Ice drilling
Instruments can be placed in the drilled holes to record temperature, pressure, speed, direction of movement, and for other scientific research, such as neutrino detection.
[12] IceCube, a large astrophysical project, required numerous optical sensors to be placed in holes 2.5 km deep, drilled at the South Pole.
[14] To understand whether a glacier is shrinking or growing, its mass balance must be measured: this is the net effect of gains from fresh snow, minus losses from melting and sublimation.
[15] The discovery of layers of aqueous water and of several hundred mapped subglacial lakes, beneath the Antarctic ice sheet, led to speculation about the existence of unique microbial environments that had been isolated from the rest of the biosphere, potentially for millions of years.
[18] Data from ice cores can be used to determine past variations in solar activity,[19] and is important in the construction of marine isotope stages, one of the key palaeoclimatic dating tools.
Some tools can also be set up to make use of ordinary household power drills, or they may include a motor to drive the rotation.
[36] In deep ice drilling it is usual to circulate the fluid only at the bottom of the hole, collecting cuttings in a chamber that is part of the downhole assembly.
Since retrieval of each segment of core requires a trip, a slower speed of travel through the drilling fluid could add significant time to a project—a year or more for a deep hole.
The fluid must contaminate the ice as little as possible; it must have low toxicity, for safety and to minimize the effect on the environment; it must be available at a reasonable cost; and it must be relatively easy to transport.
If water is used as a drilling fluid, in cold enough temperatures, it will turn to ice in the surrounding snow and firn and seal the hole.
When a core is brought to the surface, the bubbles can exert a stress that exceeds the tensile strength of the ice, resulting in cracks and spall.
The cutting tool is mounted at the bottom of the drill string (typically connected metal rods[note 1]), and some means of giving it kinetic energy must be provided.
To do this efficiently with manual labour, it is usual to set up a tripod or other supporting scaffold, and a pulley to allow the drill string to be raised by a rope.
Augers have been devised that consist of the helical cutting blades and a space for a core, without the central supporting cylinder, but they are difficult to make sufficiently rigid.
In ice, the cuttings accumulate in the space between the drillpipe and the borehole wall, and eventually start to clog the drill bit, usually after no more than 1 m of progress.
[101] The Agile Sub-Ice Geological (ASIG) drill, designed by IDDO to collect sub-glacial cores, is a recent wireline system; it was first used in the field in the 2016–2017 season, in West Antarctica.
[35][9] It was also used by the Askaryan Radio Array project in 2010–2011 at the South Pole, but was unable to drill below 63 m there because of variations in the local characteristics of the ice and firn.
[123] However, if the goal is to measure temperature in the borehole, then it is better to apply as little additional heat as possible to the surrounding ice, which means that a higher energy drill with a high water flow rate is desirable, since this will be more efficient.
[140] It is more difficult to arrange electrical power at a remote location than to generate heat via a gas boiler, so hotpoint drills are only used for boreholes up to a few hundred metres deep.
No antitorque system is needed for a thermal drill, and instead of a motor that provides torque, the power is used to generate heat in the cutting head, which is ring shaped to melt an annulus of ice around the core.
[38] The sonde of an electrothermal drill designed to run submerged in meltwater may consist almost entirely of the core barrel plus the heated cutting head (diagram (a) in the figure to the right).
Alternative designs for use in colder ice (see diagram (b) at right) may have a compartment above the core barrel, and tubes that run down to just above the cutting head; a vacuum pump sucks up the meltwater.
[153] Instead, they were adapted to use for glaciological research, reaching a depth of 1005 metres and sending temperature information back to the surface when tested in 1968 as part of the Expédition Glaciologique Internationale au Groenland (EGIG).
A SABRE probe consists of a rod that is inserted manually into snow; accelerometer readings are then used to determine the penetrative force needed at each depth, and stored electronically.
Although skates are a popular design for anti-torque and have been used with success, they have difficulty preventing rotation in firn and at boundaries between layers of different densities, and can cause problems when drilling with high torque.
The design was effective at preventing rotation of the sonde, but it proved to be almost impossible to realign the guide fins with the existing slots when tripping in.
[194][195] The most recent anti-torque system design is the use of U-shaped blades, made of steel and fixed vertically to the sides of the sonde.
When the drill is lifted, the ends of the catcher engage with the core and break it from the base, and act as a basket to hold it in place while it is brought to the surface.
[200] Casing, or lining a hole with a tube, is necessary whenever drilling operations require that the borehole be isolated from the surrounding permeable snow and firn.
[201] Low-temperature PVC tubing is not suitable for permanent casing, since it cannot be sealed at the bottom, but it can be used to pass drilling fluid through the permeable zone.
