Subglacial till and bedrock drilling

Drilling to till and bedrock of ice sheets and glaciers offers unique opportunities for examining processes acting at the bed. Samples of basal and subglacial material contain important paleo-climatic and paleo-environmental records and provide a unique habitat for life, give significant information on sediment deformation beneath glaciers and its coupling to the subglacial hydraulic system, subglacial geology, and tectonics. Retrieving bedrock samples under ice sheets and glaciers is a very difficult task. Drilling operations are complicated by extremely low temperature at the surface of, and within glaciers, and by glacier flow, the absence of roads and infrastructures, storms, winds, snowfalls, etc. Nevertheless, borehole drilling might be considered as the optimal method to access beds of the glaciers and to sample subglacial material. Four types of subglacial drilling technologies are considered: (1) non-rotary sampling; (2) non-core penetrating; (3) pipe-string rotary drilling; (4) electromechanical cable-suspended drilling. The most simple and effective systems for sampling in subglacial soft sediments or unfrozen till from pre-drilled access holes are non-rotary devices like gravity corer and piston corer. The maximal thickness of ice is determined by the length of wire rope attached to the corer and could possibly be more than 4000 m. Potentially, piston sampling can reach a maximal depth of 25 m in soft subglacial lake sediments. In stiffer sediments a hammer corer or vibrocorer should be used. To install different sensors and markers into the soft till beneath glaciers and to measure basal sliding, different types of sediment non-core penetrators were used. Typically the boreholes are pre-drilled by hot-water systems as well. To recover core of the true bedrock the rotary drilling systems are used. The experience of pipe-string rotary drilling in subglacial environment showed that drilling operations were very unstable, and the recovery of subglacial sediment was generally poor. Commercial drilling rigs for drilling up to the depth of 3000 m or more tend to be very heavy and require a large logistical load to move and support. They also require more equipment for the circulation system. Taking into account that they are not adapted for extremely unfavorable conditions in Polar Regions and also need high power consumption, these drill rigs were not considered for subglacial exploration. Electromechanical cable-suspended systems are widely used for core drilling in pure and debris-containing ice. The main feature of these systems is that an armored cable with a winch is used instead of a pipe-string to provide power to the down-hole motor system and to retrieve the down-hole unit. The use of armored cable allows a significant reduction in power and material consumption, a decrease in the time of round-trip operations, and a simplification in the cleaning of the hole from the cuttings. To penetrate frozen till and bedrock the electomechanical drills can be adapted for coring bedrock. This was confirmed by four successful penetrations into the bedrock carried out by U.S. and Russian specialists. The procedure of till and bedrock drilling and the geological description of retrieved debris-containing ice and bedrock cores are given.