Behavior of a deep hole drilled in ice at Vostok Station

A coring hole (5G) was drilled at Vostok station down to the depth of 2500m using an electro-thermal drilling system. Relationships between ice core, hole diameters, and drill speed are presented. The hole was only partially filled with fluid (of density of 860kg m^-3) so that ice pressure was not counter-balanced by the fluid column. After one year, diameter us, depth was measured. The hole deformation was found to be significant for depths below 1500m. The hole closure rate is in agreement with the law proposed by PATERSON (Rev. Geophys. Space Phys., 15,47,1977) and extrapolated to Vostok conditions as well as with previous studies from BLINOV and DMITRIEV (Antarktika, 26,95,1987). This information as well as the data from the fluid density properties is used to predict the hole closure for greater depths. Due to some drawbacks in using a high density fluid for thermal drilling operations a technical strategy for drilling toward 3000m is presented.