has the potential to significantly improve upon these accumulation estimates because of its ability to cover large regions over short time periods with relative ease at high vertical (depth) and horizontal (areal) resolutions. The current study employs GPR data to investigate the distribution and variability of accumulation at shallow depths (5 m) and at the local scale (100-m by 100-m) at two disparate locations in the accumulation zone of the Greenland ice sheet (Tunu-N and NASA-U). Beyond providing a better understanding of local-scale snow accumulation patterns on the Greenland ice sheet, the results that will be discussed also have potential implications for the interpretation and selection of ice cores as well as for space-borne remote sensing techniques aimed at deriving snow-water equivalent (SWE) from passive-microwave and/or scatterometry. http://cires.colorado.edu/~maurerj/gpr/gpr_cryosphere.html
NS14A-07
Investigations of the Subsurface by Ground Penetrating Radar (GPR); Examples From Spitsbergen.
With the IPY scientific investigations in arctic regions are more relevant then ever and electromagnetic waves are favorable for geological mapping in Polar Regions for several reasons: They have high penetration in many materials, especially dry and frozen ones, and they give good structural resolution. Ground Penetrating Radar (GPR) is a fast and versatile investigation method, in addition to being cost efficient once the initial equipment cost has been laid down. The equipment can be towed, hauled, driven or carried across virtually any terrain and can in most situations be operated by 2 people. The main problem with the method is its need for a power source and a lack of durability in harsh environments such as the Arctic. Traditionally the main use of GPR in Polar Regions have been mapping of ice thickness but the method has many other applications, the aim of this presentation is to give some examples of GPR use in the high arctic. In the spring of 2004 and 2005 some 117 km of radar line was collected on Tellbreen, a medium size valley glacier located near Longyearbyen, Spitsbergen of which 37,5 km were used for this presentation. The objective of the investigations was to make a simple 3D model of the glacier to gain more knowledge about the internal structures and drainage system. This summer 3 km of radar line was collected on what is presumed to be a cast surface. The data is still not thoroughly processed and further investigations are needed but it indicates that there has been extensive cast activity in the area and the plan is to make a high resolution 3D study of the area.
NS14A-08
Mapping the course of an englacial channel using ground-penetrating radar at Hansbreen, Svalbard
A series of surface ground-penetrating radar profiles at 100 MHz have been collected over an englacial channel system 'Crystal Cave' on the tidewater glacier, Hansbreen, Svalbard. The aim of the surveys was assess radar as a method for determining the size and shape of the channel system and its fill, and to map inaccessible parts of the channel system. It is possible to descend into the main channels of the system from moulins along its course and a detailed map of the accessible regions has been made. This mapping allows ground-truthing of the radar interpretation. The channel system consists of multiple channels at different elevations. The channels generally have low gradient sections linked by near vertical shafts. In common with other channels in Svalbard glaciers, the channels probably initiated as a supraglacial features progressively downcutting into the ice and filling with compressed snow from above. We show that ground-penetrating radar can be successfully used to determine the depth, shape, and water content of englacial channels because of the strong contrast in electrical properties between water, ice, and air. This technique has exciting possibilities for the remote monitoring of inaccessible englacial channels. Members of the Polish Station at Hornsund are thanked for their hospitality and logistic support.