Cross-borehole resistivity tomography of sea ice

The presence of brine inclusions with an alignment that is preferentially vertical means that the bulk resistivity structure of sea ice is anisotropic. This complicates the interpretation of surface resistivity soundings of sea ice. We show that consideration of the theory of resistivity measurements in an anisotropic medium suggests that cross-borehole measurements using one current and one potential electrode in each borehole should allow the determination of the horizontal component of the anisotropic bulk resistivity. A series of cross-borehole measurements made in first-year sea ice near Barrow, Alaska, as the ice warmed through the spring, yields 3D models of the resistivity structure which support this prediction. The derived models show an evolution of the resistivity structure which (1) at temperatures less than − 5 °C is broadly consistent with the expected variation with brine volume fraction predicted by Archie's Law and (2) shows evidence of a percolation transition in the horizontal component of the resistivity when brine volume fractions exceed 8–10%.