Using High Resolution Borehole Closure and Tilt to Infer the Relation Between Ice Properties and Flow

Ice deformation is sensitive to ice properties, such as crystal fabric, grain size, and impurities, which affect both ice sheet modeling and ice core interpretation. Despite numerous previous studies, in-situ data to infer details of the physical processes and feedbacks is still lacking. We use the closure and tilt of the recently drilled West Antarctic Ice Sheet (WAIS) Divide borehole to evaluate the importance of these properties on deep ice flow. This borehole offers a unique data set because it is located farther from an ice divide and has more measurable deformation than most other recent drilling projects.

We track borehole deformation by making repeat measurements with an Acoustic Televiewer in 2014/15 and 2016/17. This tool collects borehole radial shape, tilt, and azimuth data at very high resolution (depth resolutions as high as 1.4mm). The WAIS Divide coring and borehole logging efforts provide additional detailed ice properties for analysis of these deformation measurements. Here we use a 1D temperature-dependent Glen's Flow Law model as the forward model to predict borehole closure (based on density differences between the ice and borehole fluid) and tilt.

After temperature, crystal fabric emerges as the dominant factor in predicting borehole closure (normal deviatoric stress components) in this location, especially in ice within the bottom 1000m. Layers with strong vertical single-maximum fabric close more slowly than layers with more isotropic fabric. Further, borehole closure (normal stress) and tilt (simple shear) are negatively correlated where there is strong, vertical single-maximum fabric, also indicating that fabric is significant relative to the effect of impurities or grain size (which both should create positive correlations). Because of the feedbacks between ice deformation, fabric development, and climate history, the deep ice is particularly sensitive to these processes, which can be important for ice-flow modeling and interpretation of climate from deep ice cores.