Annals of Glaciology 55(68) 2014 doi: 10.3189/2014AoG68A029
199
Drilling into debris-rich basal ice at the bottom of the
NEEM (Greenland) borehole
Trevor J. POPP,1 Steffen B. HANSEN,1 Simon G. SHELDON,1 Jakob SCHWANDER,2
Jay A. JOHNSON3
1
Center for Ice and Climate, Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark
E-mail:
[email protected]
2
Oeschger Centre, University of Bern, Bern, Switzerland
3
Ice Drilling Design and Operations group, University of Wisconsin–Madison, Madison, WI, USA
ABSTRACT. After the NEEM (Greenland) deep ice-core drilling was declared terminated with respect
to developing stratigraphic climate reconstructions, efforts were turned toward collecting basal icesheet debris and, if possible, drilling into the bedrock itself. In 2010, several meters of banded debrisrich ice were obtained under normal ice-drilling operations with the NEEM version of the Hans Tausen
(HT) drill, but further penetration was obstructed by a rock in the path of the drill head at 2537.36 m.
During short campaigns in 2011 and 2012, attempts were made to penetrate further using various
reinforced ice cutters mounted on the HT drill head, tailored to cut through rock. These had some
success in penetrating coarse material, but produced severely damaged cutters. Additionally a 51 mm
diameter diamond cutting tipped rock drill was adapted to fit the NEEM drill. With this device, several
additional meters of core containing subglacial sediments, rocks and rock fragments were collected.
With these tools 1.39 m of additional material were obtained during the 2011 field season, and 7.1 m
during 2012. Subglacial water refreezing into the newly formed borehole hindered further penetration,
and the bedrock interface was not reached before final closure of the NEEM Camp.
KEYWORDS: basal ice, ice and climate, ice core, ice coring, subglacial sediments
INTRODUCTION
Gaining access to basal ice and the subglacial environment
of large ice sheets is of great scientific interest. In addition to
providing samples to examine bedrock geology, samples of
basal ice and direct observation of the subglacial environment yield information about ice-sheet/bedrock interactions
and how this zone influences the overlying ice-sheet
dynamics (e.g. Tison and others, 1993). Sampling from this
zone also provides a rich paleo-environmental archive for
many applications including cosmogenic-isotope and luminescence exposure dating, and biological studies including
ancient DNA (Willerslev and others, 2007). In the central
regions of large ice sheets, direct observation of basal ice
environments is rare, but can be gained via the deep
boreholes created by ice-core drilling. However, the basal
material lying between clean meteoric ice and the bedrock
is notoriously difficult to drill (Talalay, 2013). Ice located in
the immediate vicinity of the bedrock interface is often
loaded with rock and sediment particles of various sizes and
concentrations and/or is near the pressure-melting point.
At the NEEM (North Greenland Eemian Ice Drilling) site
in northwest Greenland, deep ice-core drilling was declared
terminated in July 2010 at 2537.36 m depth, when no
further penetration was possible with the available equipment (Popp and others, 2014). A rock embedded directly in
the path of the drill head halted penetration, and efforts to
dislodge or cut through this rock using standard ice cutters
mounted on the NEEM version of the Hans Tausen (HT) drill
resulted only in destroying the cutters. However, this depth
was not assumed to be the bedrock interface itself. The final
ice cores contained a significant amount of basal ice-sheet
material, including silty ice and some small stones (Fig. 1),
and it seemed probable that more layers of banded silty ice
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and an interval of rock/ice mixture remained before the
actual bedrock interface.
Targeted drilling campaigns continued in the NEEM
borehole for part of each of the 2011 and 2012 field seasons.
The goal was to penetrate beyond the obstruction in the
borehole and into the bedrock itself if it could be reached. To
maximize the chance of penetrating further, two approaches
were considered. First, we mounted reinforced cutters of
carbide steel and cutters with modified geometries onto our
standard ice-coring head on the HT drill. Already, our
standard cutters had succeeded in penetrating some coarse
material, so these additional reinforcements to the cutters on
the ice-core drilling head seemed promising, at least for a
rock/ice mixture. Second, we adapted a conventional rockdrilling unit with a smaller outer diameter (o.d.) to replace the
lower part of the ice-core drill below the motor section
(51 mm o.d. vs 132 mm with the NEEM version of the HT
drill). With this rock drill we intended to bypass the obstacle
if the attempts with the reinforced ice cutters failed, penetrate
ice-embedded rocks and rock fragments along the way, and
then use it to drill straight into pure rock further down.
