Editing Ice core (section)

== Coring ==
{{see also|Ice drilling}}
[[File:Brooks ice auger patent drawings 1932-retouched.png|alt=Drawing of a cylinder with two helical flanges around it and cutting teeth at the bottom|left|thumb|Ice auger patented in 1932; the design is very similar to modern augers used for shallow drilling.<ref name="Talalay-2016-2">{{harvnb|Talalay|2016}}, pp. 34–35.</ref>]]
Ice cores are collected by cutting around a cylinder of ice in a way that enables it to be brought to the surface.  Early cores were often collected with hand [[Auger (drill)|augers]] and they are still used for short holes. A design for ice core augers was patented in 1932 and they have changed little since.  An auger is essentially a cylinder with [[helix|helical]] metal ribs (known as flights) wrapped around the outside, at the lower end of which are cutting blades.  Hand augers can be rotated by a [[T-handle|T handle]] or a [[Brace (tool)|brace handle]], and some can be attached to handheld [[electric drill]]s to power the rotation.  With the aid of a [[tripod]] for lowering and raising the auger, cores up to 50&nbsp;m deep can be retrieved, but the practical limit is about 30&nbsp;m for engine-powered augers, and less for hand augers.  Below this depth, electromechanical or thermal drills are used.<ref name="Talalay-2016-2" />

The cutting apparatus of a drill is on the bottom end of a drill barrel, the tube that surrounds the core as the drill cuts downward.  The [[Drill cuttings|cuttings]] (chips of ice cut away by the drill) must be drawn up the hole and disposed of or they will reduce the cutting efficiency of the drill.<ref name="Talalay-2016-3">{{harvnb|Talalay|2016}}, p. 59.</ref> They can be removed by compacting them into the walls of the hole or into the core, by air circulation (dry drilling),<ref name="Talalay-2016-3" /><ref name="Talalay 2016">{{harvnb|Talalay|2016}}, p. 7.</ref> or by the use of a [[drilling fluid]] (wet drilling).<ref name="Talalay-2016-4">{{harvnb|Talalay|2016}}, p. 77.</ref> Dry drilling is limited to about 400&nbsp;m depth, since below that point a hole would close up as the ice deforms from the weight of the ice above.<ref>{{Cite web|url=http://www.iceandclimate.nbi.ku.dk/research/drill_analysing/drilling_techniques/deep_drilling/|title=Deep drilling with the Hans Tausen drill|date=2 October 2008|publisher=Niels Bohr Institute Centre for Ice and Climate|language=en|access-date=3 June 2017|archive-date=3 September 2017|archive-url=https://web.archive.org/web/20170903025631/http://www.iceandclimate.nbi.ku.dk/research/drill_analysing/drilling_techniques/deep_drilling/|url-status=dead}}</ref>

Drilling fluids are chosen to balance the pressure so that the hole remains stable.<ref name="Talalay 2016"/> The fluid must have a low kinematic [[viscosity]] to reduce [[Tripping (pipe)|tripping]] time (the time taken to pull the drilling equipment out of the hole and return it to the bottom of the hole).  Since retrieval of each segment of core requires tripping, 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.<ref>{{cite journal|last1=Sheldon|first1=Simon G.|last2=Popp|first2=Trevor J.|last3=Hansen|first3=Steffen B.|last4=Steffensen|first4=Jørgen P.|title=Promising new borehole liquids for ice-core drilling on the East Antarctic high plateau|journal=Annals of Glaciology|date=26 July 2017|volume=55|issue=68|pages=260–270|doi=10.3189/2014AoG68A043|doi-access=free|bibcode=2014AnGla..55..260S }}</ref> Historically, there have been three main types of ice drilling fluids: two-component fluids based on [[kerosene]]-like products mixed with [[fluorocarbon]]s to increase density; alcohol compounds, including aqueous [[ethylene glycol]] and [[ethanol]] solutions; and [[ester]]s, including [[N-Butyl acetate|n-butyl acetate]].  Newer fluids have been proposed, including new ester-based fluids, low-molecular weight [[Dimethylsiloxane|dimethyl siloxane]] oils, [[Fatty acid ester|fatty-acid esters]], and kerosene-based fluids mixed with foam-expansion agents.<ref>{{harvnb|Talalay|2016}}, pp. 259–263.</ref>

