Where does a glacier end ? GPR measurements to identify the limits between the slopes and the real glacier area. Application to the Austre Lovénbreen, Spitsbergen -- 79°N

International audienceGlacier limits are usually mapped according to a spatial discrimination based on color of remote sensing images or aerial photography. What appears like ice (white or light colored areas) at the end of the ablation period (end of summer) corresponds to the glacier, while what appears as rock (dark areas) is identified as the slope. This kind of visual discretization seems to be insufficient in the case of small arctic glaciers. Indeed, the slopes have been described as very unstable parts of glacial basins. Debris are generated by the inclination of the slopes, and reach the glacier surface. Thus, the visible limit does not correspond to the ice extension: a significant amount of ice is potentially covered by rock debris, enlarging the actual glacier surface with respect to the observed area. Hence, we apply Ground Penetrating Radar (GPR) measurements for mapping, beyond the central parts of the glacier, the steep slopes of the Austre Lovénbreen (Spitsbergen, 79°N). The aim is to assess the discrepancy between the limits extracted from remote sensing methods -- aerial photography, satellite images and derived digital elevation models -- and the GPR data which exhibit significant ice thickness at locations considered outside the glacier itself. The ice is observed to extend typically from 25 to 30 meters, and up to 100~meters, under the slopes. These measurements allow for a new determination of the rock/ice interface location following criteria beyond the visual and morphological characteristics seen from the surface, as obtained by remote sensing techniques or in-situ observations

High density coverage investigation of The Austre LovénBreen (Svalbard) using Ground Penetrating Radar

COMInternational audienceA three week field survey over April 2010 allowed for the acquisition of 120 Ground Penetrating Radar (GPR) profiles, adding to a 40 km long walk across an Arctic glacier. The profiles were acquired using a Mal°a equipment with 100 MHz antennas, walking slowly enough to record a 2.224 s trace every 30 cm on the average. Some acquisitions were repeated with 50 MHz or 200 MHz antenna to improve data quality. The GPR was coupled to a GPS system to position traces. Each profile has been manually edited using standard GPR data processing, to pick the reflection arrival time from the ice-bedrock interface. Traveltimes were converted to ice thickness using a velocity of 0.17 m/ns. Dual-frequency GPS mapping and snow coverage thickness were acquired during the same survey. Using interpolation methods, we derived the underlying bedrock topography and evaluated the ice volume

Monitoring seasonal snow dynamics using ground based high resolution photography (Austre Lovenbreen, Svalbard, 79°N)

International audienceArctic glaciers are reliable indicators of global climate changes. However, monitoring snow and ice dynamics in Arctic regions is challenging: some fast but key events can be missed since they are short in time but significant in the hydrological budget. In the context of long term monitoring with high temporal and spatial resolutions of the snow cover dynamics, automated digital cameras were installed around the Austre Lovénbreen glacier basin (Spitsbergen, Norway, 79 N). Despite data losses due to rough weather conditions and control electronics failure, a dataset of 2411 pictures (out of an expected 3294) was gathered over a 1 year hydrological period to assess the snow coverage of the glacier as a function of time with daily resolution. 73% of the total number of expected images was thus recorded, with gaps associated with temporary electronics or data storage failure. The six camera stations oriented so as to observe the glacier itself provide a surface coverage of 96%. Furthermore, geometric corrections of the pictures, using reference ground control points located on the glacier through GPS receivers, yield a quantitative information from initially qualitative images. Projecting the resulting mosaic of the images gathered from six cameras on a GIS allows for the precise monitoring of ice-related processes, and especially the snow coverage evolution over time. This paper summarizes our current understanding of such dynamics, based on the analysis of daily mosaics of images allowing for the observation of both long term evolution on the seasonal scale and the short term events on a weekly scale. Such results demonstrated over one typical full hydrological season (April-October 2009) that snow coverage evolves following discrete steps, either due to water precipitation or warm events, with a snow coverage ranging from 100% (april) to 37% (September

Deriving ice thickness, glacier volume and bedrock morphology of the Austre Lovénbreen (Svalbard) using Ground-penetrating Radar