Because we expected an ice/rock mixture before reaching
the bedrock interface, we hoped that combining these two
approaches would compensate for the facts that the ice-core
drill head was not particularly well suited for drilling in rocks,
and that the rock-drill head was not particularly well suited
for drilling through ice.
STATUS OF THE BOTTOM
A major drilling challenge when collecting basal ice-sheet
material at NEEM was that the status of the bottom was
unknown. Even though warm-ice drilling near the base at
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Popp and others: Drilling into basal ice in the NEEM borehole
Fig. 1. The final cores collected at NEEM approaching the obstructions at 2537.36 and 2538.10 m depth were drilled with the Hans Tausen
drill mounted on the NEEM deep-drill electronics, motor and gear sections. Banding silty and sediment-laden ice can be seen in (a, b). The
chips also contained sediment material (c). The cores pictured were collected in 2011 with ice cutters with carbide inserts (Fig. 4). Similar
cores with standard ice cutters were obtained to close the 2010 season.
NEEM had been relatively trouble-free to this point (Popp
and others, 2014), if many more meters were required to
reach the bottom, liquid water could increasingly interfere
with drilling as temperatures approached the ice pressuremelting point. At 2537 m the ice temperature was –3.4°C,
and while we tried to maintain a pressure-balanced borehole by adjusting the drill fluid level during drilling, the
pressure at the base of the borehole was up to 0.1 MPa less
than the pressure exerted by the surrounding ice sheet at this
depth. This under-pressure would therefore allow meltwater, if present, to enter the borehole, rather than force
borehole drill liquid into the subglacial environment.
It was also unclear from the available information how
many more meters of ice and debris remained below the
bottom of the existing borehole before the actual bedrock
interface. Interpretations of radio-echo sounding and borehole temperature profiles, as well as sonar experiments near
the bottom of the borehole, gave estimates ranging from tens
of centimeters to tens of meters before the bedrock interface
would be reached. Furthermore, we could only guess at the
character and composition of the debris-laden ice, the size
of the rocks and rock fragments we might encounter, and
how banded the sediment/rock/clear-ice mixture would be
in the remaining depths. In an attempt to ‘see’ the bedrock,
the sonar experiments were carried out at 2496.33 and
2533.12 m, i.e. 41.03 and 4.24 m above the 2010 final
depth. For this purpose an experimental borehole instrument
was constructed at the University of Bern, consisting of a
battery-operated electromagnetic pinger (gong type) that
emitted signals at a rate of �1 s–1 with center frequency of
�5 kHz where reflections are recorded with a piezo
hydrophone. The analog signal is superimposed as current
signal on the power supply of the hydrophone and digitized
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and recorded at the surface. Interpretation of these signals
was inconclusive, but suggested that the bedrock could be
<2 m away from the bottom of the borehole. This turned out
not to be the case, and the observed signals may have come
from a heavy silty layer or resulted from the fact that the dirty
ice was simply too opaque to resolve the actual bedrock/ice
interface. Nevertheless, based on the information at the
time, we operated under the assumption that <2 m
remained, and preparations and expectations were originally focused on this depth.
DRILL SET-UP
Hans Tausen drill with reinforced ice cutters
As we approached the bedrock at NEEM in late July 2010,
the HT drill configuration was set in a mode deemed most
favorable for drilling in the warm ice, during which the first
cores containing silty ice and small rocks and rock
fragments were collected with no special consideration
given to the cutters or drilling process with respect to the
coarse materials we were to encounter (Figs 1 and 2). In this
set-up, the long barrels of the NEEM drill were replaced with
the lower part of the HT drill with its 1.6 m core barrel and
1.6 m chip chamber (Popp and others, 2014). These were
mounted to the deep drill electronics, computer, motor and
gear sections to allow the same fine control and monitoring
of the cutting process at depth that we had with the long
drill. In addition to our standard hardened-steel ice cutters
(Böhler S390PM for Swiss-made cutters; Uddeholm Viking
Steel 54-56 HRC for Danish-made cutters), step cutters
(cutters with partial kerf; Zagorodnov and others, 2005)
were also mounted. Step cutters were beneficial in warm ice
because they produced coarse chips that moved efficiently
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Popp and others: Drilling into basal ice in the NEEM borehole
Fig. 2. Four images of the final 98 mm diameter ice cores drilled with the NEEM version of the HT drill equipped with cutters with carbide
inserts (see Fig. 4) containing banded clear ice, fine- and coarse-grained sediments, and embedded stones up to 2 cm in diameter.
