Improved Quantification of Microscale Ice Properties Using Borehole Geophysical Surveys
Abstract:Quantifying englacial water or air content, vital for accurate determination of ice viscosity and flow rate, is typically done using EM wave (radar) propagation speed because of its order-of-magnitude variation between ice and inclusions such as air and water. To do so at depth, Cross-Borehole (XBH) radar studies, though spatially limited, are preferred to simpler surface surveys, such as Common-Offset surveys, that rely on prior calculation of shallow ice properties. Even so, velocity measurements need to be both accurate and precise; we show that ±0.004m/ns (~2.5%) velocity error implies an uncertainty of ±0.6% in fractional water content estimate: highly significant, as published water contents of polythermal ice from radar velocities are typically between 0–2%.
Thus, we develop a rigorous approach to quantify potential uncertainties in glacier XBH velocity analysis, their consequences for micro-scale englacial property estimates, and suggest necessary standards for field practice. Major sources of potential uncertainty include first break picking, borehole geometry and most important, instrument drift. This causes up to ±0.003m/ns velocity error for only ±2ns time drift. Use of multi-offset surface calibration shots and measurements of borehole orientation and internal shape are vital. Finally, even an approximate (±4%) air content measurement (necessarily found by another method, such as seismic VSPs) can greatly improve water content calculation accuracy.
We apply our approach to XBH data from the ablation area of a polythermal mountain glacier, Storglaciären, Arctic Sweden, where we expect fast surface speeds (>0.168m/ns) due to air content, decreasing with depth due to increasing water content; instead, we find them either near-constant (0.167±0.003m/ns) or increasing with depth to 0.175m/ns, suggesting a strong, masking influence from air. Previous studies of ice properties relied on velocity measurements from a single geophysical survey method. Our study shows the need to combine geophysical survey methods, not just for result validation, but also for accurate quantification of ice properties.