page_title: Ice drilling
section_title: Breaking and retaining cores
hierarchical_section_title: Ice drilling / Drilling system components / Breaking and retaining cores
caption_attribution_description: English: Diagram showing how core dogs catch and break an ice core in ice drilling. Redrawn from Talalay, Pavel G. (April 2012). Drill heads of the deep ice electromechanical drills (Report). Jilin University, China: Polar Research Center, p. 29.
context_page_description: Ice drilling allows scientists studying glaciers and ice sheets to gain access to what is beneath the ice, to take measurements along the interior of the ice, and to retrieve samples. 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.
  Many different methods have been used since 1840, when the first scientific ice drilling expedition attempted to drill through the Unteraargletscher in the Alps. Two early methods were percussion, in which the ice is fractured and pulverized, and rotary drilling, a method often used in mineral exploration for rock drilling. In the 1940s, thermal drills began to be used; these drills melt the ice by heating the drill. Drills that use jets of hot water or steam to bore through ice soon followed. A growing interest in ice cores, used for palaeoclimatological research, led to ice coring drills being developed in the 1950s and 1960s, and there are now many different coring drills in use.
context_section_description: In ice core drilling, when an annulus has been drilled around the core to be retrieved, the core is still attached to the ice sheet at its lower end, and this connection has to be broken before the core can be retrieved.  One option is to use a collet, which is a tapered ring inside the cutting head.  When the drill is pulled up, the collet compresses the core and holds it, with loose ice chips wedged in it increasing the compression.  This breaks the core and holds it in the barrel once it has broken.  Collets are effective in firn but less so in ice, so core dogs, also known as core catchers, are often used for ice cores.
  A typical ice drill core dog has a dog-leg shape, and will be built into the drill head with the ability to rotate, and with a spring supplying some pressure against the core.  When the drill is lifted, the sharp point of the core dog engages and rotates around, causing the core to break.  Some core dogs have a shoulder to stop them from over-rotating.  Most drill heads have three core dogs, though having only two core dogs is possible; the asymmetric shearing force helps break the core.  The angle, δ, between the core dog point and the core, has been the subject of some investigation; a study in 1984 concluded that the optimum angle was 55°, and a later study concluded that the angle should be closer to 80°.  Core catchers are made from hardened steel, and need to be as sharp as possible.  The force required to break the core varies with temperature and depth, and in warm ice the core dogs may gouge grooves up the core before they catch and it breaks.  Some drills may also include a weight that can be used as a hammer, to provide an impact to help in breaking the core.
  For snow and firn, where the core material may be at risk of falling out of the bottom of the core barrel, a basket catcher is a better choice.  These catchers consist of spring wires or thin pieces of sheet metal, placed radially around the bottom of the core barrel and pressed against the side of the barrel by the core as the drill descends around it.  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.
caption: Core dog in unengaged and engaged position[197]
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