away from the drill head without packing, and required less
power per revolution to cut the same amount of ice (Popp
and others, 2014).
While the standard and step versions of the ice cutters
proved capable of drilling through some rock fragments and
other coarse material, they were destroyed in the process
(Fig. 3). Thus, two types of reinforced ice-cutter geometries
with tungsten carbide were created in an attempt to improve
performance. One approach was to mount tungsten carbide
inserts which would act as step cutters mounted on the head
of the HT drill (Fig. 4). Placed on cutter bodies based on the
normal cutter geometry, these inserts had been shown to be
capable of penetrating gneiss and granite in a controlled
manner in the workshop with a drill press. The cutter sets
made for the carbide insert were designed to create a
borehole diameter of 126 mm, 6 mm less than the nominal
borehole diameter at NEEM. This would improve the
likelihood of drilling past the obstacle sitting toward the
Fig. 3. Destroyed cutters after contact with coarse material and
stones. (a) A standard ice cutter showing erosion at its cutting edge.
(b) A destroyed version of modified step cutters mounted with a
carbide cutting insert (Fig. 4), which has been snapped off its mount
and otherwise completely worn down after contact with embedded
rock obstructions in the borehole.
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outer edge of the existing borehole while at the same time
producing fewer chips, which is desirable in warm-ice
drilling conditions. A second approach was to create a set of
cutters with carbide-reinforced tips welded directly onto the
cutting edge of standard steel cutter bodies with full kerf for
each of the three cutters in a set. In all cases these cutters
produced the nominal 98 mm diameter ice core.
Adapted rock drill
A small-diameter conventional rock drill with core-catching
collar ring was adapted to be connected to the upper sections
of the NEEM deep drill (Fig. 5). The same rock drill had been
used previously to collect bedrock at the Greenland Ice Sheet
Project 2 (GISP2) drilling site, but with different replaceable
cutting heads (Wang and others, 1994). The rock-drill bit
itself is composed of diamond cutting tips on a semi-round
crown which produces a 51 mm diameter borehole and a
33 mm diameter core (Fig. 5). The cutting head could be
Fig. 4. Cutters with carbide cutting inserts were mounted on the HT
drill head in an attempt to penetrate past the obstacle at 2537.36 m.
These cutters succeeded in collecting an additional 0.74 m of ice
cores in 2011, before meeting another impassable obstruction. In
2012 these types of cutters came up completely destroyed after one
attempt downhole. It was later discovered that despite their
destruction, they had been slowly grinding away at the stone that
had been blocking penetration (Fig. 11).
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Fig. 5. Rock-drill head with diamond tips (inset) mounted on the
NEEM tower, driven by the NEEM deep-drill motor and gear
sections. Brass centering rings were mounted to center the rock drill
in the 132 mm diameter NEEM borehole. Extension rods added in
1.5 m sections were added to reach a final depth 7.1 m below the
obstruction at 2538.1 m.
threaded onto any number of 1.5 m long extension rods to
allow deeper and deeper access as the 51 mm borehole was
extended to greater depths into which the upper end of the
drill would not fit. Centering rings were constructed and
mounted to support the long shaft as it rotated in the 132 mm
borehole and found its way into the extended narrow
borehole. The lowest of these centering rings was not fixed
to the shaft and could slide upwards as the extensions
continued deeper into the narrow borehole (Fig. 5).
