Deep drilling into the ice sheet at Vostok station, Antarctica, was started by specialists of the Leningrad Mining Institute (since 1991, St Petersburg State Mining Institute) in 1970. Five deep holes were cored: hole No. 1 to 952 m; hole No. 2 to 450.4 m; hole No. 3G (3G-1, 3G-2) to 2201.7 m; hole No. 4G (4G-1, 4G-2) to 2546.4 m; and hole No. 5G (5G-1) to 3650.2 m depth. Drilling of hole 5G-1 is not yet complete. The deep drilling at Vostok station has had successes and problems. All the deep holes at Vostok have undergone at least one offset drilling operation because of problems with lost drills. These deviations were made successfully using a thermal drilling technique. Several drilling records have been achieved at Vostok station. The deepest dry hole, No. 1 (952 m), was made during Soviet Antarctic Expedition (SAE) 17 in 1972. The deepest fluid-filled hole, No. 5G-1, made by a thermal drill (TBZS-132), reached 2755 m during SAE 38 in 1993. The deepest fluid-filled hole in ice, No. 5G-1, was drilled with a KEMS-132 electromechanical drill and was stopped above Vostok Subglacial Lake at 3650.2 m depth during Russian Antarctic Expedition (RAE) 51 in 2006.In the summer season of the 15th SAE, the first drilling shelter was constructed on two steel sleds (Fig. 3), providing a work area measuring 15 Â 2.9 Â 2.5 m. A 9.7 m round tower, measured from the top of the hole, was constructed in the center of the work area and wrapped in rubberized cloth for protection ( Fig. 4).

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IntroductionCore drilling is considered to be the most effective method of studying glaciers and sub-glacial rock. Thermal drills suspended on cables are very simple in construction and enable the drilling of deep bore-holes in firn and ice (Kudryashov and others 1991a). However, mechanical dri 11 ing is characterized by lower energy consumption and a higher rate of penetration (Ueda and Garfield 1970; Gundestrup and others 1984). Moreover, drilling of subglacial rock and ice containing mineral inclusions becomes possible only by using electromechanical drills suspended on cables. Drilling equipmentDeveloped at the St Petersburg Mining Institute, the Core Electromechanical Drill 112 (KEMS-112) (Bobin and others 1988) is designed for core drilling in glaciers and sub-glacial rock to a depth of 4000 m at temperatures down to -60°C. The drill consists of a core chamber, including a drill head and inner collector of ice slurry; an electric motor drive with gear reducer; an electric rotary pump; an anti-torque system; a hammer block to induce core breakage and to aid the extraction of a stuck drill; a slip-ring assembly; and a cable termination. The independent, smoothly regulated electric drive of the pump provides continuous circulation of hole liquid during drilling and different technological operations (such as bore-hole cleaning). The construction of the inner slurry collector provides regular and complete filling with chips of a high average density (400-550 kg m 3 ).The outer diameter of the drill head for different modifications is 112-116 or 132-135 mm, with a core diameter of 85-89 or 107 mm, respectively. The length of the core barrel is 2-4 m; the total length of the drill, depending on the core barrel, is 7-12 m; and its weight is 120-180 kg. The power of the driving three-phase 220 V AC motor is 2.2 kW. The voltage on the DC motor of the pump can be changed from 15 to 27 V, with a maximum current of 12 A. A two-stage gear-reducer lowers the nominal 2800 rpm of the driving motor to 230 rpm in the core chamber.The diameter of the cable, type KG 7-95-180, is 16.5 mm, the breaking strength is 95 kN, and the weight in the air is 865 kg km-'. The cable has six teflon-insulated conductors, each with an area of 2.5 mm 2 , and a central co-axial conductor of the same cross-section.For drilling operations, a special complex of surface technical devices has been developed (Chistyakov and others 1988),including amovable, warmed, drilling building with a metal rig, a winch with an electric driving unit, a diesel-electric generator (operational power is 20 kW), a unit for regular drill transference during the drilling process, an assembling/disassembling drill device for core and ice slurry recovery, and a control desk by the drill and surface equipment. DrillingThe tests of the electromechanical drill KEMS-112 were carried out in three stages. The initial stage tested the gear reducer, the system for the circulation of hole liquids and the slurry collector, the anti-torque system, the core-break system, and element...

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