Rotary drilling is the main method of drilling for minerals and it has also been used for ice drilling. It uses a [[Drill string|string of drill pipe]] rotated from the top, and drilling fluid is pumped down through the pipe and back up around it.  The cuttings are removed from the fluid at the top of the hole and the fluid is then pumped back down.<ref name="Talalay-2016-4" /> This approach requires long trip times, since the entire drill string must be hoisted out of the hole, and each length of pipe must be separately disconnected, and then reconnected when the drill string is reinserted.<ref name="Talalay-2016-3" /><ref name="Talalay-2016-5">{{harvnb|Talalay|2016}}, p. 101.</ref> Along with the logistical difficulties associated with bringing heavy equipment to ice sheets, this makes traditional rotary drills unattractive.<ref name="Talalay-2016-3" /> In contrast, [[Wireline (cabling)|wireline]] drills allow the removal of the core barrel from the drill assembly while it is still at the bottom of the borehole.  The core barrel is hoisted to the surface, and the core removed; the barrel is lowered again and reconnected to the drill assembly.<ref>{{harvnb|Talalay|2016}}, p. 79.</ref> Another alternative is flexible drill-stem rigs, in which the [[drill string]] is flexible enough to be coiled when at the surface.  This eliminates the need to disconnect and reconnect the pipes during a trip.<ref name="Talalay-2016-5" />[[File:Drilling mechanical-drill-head.jpg|thumb|Mechanical drill head, showing cutting teeth|alt=The bottom of an ice drill]] The need for a string of drillpipe that extends from the surface to the bottom of the borehole can be eliminated by suspending the entire downhole assembly on an armoured cable that conveys power to the downhole motor.  These cable-suspended drills can be used for both shallow and deep holes; they require an anti-torque device, such as [[Leaf spring|leaf-springs]] that press against the borehole, to prevent the drill assembly rotating around the drillhead as it cuts the core.<ref>{{harvnb|Talalay|2016}}, pp. 109–111.</ref> The drilling fluid is usually circulated down around the outside of the drill and back up between the core and core barrel; the cuttings are stored in the downhole assembly, in a chamber above the core.  When the core is retrieved, the cuttings chamber is emptied for the next run.  Some drills have been designed to retrieve a second annular core outside the central core, and in these drills the space between the two cores can be used for circulation.  Cable-suspended drills have proved to be the most reliable design for deep ice drilling.<ref>{{harvnb|Talalay|2016}}, pp. 173–175.</ref><ref>{{harvnb|Talalay|2016}}, pp. 252–254.</ref>

Thermal drills, which cut ice by electrically heating the drill head, can also be used, but they have some disadvantages.  Some have been designed for working in cold ice; they have high power consumption and the heat they produce can degrade the quality of the retrieved ice core.  Early thermal drills, designed for use without drilling fluid, were limited in depth as a result; later versions were modified to work in fluid-filled holes but this slowed down trip times, and these drills retained the problems of the earlier models.  In addition, thermal drills are typically bulky and can be impractical to use in areas where there are logistical difficulties.  More recent modifications include the use of [[antifreeze]], which eliminates the need for heating the drill assembly and hence reduces the power needs of the drill.<ref>{{cite journal|last1=Zagorodnov|first1=V.|last2=Thompson|first2=L.G.|title=Thermal electric ice-core drills: history and new design options for intermediate-depth drilling|journal=Annals of Glaciology|date=26 July 2017|volume=55|issue=68|pages=322–330|doi=10.3189/2014AoG68A012|doi-access=free|bibcode=2014AnGla..55..322Z }}</ref> Hot-water drills use jets of hot water at the drill head to melt the water around the core. The drawbacks are that it is difficult to accurately control the dimensions of the borehole, the core cannot easily be kept sterile, and the heat may cause [[thermal shock]] to the core.<ref>{{Cite book|title=Exploration of Antarctic Subglacial Aquatic Environments: Environmental and Scientific Stewardship|last=National Research Council of the National Academies|publisher=National Academies Press|year=2007|isbn=978-0-309-10635-1|location=Washington DC|pages=82–84}}</ref>

