Improved crosshole radar data collection for the measurement of porescale water

Abstract:

Characterising the distribution and magnitude of moisture content in the subsurface is important for a wide range of environments. In a glacial environment, porosity changes define the snow-firn, and firn-ice transitions, and the boundary between any cold (water-free) and temperate ice; these boundaries each have a large spatial variability. Crosshole radar (XHR) has been successfully applied in a wide range of near-surface investigations, allowing repeatable measurements of material volumes remaining under natural stresses. Porescale water quantities are inferred from bulk velocity variations caused by differences in electromagnetic properties between the water and the surrounding material. An increase in glacier porescale water from 0 to 0.8% will soften the ice and triple the strain rate. Hence, highly precise radar velocities are required, as ±0.003 m/ns is equivalent a water-content of ±0.4 volumetric %. We rigorously assess the sources of uncertainty in XHR surveys, their consequent effect on the measured water-content, and hence propose several revisions for improved data acquisition. We analyse two field data examples acquired with contemporary and improved techniques. We find contemporary data acquisition produces average velocity uncertainties of ±3.0% (±0.005 m/ns), equivalent to ±0.8 volumetric % water-content. Improved techniques reduce this uncertainty to ±1.5% (±0.003 m/ns), equivalent to ±0.4 volumetric % water-content. Measurement of the borehole diameter when hot-water drilling can further reduce this uncertainty to ±0.8% (±0.001 m/ns), or ±0.2 volumetric % water-content. The property differences between water, air and ice mean radar velocity variations are dominated by water changes. Although XHR provides a highly precise measurement, we find that accurate determination of water-content is not possible using this technique alone, without prior knowledge of the air-content; this could be quantified from a co-located, complementary borehole technique, e.g. Vertical Seismic Profile.