Linking micro-physical properties to macro features in ice sheets with geophysical techniques
Zusammenfassung der Projektergebnisse
The Emmy Noether group achieved major scientific advances in all work packages of the project since its start. Regarding the first work package "Frequency-dependent dielectric properties of ice samples and application to internal radar-reflector origins", the dielectric backscattering coefficients of ice in the radio-frequency range (10 MHz–1.5 GHz) has been continuously measured for the first time ever. The deduced dielectric permittivity does not show any clear frequency dependence within measurement accuracy. The developed techniques was applied to artificial snow and ice samples as well as ice-core sections from the Alpine drill site Colle Gnifetti. Results obtained from coaxial cell measurements in the MHz-range were compared with well established dielectric-profiling in the kHz-range as well as with simple parameterisations of dielectric properties based on density. The measurements agree within the range of uncertainties. This implies that DEP measurements can well be used to determine properties in the MHz-range for radio-wave propagation. A major remaining experimental difficulty is to further reduce the accuracy of the coaxial cell, coming along with the sample preparation. We recommend a different set-up than coax-cell for routine measurements at MHz frequencies. These were appropriate for first measurements, but are too labour intense for operational applications along ice cores. We suggest to either convert DEP benches to measure in the MHz range or switch to simpler geometries. In addition to ice core, a borehole-style DEP would also allow for characterising dielectric tensor in-situ in a comparably fast way. One major achievement in the second work package "Internal seismic reflectors: detection and interpretation" is the confirmation of the hypothesis that internal seismic reflectors exist at ice domes. The site chosen for the pilot study, the local dome Halvfarryggen, Antarctica, showed several layers which are interpreted as abrupt changes in the orientation of the ice’s crystal fabric. Together with internal layering observed from radar and ice-flow modelling in collaboration with partners from the British Antarctic Survey, it has been possible to determine that the ice dome has been stable at least over the complete Holocene. This finding is of importance to deduce the overall deglaciation history of the Antarctic ice sheet in the Dronning Maud Land sector from the Last Glacial Maximum into the Holocene. A major technical breakthrough has been the transfer of the vibroseis method to ice-covered areas. Vibroseismics is used alternatively to explosives as a seismic sources: a plate is pushed onto the surface and excited to vibrations in a predefined frequency range over some seconds. As part of a new collaboration with the University of Bergen, a conventional 16 ton vibroseis truck put on skis was provided. In a second stage, an medium-weight state-of-the-art vibroseis buggy was used on a new type of sled. During our Antarctic pilot studies, it was possible to employ the vibroseis sources together with a snow streamer for operational seismic surveys. This combination allows "fast" seismic surveys with a production rate of 20 km/day multi-fold coverage and 40 km/day for single-fold coverage. This exceeds production rates of on-shore industry operations. With explosive charges, production rates were about five times less. The results enable us to extend our scientific investigations beyond the ice into the subglacial geology: vibroseis data show that sedimentary structures underneath ice shelves and sheets can be imaged in a comparable quality as with explosive seismics. Ice-internal seismic layers can be imaged as well. In the Antarctic field season 2013/14, a total of 400 km of seismic profiles were acquired in the Ekströmisen catchment in only three weeks. Such a data coverage is unprecedented and is the start new opportunities for ice sheet characterisation — in fact a game changer. During our Alpine pilot studies, it has been demonstrated that light vibroseis sources (<100 kg) work excellently on cold firn, too. By combining data from pressure- and shear-wave microvibrators, it has been possible, in collaboration with LIAG Hannover, to deduce the spatial variation of elastic properties within the glacier. Overall, we know now that vibroseismics is the method of choice over a large peak-force range (from 450 N to 100 kN) for seismic profiling on ice masses. It opens the way for extended geophysico-glaciological surveys on ice sheets and shelves. The third work package "Development of inversion schemes for deducing physical property profiles from geophysical data" revealed the origin of features related to certain patterns observed in radar data of ice sheets. For instance, a zone just above the ice-sheet’s bed, which is free of radar reflections (echo-free zone), could be related to disturbances in the layering on the centimeter-scale, whereas a certain polarimetric radar backscatter pattern is instead likely related to climate transitions. A by-product of the group’s radar studies has been the establishment of a now patented autonomous upwardlooking radar technique, which allows the observation of temporal changes in the radar signature of a snow and firn cover. It is now possible to observe the onset of melting and propagation of the melt front and other features in the snow cover from below. Current applications in the spin-off project MUSI are related to deducing the stability of slopes in respect to snow avalanches. The general technical and logistic difficulties were expected and already considered in the proposal stage of the project. In contrast, the expenditure of time was largeley underestimated for administrative tasks. Among these are transferring funds between DFG, AWI and the University of Heidelberg; purchasing of equipment (including solicitations, negotiating and warranty issues); governmental authorisations and reporting; organisation of logistics and issues related to personnel management and qualification. Regarding the presentation of results in public media, the EKSEIS traverse was accompanied by a blog, now called "eisblog", run by the Helmholtz association’s media office in conjunction with AWI’s public relation department. Olaf Eisen provided an blog entry about every day. Given the type and amount of comments, the blog was well received and is now continued for all glaciological activities, including field excursions for students. It later led to the invitation of Olaf Eisen to the public presentation of climate issues from a glaciology point of view in the "Science Notes", a project which especially addresses pupils and young students for contemporary scientific debates and issues.