When drilling in temperate ice, thermal drills have an advantage over electromechanical (EM) drills: ice melted by pressure can refreeze on EM drill bits, reducing cutting efficiency, and can clog other parts of the mechanism.  EM drills are also more likely to fracture ice cores where the ice is under high stress.<ref>{{cite journal|last1=Schwikowski|first1=Margit|last2=Jenk|first2=Theo M.|last3=Stampfli|first3=Dieter|last4=Stampfli|first4=Felix|title=A new thermal drilling system for high-altitude or temperate glaciers|journal=Annals of Glaciology|date=26 July 2017|volume=55|issue=68|pages=131–136|doi=10.3189/2014AoG68A024|doi-access=free}}</ref>

When drilling deep holes, which require drilling fluid, the hole must be [[Casing (borehole)|cased]] (fitted with a cylindrical lining), since otherwise the drilling fluid will be absorbed by the snow and firn.  The casing has to reach down to the impermeable ice layers.  To install casing a shallow auger can be used to create a pilot hole, which is then [[Reaming|reamed]] (expanded) until it is wide enough to accept the casing; a large diameter auger can also be used, avoiding the need for reaming.  An alternative to casing is to use water in the borehole to saturate the porous snow and firn; the water eventually turns to ice.<ref name="Talalay-2016-6" />

Ice cores from different depths are not all equally in demand by scientific investigators, which can lead to a shortage of ice cores at certain depths.  To address this, work has been done on technology to drill replicate cores: additional cores, retrieved by drilling into the sidewall of the borehole, at depths of particular interest.  Replicate cores were successfully retrieved at WAIS divide in the 2012–2013 drilling season, at four different depths.<ref>Anonymous (30 June 2017), Ice Drilling Design and Operations: Long Range Technology Plan, p. 24.</ref>

=== Large coring projects ===
The logistics of any coring project are complex because the locations are usually difficult to reach, and may be at high altitude.  The largest projects require years of planning and years to execute, and are usually run as international consortiums.  The [[East Greenland Ice-Core Project|EastGRIP]] project, for example, which as of 2017 is drilling in eastern Greenland, is run by the [[Centre for Ice and Climate]] ([[Niels Bohr Institute]], [[University of Copenhagen]]) in [[Denmark]],<ref>{{Cite web|url=http://eastgrip.org/|title=EastGrip – The East Greenland Ice-core Project|last=Petersen|first=Sandra|date=23 February 2016|website=East Greenland Ice Core Project|language=en|access-date=17 June 2017}}</ref> and includes representatives from 12 countries on its steering committee.<ref>{{Cite web|url=http://eastgrip.org/partners/|title=Partners|last=Madsen|first=Martin Vindbæk|date=14 April 2016|website=East Greenland Ice Core Project|language=en|access-date=17 June 2017|archive-url=https://web.archive.org/web/20170628113003/http://eastgrip.org/partners/|archive-date=28 June 2017|url-status=dead}}</ref> Over the course of a drilling season, scores of people work at the camp,<ref>{{harvnb|Dahl-Jensen|Kirk|Larsen|Sheldon|Steffensen|2016}}, pp. 17–19.</ref> and logistics support includes airlift capabilities provided by the [[Air National Guard|US Air National Guard]], using [[Lockheed C-130 Hercules|Hercules transport planes]] owned by the [[National Science Foundation]].<ref>{{Cite web|url=http://eastgrip.org/about/|title=About EastGRIP|last=Petersen|first=Sandra|date=23 February 2016|website=East Greenland Ice Core Project|language=en|access-date=17 June 2017|archive-url=https://web.archive.org/web/20170628113110/http://eastgrip.org/about/|archive-date=28 June 2017|url-status=dead}}</ref> In 2015 the EastGRIP team moved the camp facilities from [[North Greenland Eemian Ice Drilling|NEEM]], a previous Greenland ice core drilling site, to the EastGRIP site.<ref>{{harvnb|Dahl-Jensen|Kirk|Larsen|Sheldon|Steffensen|2016}}, pp. 8–9.</ref> Drilling is expected to continue until at least 2020.<ref>{{Cite magazine|url=http://www.newyorker.com/magazine/2016/10/24/greenland-is-melting|title=When a Country Melts|last=Kolbert|first=Elizabeth|date=24 October 2016|magazine=The New Yorker|access-date=17 June 2017}}</ref>