Having adapted this rock drill to our drill system without
modifying the drill motor and gear sections, we were
advised that it would be suboptimal for the desired weighton-bit (WOB) or rotation speed characteristics for efficient
cutting. For such a system rotation, speeds of >500 rpm
(Wang and others, 1994) and WOB exceeding 200 kg
(personal communication from Geo Engineering A/S, 2010)
were recommended. By not meeting these criteria, we ran
the risk that the cutting head, rather than biting into stone,
would simply skate on the rock surfaces, resulting in
premature deterioration of the cutting edges. The rotation
speed was fixed at 80 rpm by the motor and gear sections we
had available. However, to increase cutting efficiency we
were able to add up to three dead-weight sections of 70 kg
each above the motor section to increase the weight of the
drill. When cutting we could apply up to 240 kg on the bit,
which was the full weight of the drill including the extra
dead-weight sections in the buoyant liquid column. Under
these circumstances a new drill head would last up to about
ten runs in the sediment/rock/ice mixture before being
replaced. We gained confidence in this set-up with a test
drilling at the surface into granite which showed the drill
was able to penetrate at a rate of �1 cm in 2 min.
For drilling in the fluid-filled borehole the design and
implementation of a suitable pumping system to circulate
liquid at the cutting head was also required. The pump
section was created around a commercially available orbital
motor (Sauer-Danfoss OMM50), where the turning motor
functioned in this case as a hydraulic pump (Fig. 6 inset).
The motor’s rotation was driven by the drill motor attached
via a gear designed to turn the motor three times faster than
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Popp and others: Drilling into basal ice in the NEEM borehole
Fig. 6. The pump and gear section for the adapted rock drill. The
upper part of the gear section is constructed to be mounted onto the
rotating bearing at the lower end of the deep-drill motor and gear
section, which, in turn, turns pump gear (blue) and the hydraulic
pump (internal mechanism view shown inset at lower right) which is
housed inside the green section. Depending on the orientation at
which it is mounted, the flow direction can be reversed. With the
speed of the outer toothed wheel set by the drill motor, the pump
gear was designed to turn three times as fast as the rotating drill shaft.
The large holes in the green section are lined with a fine-mesh filter
to protect the pump from the rock dust and sediment produced
during cutting while allowing liquid to flow through the pump.
the rotation of the drill tube (Fig. 6). The upper part of the
pump gear section was constructed to be mounted onto the
rotating bearing at the lower end of the deep-drill motor and
gear section, which, in turn, turns the pump gear and the
hydraulic pump itself. The connection to the NEEM deepdrill motor section is essentially made in the same way that
the hollow shaft is mounted inside the chips chamber in
normal ice drilling with HT-type drills (Johnsen and others,
2007), where it hangs on three engaged spring-loading
screws. Given the nominal rotation speed of 80 rpm
available with the existing drill motor and a pumping
capacity of 50 mL rev–1 we could expect a maximum flow of
�12 L min–1. To protect the pump from fine grit and coarse
particles created during drilling, the entire pump section
body was wrapped in a fine-mesh filter before each run
(Fig. 7), and the debris trapped in the filter could also be
collected as sample.
Below the pump, a connection piece to the drill head and
extension rods was designed to mimic the bayonetted ‘Super
Banger’ connection used in the normal operation of HT-type
drills (Fig. 7). With initial concerns that our cable might not
be strong enough to break a rock core with a static pull, this
feature allows the full weight of the drill to act as an efficient
hammer giving a sharp impact to assist in breaking the core.
As with normal ice-drilling operations it also allows the
rock-drill head (and its extension rods) to be detached
downhole in order to recover the rest of the drill if the drill
becomes stuck.
COLLECTING BASAL ICE-SHEET MATERIAL AND
DRILL PERFORMANCE
As the NEEM borehole approached the bedrock interface in
2010, penetration problems during drilling were primarily
due to the silty sediment material, rock fragments and rocks
Popp and others: Drilling into basal ice in the NEEM borehole
embedded in the ice, and the destruction these caused to the
cutters once they were encountered (Fig. 3a). Nevertheless,
with our standard hardened-steel ice cutters, a version with
full cutting kerf and a step-cutter version, the drill demonstrated it was capable of penetrating silty ice and, in a few
cases, directly through small stones embedded in the ice
(Fig. 2). In one case the outer edge of a stone with a diameter
of �2 cm, found embedded in an ice core, appeared to have
been shaved by the cutting action of the ice cutters,
although at the expense of the cutters (Fig. 2a and c). So
already in 2010 without regard to drilling procedures
beyond dealing with the warm-ice conditions, several cores
with layers of banded silty ice and rock fragments were
collected. Together with the chips produced by the cutting
action in dirty ice, a significant quantity of debris-rich basal
ice was already available, along with a pile of damaged
cutters. Finally, at 2537.36 m depth, our cutters failed to
penetrate at all and came to the surface completely
destroyed (Fig. 3). Obstacles, which were presumably rocks
embedded in the ice directly in the path of the rotating head,
were severely eroding only the outer half to two-thirds of
each cutter. This led us to believe that if this stone could be
drilled through or dislodged, more ice and sediment
material would likely be found below.
