Deep ice coring at Vostok Station (East Antarctica) by electromechanical drill

A deep bore-hole 5 G at Vostok Station (East Antarctica) from the depth of 2755m was drilled by electromechanical drill KEMS-132. During wintering of the 40th (1995) Russian Antarctic Expedition and summer seasons of the 41st (1995/1996), 42nd (1996/1997) and 43rd (1997/1998) RAE the bore-hole was deepened from 2755 to 3623m. At present the bore-hole 5 G has a complicated stepwise structure. The casing with an inner diameter of 165mm insulates the upper 120m of the hole from permeable firn. From 120m to 2200m the diameter of the hole is 153mm. In the deeper sections of the hole its diameter decreases to 139mm (between 2200 and 3095m). 138.4mm (3095-3321m), 137.9mm (3321-3500m)-136.2mm (3500-3570m), and 135mm (3570-3623m). The drilling fluid, a mixture of kerosene and Forane F-141b as densifier, has an average density of 928kg/m^3. Its level in the hole is maintained at a depth of 95m. The difference between the overburden pressure of ice and the hydrostatic pressure of the fluid at the bottom of the hole is estimated to be about 0.1MPa. Accordingly, the rate of bore-hole closure at the bottom is calculated to be less than 0.1mm/year. Data regarding the technology of drilling by electromechanical drill KEMS-132 (description of drilling complex, electromechanical drill, casing, stability of the bore-hole) are given.