Projektbezogene Publikationen (Auswahl)
- (2012) Permittivity of Ice at Radio Frequencies: Part II. Artificial and Natural Polycrystalline Ice, Cold Regions Science and Technology
Bohleber, P., Wagner, N. and Eisen, O.
(Siehe online unter https://doi.org/10.1016/j.coldregions.2012.05.010) - (2012) Potential mechanisms for anisotropy in ice-penetrating radar data, Journal of Glaciology, 58 (209), pp. 613-624
Drews, R., Eisen, O., Steinhage, D., Weikusat, I., Kipfstuhl, S. and Wilhelms, F.
(Siehe online unter https://doi.org/10.3189/2012JoG11J114) - (2013) Characterizing the glaciological conditions at Halvfarryggen icedome, Dronning Maud Land, Antarctic, Journal of Glaciology, 59 (213), pp. 9-20
Drews, R., Martín, C., Steinhage, D. and Eisen, O.
(Siehe online unter https://doi.org/10.3189/2013JoG12J134) - (2013) Determining the age distribution of Colle Gnifetti, Monte Rosa, Alps, by combining ice cores, ground-penetrating radar and a simple flow model, Journal of Glaciology, 59 (213), pp. 179-189
Konrad, H., Bohleber, P., Wagenbach, D., Vincent, C. and Eisen, O.
(Siehe online unter https://doi.org/10.3189/2013JoG12J072) - (2013) Investigating englacial reflections with vibro- and explosive-seismic surveys at Halvfarryggen ice dome, Antarctica, Annals of Glaciology, 54 (64), pp. 189-200
Hofstede, C., Eisen, O., Diez, A., Jansen, D., Kristoffersen, Y., Lambrecht, A. and Mayer, C.
(Siehe online unter https://doi.org/10.3189/2013AoG64A064) - (2013) Joint interpretation of explosive and vibroseismic surveys on cold firn for the investigation of ice properties, Annals of Glaciology, 54 (64), pp. 201-210
Diez, A., Eisen, O., Hofstede, C., Bohleber, P. and Polom, U.
(Siehe online unter https://doi.org/10.3189/2013AoG64A200) - (2014) Influence of crystal anisotropy on seismic velocity analysis, Annals of Glaciology, 55 (67)
Diez, A., Eisen, O., Weikusat, I., Eichler, I., Hofstede, C., Bohlen, T., Polom, U.
(Siehe online unter https://doi.org/10.3189/2014AoG67A002) - (2014) Reassembling Gondwana: A new high quality constraint from vibroseis exploration of the sub-ice shelf geology of the East Antarctic continental margin, Journal of Geophysical Research–Solid Earth
Kristoffersen, Y., Hofstede, C., Diez, A., Blenkner, R., Lambrecht, A., Mayer, C., Eisen, O.
(Siehe online unter https://doi.org/10.1002/2014JB011479) - (2014) Seismic wave propagation in anisotropic ice - Part 1: Elasticity tensor and derived quantities from ice-core properties, The Cryosphere Discussion, 8 (4), pp. 4349-4395
Diez, A. and Eisen, O.
(Siehe online unter https://doi.org/10.5194/tc-9-367-2015) - (2014) Seismic wave propagation in anisotropic ice - Part 2: Effects of crystal anisotropy in geophysical data, The Cryosphere Discussion, 8 (4), pp. 4397-4430
Diez, A., Eisen, O., Hofstede, C., Lambrecht, A., Mayer, C., Miller, H., Steinhage, D., Binder, T. and Weikusat, I.
(Siehe online unter https://doi.org/10.5194/tc-9-385-2015)