During 11 days in the 2011 field season and 19 days in
the 2012 field season, we deployed either the HT drill with
carbide cutter reinforcements or the adapted rock drill in a
sequence reacting to the conditions encountered run to run.
A version of our trials in the 2011 season was publicized
online at the time and reproduced in Talalay (2013) and can
be found in its original form in the NEEM field season report
(Ice and Climate Group, 2011). Here we develop and
complete that discussion, as well as describing the
continuation of our efforts during the 2012 season.
The short 2011 campaign was started with the adapted
rock drill with a single 70 kg dead weight. This provided a
maximum load on the cutter head of �120 kg in the drilling
fluid. Over two runs we drilled through �80 cm of coarsegrained ice of unknown origin, which was likely refrozen
meltwater and not an extension of the borehole depth. Next
we deployed the HT drill, making several runs with the new
cutters with carbide inserts mounted on the standard icedrilling head. During the first run a short circuit in the
anti-torque (AT) section pointed to the presence of conductive material in the borehole, which was later shown to be
subglacial water. After improving the insulation in the AT
section, 74 cm of heavily silt-loaded ice cores were retrieved
in three runs with the carbide inserts. A broken carbide
insert and ground adapters for the inserts indicated that we
had encountered another larger rock piece. As it turned out,
these cores were to be the final 98 mm cores retrieved at
NEEM, bringing the final logging depth in 2011 to
2538.10 m. Despite further attempts with carbide-reinforced
ice cutters on the ice-coring drill head, beyond 2538.10 m
only material from 33 mm cores from the rock drill were
collected at greater depths.
At this point, we switched back to the rock drill with two
additional 70 kg dead weights added on top of the motor
section, resulting in a maximum load on the cutter head of
240 kg. Until now, the fluid flow direction had been chosen
to suck the drilling fluid into the core barrels with the
intention of collecting the rock cuttings and bringing them to
the surface. However, it turned out that the clearances in the
rock drill, especially the annulus between the outer and inner
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203
Fig. 7. Bayonetted coupling between the pump section and
extension rods of the adapted rock drill. The pump section at the
top of the right photo is covered in a fine-mesh filter that was
replaced after each run when it became coated in sediment
particles. The extension rods threaded onto the lower part of the
bayonet coupling. The bayonet could be used to provide a hammer
effect when making a hard-core break, or be released if the drill
head was stuck, thus recovering all of the upper parts of the drill
sonde. Fortunately this never needed to be used in the rock drilling
at NEEM.
barrels, were too small for this flow direction (reverse flow in
rock drilling), leading to rapid clogging with ice chips and silt.
We also noted refrozen water at the drill head, which was
also a clear sign of liquid water in the hole, and explained the
short circuit in the AT (Fig. 8a). This was confirmed when we
lowered the video camera into the hole and found a layer of
�0.6 m of basal water at the bottom of the hole.
Fortunately, upon its set-up the orbital motor pump could
be configured to reverse the flow direction. We changed the
fluid flow direction to forward flow mode, and stable drilling
with the rock drill was found to be possible when pumping
liquid from inside-out through the core tube. In this mode,
cuttings were flushed from the drill head area from inside
the core tube and transported by the drill liquid in the space
between the outside of the drill tube and the borehole wall,
before being filtered at the pump and recirculated back
inside the drill tube to the head. In this way, however, we
were not collecting most of the cuttings and they were left
downhole, except those that ended up stuck to the filter
surrounding the pump section.