International audienceThe Austre Lovénbreen is a 4.6 km2 glacier on the Archipelago of Svalbard (79°N) that has been surveyed over the last 47 years in order of monitoring in particular the glacier evolution and associated hydrological phenomena in the context of nowadays global warming. A three-week field survey over April 2010 allowed for the acquisition of a dense mesh of Ground-penetrating Radar (GPR) data with an average of 14683 points per km2 (67542 points total) on the glacier surface. The profiles were acquired using a Mala equipment with 100 MHz antennas, towed slowly enough to record on average every 0.3 m, a trace long enough to sound down to 189 m of ice. One profile was repeated with 50 MHz antenna to improve electromagnetic wave propagation depth in scattering media observed in the cirques closest to the slopes. The GPR was coupled to a GPS system to position traces. Each profile has been manually edited using standard GPR data processing including migration, to pick the reflection arrival time from the ice-bedrock interface. Snow cover was evaluated through 42 snow drilling measurements regularly spaced to cover all the glacier. These data were acquired at the time of the GPR survey and subsequently spatially interpolated using ordinary kriging. Using a snow velocity of 0.22 m/ns, the snow thickness was converted to electromagnetic wave travel-times and subtracted from the picked travel-times to the ice-bedrock interface. The resulting travel-times were converted to ice thickness using a velocity of 0.17 m/ns. The velocity uncertainty is discussed from a common mid-point profile analysis. A total of 67542 georeferenced data points with GPR-derived ice thicknesses, in addition to a glacier boundary line derived from satellite images taken during summer, were interpolated over the entire glacier surface using kriging with a 10 m grid size. Some uncertainty analysis were carried on and we calculated an averaged ice thickness of 76 m and a maximum depth of 164 m with a relative error of 11.9%. The volume of the glacier is derived as 0.3487±0.041 km3. Finally a 10-m grid map of the bedrock topography was derived by subtracting the ice thicknesses from a dual-frequency GPS-derived digital elevation model of the surface. These two datasets are the first step for modelling thermal evolution of the glacier and its bedrock, as well as the main hydrological network

APPLICATION DU MODELE INVERSE AUX INTERACTIONS EAU-ROCHE DANS LES EAUX SOUTERRAINES DES METAGRANODIORITES, SUD-EST COTE D’IVOIRE

In Ivory Cost, few studies have been performed on the process of water-rock interactions in aquifers of basement. The study aim is to provide, from an inverse model by usgs program “phreeqci”, orders of magnitude of the mass transfer from each mineral during the process of water mineralization in the aquifer of metagranodiorites. We use mineralogy and geochemistry of water and rocks. The mineralogical study indicated paragenesis of plagioclase-feldspar-chlorite-biotite and amphibole. Kaolinite is assumed to be the clay product of silicate minerals hydrolysis in the study area. The simulation provided dissolution rate of 8.3 10-4 mol l-1 for plagioclase, 1.7 10-4 mol l-1 for chlorite, 9.4 10-5 mol l-1 for biotite and 2.3 10- 5 mol l-1 for amphibole during water-rock interaction process occurred 15,000 years ago

APPLICATION DU MODELE INVERSE AUX INTERACTIONS EAU-ROCHE DANS LES EAUX SOUTERRAINES DES METAGRANODIORITES, SUD-EST COTE D’IVOIRE

In Ivory Cost, few studies have been performed on the process of water-rock interactions in aquifers of basement. The study aim is to provide, from an inverse model by usgs program “phreeqci”, orders of magnitude of the mass transfer from each mineral during the process of water mineralization in the aquifer of metagranodiorites. We use mineralogy and geochemistry of water and rocks. The mineralogical study indicated paragenesis of plagioclase-feldspar-chlorite-biotite and amphibole. Kaolinite is assumed to be the clay product of silicate minerals hydrolysis in the study area. The simulation provided dissolution rate of 8.3 10-4 mol l-1 for plagioclase, 1.7 10-4 mol l-1 for chlorite, 9.4 10-5 mol l-1 for biotite and 2.3 10- 5 mol l-1 for amphibole during water-rock interaction process occurred 15,000 years ago

Etude du transfert des solutions et des interactions eaux-roches en zone non saturee sous climat periglaciaire - Presqu'ile de Brogger (79deg.N) - Svalbard

SIGLEAvailable from INIST (FR), Document Supply Service, under shelf-number : T 83225 / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc