Porosity of a sea-ice pressure ridge keel estimated on the basis of surface nuclear magnetic resonance measurements

The volume fraction of liquid in sea ice (which typically accounts for well over 90% of the total porosity) is an important physical quantity because it determines the mechanical and transport properties of sea ice and for high-porosity sea ice it is an important component of the ice mass budget. Of particular importance, but poorly studied, is the distribution of pore space and liquid-filled voids in sea-ice pressure ridges. Up to 30% of the Arctic sea ice volume consists of deformed ice in the form of pressure ridges. These are three dimensional and non-consolidated structures built up of crushed ice floes, and are usually between 2 and 20 meters thick. Below sea level larger voids between ice floe fragments and pores (cm to sub-mm scale) in a pressure ridge are filled with seawater or brine, such that its total porosity equals its liquid water content. Determining the volume fraction of these inclusions is of great importance for estimates of the sea-ice mass budget and thickness distribution, as determined by other geophysical measurements, such as electromagnetic induction sounding (EM) or submarine sonar. A powerful hydrogeophysical method for directly estimating liquid water content (and porosity) is surface nuclear magnetic resonance (surface NMR). The idea of applying surface NMR to aquifer investigations arose in the 1960s but the first effective equipment was not designed and built until the early 1990s. In this study we explore the utility of surface NMR tomography to provide volume integrated measures of liquid water content in sea-ice pressure ridge keels. We further highlight challenges which need to be addressed to make surface NMR a practical sea-ice field tool in the future. A numerical modeling study was performed prior to the field survey, which showed that surface NMR is in theory suitable for determining the water content distribution in pressure ridges. The surface NMR fieldwork and the numerical modeling study are discussed in greater detail in Nuber et al. (2012). In addition to the work of Nuber and co-workers, the present expanded abstract presents the results of an electrical resistivity tomography survey carried out over the same pressure ridge. The method of surface NMR is explained in detail by Hertrich et al. (2008) and will not be repeated here.