Firn permeability impacts on pressure loss associated with rapid air movement drilling


The polar ice sheets of Greenland and Antarctica are sensitive indicators of weather and climate, and also serve as natural archives of past climate; evidence from within and below the ice is important for scientific investigations. A variety of ice drilling techniques have been used for a wide array of scientific applications, including mechanical ice coring, hot water drilling of access holes, and compressed air-drilling of holes to contain explosive charges for seismic measurements. Creating access holes using compressed air-drilling in ice has rapid rates of penetration in some locations on an ice sheet, yet the technique has failed at shallower depths in other locations. This paper investigates the interaction of the properties of the aged snow, called ‘firn’, with pressure loss associated with compressed air-drilling. Specifically, we report on numerical studies examining the impact of firn layering and firn permeability on air-drill induced pressure changes inside the borehole and in the surrounding firn. Results indicate borehole pressure loss is highest in firn that has bands of higher air permeability layers. A loss of borehole pressure would lead to decreased ice-chip air transport up through the borehole, causing degraded drill performance as ice chips clog the drill bit, stalling the drill. Numerical results show that the presence of higher-permeability layers near the bottom of the borehole may lead to sufficient pressure loss to impede drilling. In contrast to other kinds of ice drilling, where casing the borehole near the surface is common, casing the bottom of the borehole may be necessary for rapid air movement drilling in areas where high permeability bands of layers exist at depth.

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Cold Regions Science and Technology



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