Cross-borehole resistivity tomography of sea ice

Abstract

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%.

Introduction

Pockets of brine trapped within sea ice have a determining influence on its physical (Trodahl et al., 2001, Eicken, 2003) and biological (Krembs et al., 2000) properties, and, through these, on global climate (Saenko et al., 2002, Beckmann and Goosse, 2003) and ecology. In particular the bulk properties of sea ice are sensitive to the degree of connectivity of the brine pockets within the ice-brine composite. The connectivity permits the flow of brine and, in turn, of nutrients and heat. Thus, for example, the heat flow in sea ice shows anomalies associated with brine and meltwater movement (Lytle and Ackley, 1996, Pringle et al., 2007).

In theory the internal structure of sea ice can be studied using any transport property to which the brine and ice components contribute differently. Fluid transport, which requires a direct measurement of brine permeability, has been studied by several authors using NMR techniques (e.g. Callaghan et al., 1998, Callaghan et al., 1999, Eicken et al., 2000, Mercier et al., 2005) as well as field-based methods (e.g. Freitag and Eicken, 2003). However, there are severe difficulties in making accurate measurements under the required in situ constraint. An alternative method to try and resolve the internal structure of sea ice utilizes the electrical resistivity of the ice, which can, in principle, be measured using the direct current (dc) resistivity geophysical technique. DC resistivity soundings, made on the surface of the sea ice, have previously been discussed by Fujino and Suzuki (1963), Thyssen et al. (1974), Timco (1979) and Buckley et al. (1986). Much of this work focussed on the ability of resistivity measurements to determine the total ice thickness, although Timco (1979) attempted to relate the measured resistivity to the microstructure of the ice by developing a model for the way in which the bulk resistivity depended upon the geometry of brine inclusions. However, the observed preferential vertical orientation of brine pockets leads to the bulk resistivity being anisotropic and this causes significant complications in the interpretation of surface based resistivity data.

In this paper we briefly review the theory behind resistivity measurements in a medium in which the bulk resistivity is anisotropic. We use this to show that, in contrast to surface measurements of resistivity, which are sensitive only to the geometric mean of the principal resistivities, cross-borehole resistivity measurements should be able to not only determine an accurate thickness of sea ice, but also yield measurements of the horizontal component of the bulk resistivity. We demonstrate this by presenting the results of a series of field measurements made on first-year sea ice at Barrow, Alaska over the period during which the sea ice underwent substantial warming during the spring.