ALBERT

Description: The measured density and conductivity of a 181 m ice core are used to calculate a profile of complex reflection coefficients. It is confirmed that the amplitude reflection coefficients due to the conductivity variations are negligible in comparison to the amplitude reflection coefficients due to the density variations. Using the magnitudes of these reflection coefficients we obtain a profile of the expected power being returned to the surface. The modeled power return is compared with the results of a short pulse electromagnetic reflection survey near the drill site. We also convolve the profile of complex reflection coefficients with a depth invariant input wavelet to produce a synthetic radargram for the top part of the ice sheet. The phase change given to wavelets reflecting from the conductivity variations has a negligible effect on the synthetic radargram. Using this simple model we are unable to match the positions of the internal layers in the synthetic radargram with those in the reflection survey.

Description: Since the austral summer 1994/95 the Alfred Wegener Institute (AWI) hascarried out airborne radio echo sounding measurements (RES) in Antarctica withits newly designed RES system. Since 1995/96 an ongoing presite survey foran ice coring drill site in Dronning Maud Land (DML) has been carried out.This survey is part of the EPICA programme (European Project for Ice Coring inAntarctica). It covers an area of 948000 km2 with more than 49500 km ofairborne RES obtained from more than 200 hours of flight operation flown from1994-1997. Within this paper first results of the airborne RES survey will begraphically summarized as newly derived maps of the ice thickness and thesubglacial topography as well as a 3D view of surface and subglacial bed andoutcrop topography, revealing a total icevolume of 1.48 million km3.

Description: Gerade ist das neue Polarflugzeug Polar 6 in Bremerhaven eingetroffen und wird für seinen ersten Einsatz in der Antarktis ausgerüstet. Die eingebaute Messtechnik dient beispielsweise dazu, die Erdoberfläche der Antarktis zu vermessen, die unter kilometerdickem Eis verborgen liegt. Und auch das Eis selber ist Gegenstand der Polarforschung: Wo ist der über Jahrtausende gepresste Schnee möglichst ursprünglich geschichtet und nicht umgelagert? Hier sind gute Orte für Eiskerntiefenbohrungen, über die Informationen über die Klimavergangenheit liefern. Der Geophysiker Dr. Daniel Steinhage hat seit 1995 an über 15 Flugzeugexpeditionen des Alfred-Wegener-Instituts in Arktis und Antarktis teilgenommen. Anfang Dezember führt ihn seine nächste Expedition in die Antarktis. In seinem Vortrag stellt er die Forschungsflugzeuge Polar 1 bis Polar 6 vor und berichtet über die Forschungsergebnisse, die ausschließlich durch den Einsatz dieser Maschinen erzielt werden konnten. Steinhage berichtet zusätzlich, wie sowohl die Maschinen als auch das Messgerät stetig weiterentwickelt werden, um aktuellen wissenschaftlichen Fragen auf den Grund zu gehen.