Continuing in this mode with the rock drill, we retrieved
an additional 0.65 m of silty and sediment-rich ice. The core
diameter varied between 20 and 30 mm (33 mm nominal),
pointing to a wobbling head and possibly drilling by partial
melting of the ice. Penetration was slower in sections with
more ice, and faster when more coarse material was present.
This could be monitored by following the motor current
needed to turn the drill, with higher and steady motor
currents (>10 A) when rocks were engaged. With the last
attempts at the close of the 2011 campaign, it was believed
that bedrock had in fact been reached because during the
final runs drilling current was high and stable throughout the
run, which we interpreted as characteristic of rock penetration. The screen mounted around the pump was clogged
with a thick layer of very fine silt, which also indicated that
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Fig. 8. Rock drill with refrozen water. (a) Refrozen water adheres to
the drill head and refreezes as the drill is hoisted through a column
of liquid water in the borehole. (b) Refrozen water as part of a core
from the borehole that had refilled with subglacial water and
refroze in 2011, and was drilled again in 2012.
we were cutting rock. However, after �60 cm of penetration
it had not been possible to bring any core material to the
surface at that time, as three attempts to catch the ‘core’
failed. The 1.39 m of core retrieved plus �0.6 m of further
penetration below the 2010 borehole in 2011 fit our
assumption that we had on the order of 2 m remaining to
reach the bedrock interface, so we left believing that bedrock
had been reached, although not brought to the surface.
Starting again in 2012 there was constant drama in the
control room as we anticipated reaching our goal, and there
was surprise that turned to resignation when we continued
for >7 m without reaching the bedrock interface. We started
the campaign with a single run with the HT drill head with
Popp and others: Drilling into basal ice in the NEEM borehole
the welded carbide-plate reinforced cutters. The welded tips
snapped off immediately, leaving only destroyed and
unusable cutter bodies to add to the steel carnage. At this
point, we concluded that the obstruction could not be
penetrated with any type of cutters we had available to
mount on the HT drill head, so we switched immediately to
the adapted rock drill and performed 18 runs during which
7.1 m was drilled below the 2011 logging depth of
2538.1 m. The first several runs re-drilled the 51 mm
diameter portion of the 2011 borehole, which had filled
with refrozen meltwater, yielding frozen water cores
(Fig. 8b), but also provided sediment-rich ice chips that
could be collected as sample. The characteristic high and
stable motor current of the final runs in 2011 was not
immediately present in the re-drilling depths, but did occur
several times at greater depths and was always associated
with the presence of rocks in the mixed ice/rock cores
collected (Fig. 9).
As in 2011, the rock drill very slowly penetrated the rock/
ice mixture and captured up to 70 cm of material per
attempt. The cores brought to the surface were a variety of
banded clear glacial ice with sediment layers and rocks
(Fig. 9a). As we got deeper, more pebbles and larger rock
fragments were collected within the small-diameter cores
(Fig. 9b), with many different size fractions that could be
sorted (Fig. 9c), though still with inclusions of clear glacial
or accreted ice in each core. Preliminary assessment
suggests the rocks and rock fragments were granite type
with quartz minerals, along with silt, clay and fine-grain
sand sediments. The exact mineralogy, basal ice properties,
rock and particle size and type distribution, and general
geologic framework are under investigation (personal
communication from J.-L. Tison and T. Goossens, 2014).
Typical run times at the bottom (excluding hoisting and
lowering) were on the order of 1–3 hours depending on the
material present. In extreme cases, drill rotation was
allowed to continue for up to 6 hours, which was pushing
the limit for the heat generated (up to 80°C) inside the drill
motor section after prolonged constant operation. Much of
this drill time was used in ‘reaming’ the 51 mm borehole to
regain access to the bottom, squeezing the long extensions
back into the narrow borehole into which subglacial water
Fig. 9. Cores from the rock drill with (a) banded glacial ice and sediments and (b) an ice/rock mix, including pebble-sized stones, and (c) an
assortment of stones from the rock cores.
