Deep Drilling Of Ice Cores At Vostok Station

The first successful attempts to drill deep holes at Vostok Station with thermo-electrical drills equipped with a pump to remove meltwater from the bottom of the hole, also capable of extracting a core, began in 1970.

The Leningrad Institute of Mining and its Department of Drilling, led by Professor Kudryashov, became a main participant in the construction, manufacture, and deep-core drilling at Vostok Station.

The idea of making a borehole by simply melting ice under a hot ring powered by electricity was not only simple but also a natural process. Nobody could imagine then that drilling to the bottom of the ice sheet would take more than 30 years of dedicated work in Leningrad and at Vostok Station itself. Professor Kudryashov, Principal Investigator on the project for 25 years passed away, never having the pleasure of seeing its successful completion (it is interesting to mention that his team, until about 1997, did not believe in the existence of the lake and had prepared themselves and their equipment to reach the bottom and to drill solid bedrock beneath the ice sheet).

The first "dry" borehole drilled at Vostok Station with the thermo-electrical drilling unit in 1970 achieved a depth of 500 m. Temperature measurements showed an increase from —57°C at a depth of 20 m to —53.5°C at 500 m. A total of 293 trips down and up were performed to reach this depth and an average traverse at a single trip (close to a length of the core) was 1.73 m. The average speed of drilling was about 1.3mh—In May 1972 the "dry" borehole at Vostok Station reached a depth of 952.5 m.

The time involved for the drilling tool to reach the bottom at depths of 500 m was significant. It was clear that the borehole was closing under the surrounding pressure and the borehole would have to be filled with some liquid, heavy enough to compensate for the pressure and prevent borehole closure. An aviation fuel including a mix of a type of kerosene, tetrachloric hydrogen, tetrabromethane, and freon was chosen. All components were well mixed, and the resulting mixture had a low temperature stability and did not interact with the water or ice. The viscosity of the mixture was low, which was important because of the hundreds of round trips required to make a deep borehole.

The drilling device consisted of a 10 m long tube of steel with outside/inside diameters of 180/130mm. The lower end had a ring heating element, which was powered electrically (approximately 3.5 kW, Figure 7.1). The tube was kept vertical on an armored logging electrical cable. When it was placed on the snow or ice surface and the heating device was turned on, the drill began to melt its way downward. A core with a diameter slightly less than the inner diameter of the tube entered the tube until the space was completely filled. Melted water was then pumped from the bottom of the borehole into a section of the tube above the core container. This section served as a meltwater container. The upper part of the tube contained electrical equipment for the drill. When the core container was full, a cable winch at the surface of the ice sheet raised the drill and core. The "teeth" of a specially designed core-cutting device installed at a lower part of the tube cut off the core from the bottom of the borehole and prevented it from falling out while the drill was raised to the top of a drilling mast. The mast was protected from the weather elements by a protective cover. The mast, winch, and logging device with control and monitoring equipment, were mounted inside a drill building, which consisted of two movable steel houses on sleds. The core and meltwater were removed from the

Figure 7.1. The thermo-electric drilling device used to drill deep holes at Yostok Station (adapted from Kudryashov et al., 1982): 1 - heater ring, 2 - core lifter, 3 - core barrel, 4 -drain adapter, 5 - water pipes, 6 - heater power cable, 7 - water heater, 8 - water tank, 9 -water pipe, 10 - pump adapter, 11 - pump, 12 - electrical section, 13 - cable termination, 14 -end cap, and 15 - electro-mechanical cable.

drill and the drill returned to the bottom of the borehole for another round of drilling. The temperature inside the building was maintained above 0°C with the aid of two heaters; the outside temperature was less than —70°C. The temperature inside the core drilling device and the cable just after lifting the device from the hole was below 0°C.

