Temporal variations in sea ice resistivity: Resolving anisotropic microstructure through cross-borehole DC resistivity tomography

The distribution and connectivity of brine pockets in first year sea ice has a determining influence on the bulk properties of the ice and its interaction with the environment. The structure of the brine network depends upon both temperature and salinity, and a full understanding of the temporal evolution of sea ice physical properties requires measurements that are sensitive to the microstructure and can also be made without disturbing the natural state of the ice. Direct current resistivity techniques are suited to this as the brine fraction is orders of magnitude more conductive than solid ice. However, due to the preferential vertical alignment of brine inclusions, the bulk resistivity of first year sea ice is anisotropic. Although this makes the interpretation of surface resistivity soundings extremely difficult, consideration of the theory of resistivity measurements in an anisotropic medium shows that the anisotropic resistivity structure may be resolved through cross‐borehole measurements. Borehole pairs with one current and one potential electrode in each hole allow the determination of the horizontal component of the anisotropic bulk resistivity (ρH). Use of four boreholes allows an estimate of the geometric mean resistivity (ρm) to be derived. Combining these measurements allows calculation of the vertical resistivity (ρV). This is illustrated by measurements made in first year sea ice near Barrow, Alaska in April–June 2008. Over this period significant changes in resistivity are observed which may be shown to be related to both the brine volume fraction and the microstructure of the ice.