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205
Popp and others: Drilling into basal ice in the NEEM borehole
Fig. 10. Concrete diamond core bit with threaded attachment to
mount to rock-drill extension rods and bayonetted connection to the
pump section. This head was attached to the rock-drill extension
rods, and ultimately dislodged the stone obstruction at 2538.1 m.
would gradually fill and refreeze. This water, presumably
together with the water created by the heat during cutting,
could be seen on or in the drill when it was brought to the
surface (Fig. 8). Upon hoisting, core breaks were not strong
(<10 kN), and the cores often broke at a sediment/rock/ice
interface.
During the 18th run, a large stone (�3 cm � 5 cm) was
brought to the surface, wedged in the otherwise empty core
barrel. Thinking that this could have been the obstruction
that halted penetration with the HT drill head, the HT drill
was redeployed with cutters with carbide inserts (Fig. 4), and
after a single run these came up in a severely eroded state,
with inserts snapped off their mount under the press of the
drilling action (Fig. 3b).
Eventually, the rate at which subglacial water entered the
borehole and refroze, together with our need to close the
NEEM camp, became the limiting factors in attempts to
reach the bedrock. After a 10 day hiatus, used to obtain
more 1.5 m extension rods after we had exhausted our
supply in camp, the newly formed 51 mm diameter, 7.1 m
deep borehole had already been closed by refrozen water.
Further penetration with the adapted rock drill was not
possible in the time we had left before closing the camp. We
had not reached the bedrock interface, nor had we
encountered pure stone in the form of a larger boulder.
In the short time remaining we decided to attack the
obstruction at the bottom that had blocked further attempts
to penetrate with the HT drill head. If this obstruction could
be dislodged, we would then try to penetrate further with the
full diameter of the HT drill before switching back to the
rock drill at greater depth as required. To dislodge the stone,
a concrete diamond core bit was constructed and attached
to the drill motor section and the rock-drill pump via the
threaded end of an extension rod (Fig. 10). The concrete
diamond bit had an o.d. of 127 mm and a 4 mm cutting kerf.
After one run it was clear that we had penetrated at least
partially through an obstruction. Since this cutting head was
meant to be a blunt tool, it had no capability to pick up any
core material. Afterwards, we deployed our conical reamer
in the hope that if the stone had been dislodged we could
center it in the borehole and then drill around it with the HT
drill. The HT drill with the standard ice-cutting drill head
was deployed, but again with no success in penetrating, and
more destroyed cutters.
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Fig. 11. The cm rock fragment which apparently stopped penetration using the HT drill was eventually dislodged from the
borehole wall by mounting the concrete diamond core bit to the
rock drill (surface 1). The stone was later brought to the surface
when it was jammed within the rock-drill head after being partially
drilled through and repositioned so it lay free at the bottom of the
borehole (surface 2). Examination of the stone at the surface
indicates that attempts to drill through the stone using the HT drill
with carbide step cutter inserts had in fact been effective. These
cutters had been slowly penetrating the stone before their eventual
destruction under this action (surface 3).
Nevertheless, at the end of all the drilling activities at
NEEM we had the pleasure of bringing to the surface what
we believe to have been this final obstruction, which was a
rock �3–4 cm in diameter, caught within the rock-drill
head. After being partially drilled through by the concrete
diamond cutting head, this rock had indeed been dislodged
from the borehole wall and repositioned so it apparently lay
free at the bottom of the borehole, but not such that it could
be avoided by the HT drill. Examination of the stone showed
the signs of its having been attacked by three different tools:
the HT drill with carbide insert cutters, the concrete
diamond cutting head that dislodged it, and finally the rock
drill into which it was jammed and in which it was brought
to the surface (Fig. 11). Most encouraging for future
development, perhaps, was that the HT drill with carbide
inserts mounted as step cutters or as tips appears to have
been mildly effective at working itself through this stone.
Under different circumstances this cutter design may be
effective for drilling dirty ice, particularly ice with a high
concentration of small rock fragments. In this case,
however, despite the slow penetration into rock with the
inserts, which we were unaware of at the time, the stainlesssteel cutter body holding the inserts was severely damaged,
limiting their use if we were to attack the stone with a large
quantity of replaceable inserts.
