Properties of grain boundary networks in the NEEM ice core

The microstructure along the entire NEEM ice core (North-West Greenland, 2537 m length) drilled in 2008-2011 has been analyzed based on a large data set of sublimation groove images. The sublimated surface of vertical section series (six consecutive 6 x 9 cm2 sections in steps of 20 m – in total about 800 images) have been scanned by a Large Area Scanning Macroscope. In these cross-section images 10-15 micron wide grain boundary grooves and air bubbles appear dark, whereas the inside of grains appears gray (further developed by [S. Kipfstuhl et al., 2006, Journal of Glaciology, 52, 398-406]). A dedicated method of automatic image analysis has recently been developed to extract and parameterize the grain boundary networks of this set [T. Binder et al., 2013, Journal of Microscopy, in press]. In contrast to the microstructure obtained from thin sections between crossed polarizers in transmitted light, sublimation groove images in reflected light allow to include small grains (equivalent radius of 65 micron) in the size distribution. It has become possible to extract continuous curvature values of grain boundaries, an estimate of the lower bound of the stored strain energy and the dislocation density. In this contribution we give an overview on profiles of different calculated parameters related to deformation and recrystallization mechanisms. In older glaciological studies the value of the lower cut-off for grain sizes considered for calculation of a mean grain size has been arbitrary. We suggest to compare different definitions of the lower cut-off in the size. With respect to the important question which processes are dominating the grain size evolution in the late- to middle-Holocene, high sensitivity to the definition of this cut-off has been found [T. Binder et al., 2013, Materials Science Forum, 753, 481-484]. Between 250 m and 1000 m depth the curvature of grain boundaries steadily increases and grains become more irregularly shaped which correlates with increasing pressure of air bubbles. In the NEEM ice core the depth of the transition from air bubbles to clathrate hydrates clearly can be separated from the depth where the transition from Holocene to the last glacial takes place. In this way, we found that the shape of grains is highly influenced by air bubbles, whereas the size of the grains is more sensitive to climatic transitions

A comparison of state-of-the-art image evaluation techniques for analysis of opaque flows

In this paper we compare state-of-the-art fluid motion estimation algorithms with respect to the application to opaque flow images, such as Particle Image Velocimetry (PIV), Optical Flow and specialized variants of these. In order to compare the outcome of the algorithms we perform turbulent jet experiments with different Reynolds numbers to generate opaque images with a known average flow field as a reference to estimate the errors. Thereafter the algorithms are used to estimate the velocity of a submerged oil/gas plume that was recorded by an observation camera. We will show that the estimates strongly depend on the choice of algorithm, especially for field test images where cloud-like image structures are the only information to retrieve the motion

Combined transmission and reflection optical microscopy of ice core sections

Microstructure analysis of ice cores is vital to understand the processes controlling the flow of ice on the microscale. To quantify the microstructural variability (and thus occurring processes) on centimeter, meter and kilometer scale along deep polar ice cores, a large number of sections has to be analyzed. In the last decade, two different methods have been applied: On the one hand, transmission optical microscopy of thin sections between crossed polarizers yields information on the distribution of crystal c-axes. On the other hand, reflection optical microscopy of polished and controlled sublimated section surfaces allows to characterize the high resolution properties of a single grain boundary, e.g. its length, shape or curvature. Based on a polar and an alpine ice core we applied both methods to the same set of sections. This enables us to combine all information on crystal orientation and (sub-)grain boundaries. In this contribution we introduce the method of combined transmission-polarization and reflection microscopy as well as an image processing framework for processing and matching both image types [T. Binder et al., 2013, Journal of Microscopy, in press]. The information content of both analysis methods is limited and influenced by different types of artifacts. It is exemplary shown how the combination allows to compensate for deficiencies of one method. The gray values in images of the grain boundaries on polished ice core sections are influenced by the duration of surface sublimation and the energy/misorientation of the grain boundaries in the section. By combining these gray values with the misorientation obtained from the corresponding thin section imaged between crossed polarizers we try to validate the information content of gray values on the basis of large data sets. This approach is compared to X-ray Laue diffraction measurements (yielding full crystallographic orientation) which validated the sensitivity of the surface sublimation method [I. Weikusat et al., 2011, Journal of Glaciology, 57, 111-120]. As microscopy in transmission mode acquires volume information and microscopy in reflection mode gains information on the surface, an “optimal” matching of both images contains displacements of grain boundary sites. We try to quantify this inaccuracy which can also be interpreted as orientation of the grain boundary surface in 3D