The development, manufacture, and testing of a new drilling device, capable of work in boreholes filled with this liquid, plus the construction of a new 14-m mast and advanced new winch and connecting equipment took more than 5 years. It was not until 1980 that this new drilling installation was transported from Mirny Station

et al., 2002): 1 - drilling building, 2 - tower, 3 - hoisting winch, 4 - cable, 5 - control deck, 6 -drilling device, 7 - drill-handling equipment, 8 - DC electric power generator, 9 - electric motor, 10 - worm reducer, 11, 12 - pulleys, and 13 - geophysical winch.

to Vostok Station on a tractor train. The filled borehole was drilled using this equipment to a depth of 1,415m in 1981 and in August 1985, in the middle of the austral polar night, a depth of 2,002 m was achieved. The drilling complex at Vostok Station used for the deepest borehole is shown in Figure 7.2.

The length of the drilling system is 18 m, its width 4m, and height 15 m. An electrical motor for the winch uses 20 kW, the heating elements 12 kW, and lights 5kW. The average speed of pulling/lowering operations at a maximum depth of the borehole (up to 4,000m) with a 16mm diameter armored cable is 0.7ms"1 (Kudryashov et al., 2000).

The drilling of the deep borehole number 5G, with its base 130 m above the lake, was started at the surface in 1990 by the 35th Soviet Antarctic Expedition (SAE) (Kudryashov et al., 2000) using the TELGA thermo-electric drill. A TBZS thermoelectric drill was used for holes filled with liquid for deeper horizons (Kudryashov et al., 2000). In 1993 thermal drilling of the hole was terminated at a depth of 2,755 m.

In 1994 drilling operations were suspended due to financial and logistical difficulties - since that time ice coring and ice core studies have been continued as a collaborative effort between Russia, France, and the U.S.A.

Figure 7.3. Electro-mechanical ice core drilling device KEMS (length 8-13 m, diameter 132 mm, length of core barrel 3 m, weight 240 kg) for drilling the lower part of the deep hole at Vostok Station (from Kudryashov et al., 2000): 1 - drill head, 2 - core barrel, 3 - chip chamber including chip filter, 4 - reducer, 5 - driving electric motor, 6 - pump, 7 - anti-torque system, 8 - hammer block, 9 - electric chamber, 10 - cable suspension clip, and 11 - cable.

The last phase of drilling operations was performed with an electro-mechanical device (KEMS), which was developed and manufactured in Leningrad to drill into the bedrock below the ice sheet under Vostok Station (Figure 7.3). This device had a ring of cutting "teeth" powered by an electrical motor instead of a heated ring and anti-torque leaf springs in the upper part of the drill.

Drilling at Vostok Station was stopped in January 1996, when the depth of the borehole was 3,300 m. Termination of the drilling and conservation of the hole have been undertaken according to a Scientific Committee on Antarctic Research (SCAR) recommendation to prevent any danger of possible contamination of the lake in spite of the fact that some hundreds of meters of undrilled ice remain above the lake water. But under pressure from scientists interested in receiving the deepest possible ice core for study, sessions of core drilling proceeded in the 1997-1998 field season (43rd Russian Antarctic Expedition), and hole 5G reached a depth of 3,623 m, which remained the situation in autumn 2004.

It was shown before the termination of drilling that stable isotopes, dust, and electrical conductivity measurements (ECM) are well preserved in the ice from the surface of the ice sheet down to the depth in borehole 5G, offering a continuous climate record for the last 400,000 years. It was shown that there were four distinct climatic periods of cooling and warming within this period of time. Results of the study of this core by specialists in Russia from the St. Petersburg Arctic and Antarctic Research Institute and Mining Institute and the Institute of Geography of the Russian Academy of Sciences, and in France at the Laboratoire de Glaciologie et de Geophysique de l'Environnement, Grenoble and the Laboratoire de Modelisation du Climat et de l'Environnement, Saclay under the framework of Russian-French collaboration on Vostok core investigation are remarkable. Some results are shown in Figures 7.4 and 7.5. It was also shown that close to this depth the isotopes and ECM signals became smooth and were no longer an effective means for deciphering the glacial-interglacial changes. This meant that a two-decade-long core which provided access to the most extended and undisturbed paleo-environmental series extending 400,000 years, along with the last four climatic cycles, came to an end.

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