Description: Airborne and ground-based radar have been used extensively in the past to measure ice thickness and to investigate the internal structure of ice sheets in terms of layering. The main reflection mechanisms for internal reflections are changes in density, conductivity, and crystal orientation fabric, which alter thepermittivity of the ice. Linking the different mechanisms to the individual reflection horizons enables thededuction of glaciological parameters like accumulation rates or age-depth estimates. If no sample material from snow pits or ice-cores are available, multi-frequency and multi-polarization measurements must be applied to distinguish between the different reflection mechanisms. The backscattered power of horizons caused by changes in conductivity varies with the center frequency whereas in the case of horizons originating from changing crystal orientation the backscattered power is dependent on the polarization plane of the carrier signal.In this study we examine a sample data set near the German summer station Kohnen (drill site for theEPICA-EDML ice core) on the Antarctic plateau. The data were acquired with an airplane sliding on ground, producing varying incident polarization with a circular profile and several cross profiles with different headings. We find that the backscattered power changes with varying antenna orientation (i.e. polarization). In the upper third of the ice column the backscatter has two maxima with a 180° symmetry. The maxima align with the direction of minimal surface strain. At approximately 900 m depth the anisotropy is shifted by 90° in heading azimuth, with the maxima now being parallel to the maximum in surface strain. This dataset is unique, as airborne systems (primarily designed for the sounding of ice thickness) are usually not used for ground-based applications. The observed anisotropy appears clearly and is intriguing as the reason for it is entirely unknown. As primary suspects we consider the role of changing crystal orientation and ellipsoidal shaped air bubbles. The effect is visible from 200 1400 m. It appears distributed along the entire interval, and not restricted to individual layers. It seems that the polarization dependence becomes visible by a changing background level of the acquired signal, which is otherwise largely dominated by layer-like, polarization independent reflections. Hence we apply a (semi-analytical) volume scattering model in order to understand the different reflection mechanisms better. From ice-core measurements it is known that the crystals in the upper hundred meters are only weakly aligned (if at all), and it is unclear how the crystal orientation changes overshort depth intervals (~10 m). The rotation of the anisotropy coincides with the clathrate transition in the ice core and thus we first focus on the effect of anisotropic air bubbles. In an in-coherent approach we treat the ice matrix as a random medium and use the vector radiative transfer theory to incorporate boundary conditions. In a second step we model the effect of crystal orientation to estimate both, the degree of alignment and the statistical variance in the permittivity tensor needed to generate the observed pattern in backscatter. Doing so, we eventually aim at pinning down the mechanisms for the anisotropy in the upper interval, lower interval and the interrelation of the two by a shift of 90°.Anisotropic air bubbles as well as aligned crystal orientation allow to deduce stress and strain rates and a potential change thereof along depth. So far it is largely unclear, how surface strain rates relate with strain rates within the ice. If one of the two suspected mechanisms can be excluded or confirmed, this study may serve as a case study for future polarimetric surveys with low-frequency radars, in order to supply ice-sheet modelling with adequate boundary conditions - including changes in the internal structure of ice sheets along depth.

Description: Radar data (center frequency 150 MHz) collected on the Antarctic plateau near the EPICA deep-drilling site in Dronning Maud Land vary systematically in backscattered power, depending on the azimuth antenna orientation. Backscatter extrema are aligned with the principal directions of surface strain rates and change with depth. In the upper 900 m, backscatter is strongest when the antenna polarization is aligned in the direction of maximal compression, while below 900m the maxima shift by 90◦ pointing towards the lateral flow dilatation. We investigate the backscatter from elongated air bubbles and a vertically varying crystal-orientation fabric (COF) using different scattering models in combination with ice-core data. We hypothesize that short-scale variations in COF are the primary mechanism for the observed anisotropy, and the 900m boundary between the two regimes is caused by ice with varying impurity content. Observations of this kind allow the deduction of COF variations with depth and are potentially also suited to map the transition between Holocene and glacial ice.

Description: Halfvarryggen is a small ice dome bordering the catchment area of Ekströmisen. It is a candidate for drilling one IPICS 2k/40k ice core. Near the actual dome, three ice divides merge in a triple point. Halvfarryggens internal structure has been imaged with airborne radio-echo sounding (RES) in the past years, which indicate upwarping internal layers, so-called isochrone arches (or partly Raymond bumps), which develop into a double bump at larger depths. Modelling studies (Martin et al., 2009, JGR-F) indicate that the crystal orientation fabric (COF) at larger depths at ice domes like Halvfarryggen should be highly anisotropic. As changes in COF not only change the dielectric permittivity, but also the acoustic impedance contrast, such changes should also be detectable with seismic methods. We present an overview and preliminary results of a geophysical LIMPICS campaign at Halfvarryggen in 2009/10, involving reflection seismics, shallow ground-penetrating radar (GPR) and other glaciological studies. Main scientific goals were to detect internal reflection horizons along a profile nearby the ice dome with seismics and GPR, image the internal layer architecture along hexagons crossing all three ice divides, detect the ice-bed interface and image the upper tens of meters of the underlying bedrock.