FINAL COMMENTS
Although the ice/bedrock interface was not reached at
NEEM, an additional >7 m of silty ice, rocks and other
coarse materials were collected below the original obstruction that terminated normal drilling operations at 2537.36 m
and again at 2538.1 m in 2011, with a combination of
reinforced ‘ice’-drilling cutters and the adapted rock drill. In
206
the end, we still did not know how far we were from the
bedrock interface. Nevertheless, the rock-drill head proved
quite effective at penetrating the rock/ice mixture. The
smaller diameter of the rock-drill head allowed us to bypass
obstacles in the borehole that stopped penetration with the
HT drill head, and continue deeper. To deal with meltwater
in the subglacial environment, it seems feasible that
injecting ethanol around the drill head could make the tool
even more effective in warm-ice environments by avoiding
refreezing meltwater (e.g. Johnsen and others, 2007).
Alternatively, creating an over-pressure by increasing the
drill fluid height could prevent meltwater from seeping into
the borehole. Such a scenario will be unacceptable,
however, if maintaining a ‘clean’ subglacial environment
is a priority. But, with the rock drill now adapted to be used
with the NEEM (or any HT-type) drill, it will be a tool we
keep ‘on the shelf’ for future drillings that approach the
bedrock foundation of ice sheets or glaciers. With the
combined capabilities of the adapted rock drill, effective
reinforced cutters of the ice drill, and blunt tools when
necessary, it seems plausible that the system used at NEEM
could succeed in collecting not only debris-rich basal ice,
but also the bedrock itself. Tensile strength of granite for a
rod of comparable size to the rock-drill core diameter is on
the order of 10 MPa, which is well within the limits of the
force available with the winch and cable used at NEEM.
ACKNOWLEDGEMENTS
We thank the many logistical hands at NEEM, in Copenhagen
and at Ice Coring and Drilling Services, Madison, WI, that
supported us during our long shifts, sometimes in a darkened
drill trench, in the final seasons at NEEM collecting basal ice
and debris. NEEM is directed and organized by the Center for
Ice and Climate at the Niels Bohr Institute, and the US
National Science Foundation (NSF) Office of Polar Programs.
It is supported by funding agencies and institutions in
Belgium (FNRS-CFB and FWO), Canada (NRCan/GSC),
https://doi.org/10.3189/2014AoG68A029 Published online by Cambridge University Press
Popp and others: Drilling into basal ice in the NEEM borehole
China (CAS), Denmark (FIST), France (IPEV, CNRS/INSU,
CEA and ANR), Germany (AWI), Iceland (RannIs), Japan
(NIPR), Korea (KOPRI), The Netherlands (NWO/ALW),
Sweden (VR), Switzerland (SNF), United Kingdom (NERC)
and the USA (US NSF, Office of Polar Programs).
REFERENCES
Ice and Climate Group, Niels Bohr Institute (2011) North Greenland Eemian ice drilling (NEEM) 2007–2011: NEEM bedrock
core drilling and last processing. (Field season report 2011)
NEEM Steering Committee and Danish and Greenlandic
authorities http://neem.dk/documentation/2011/Field_season_
report_2011_draft1_.pdf
Johnsen SJ and 16 others (2007) The Hans Tausen drill: design,
performance, further developments and some lessons learned.
Ann. Glaciol., 47, 89–98 (doi: 10.3189/172756407786857686)
Popp TJ, Hansen SB, Sheldon SG and Panton C (2014) Deep icecore drilling performance and experience at NEEM, Greenland.
Ann. Glaciol., 55(68) (doi: 10.3189/2014AoG68A042) (see
paper in this issue)
Talalay PG (2013) Subglacial till and bedrock drilling. Cold Reg.
Sci. Technol., 86, 142–166 (doi: 10.1016/j.coldregions.2012.
08.009)
Tison J-L, Petit JR, Barnola JM and Mahaney WC (1993) Debris
entrainment at the ice–bedrock interface in sub-freezing
temperature conditions (Terre Adélie, Antarctica). J. Glaciol.,
39(132), 303–315
Wang ZW, Collins J and Huang SL (1994) Low power diamond
rock coring parameters. In Ice Drilling Technology. Proceedings
of the 4th International Workshop on Ice Drilling Technology,
20–23 April 1994. (Mem. Natl Inst. Polar Res., Special issue 49),
99–112
Willerslev E and 29 others (2007) Ancient biomolecules from deep
ice cores reveal a forested southern Greenland. Science,
317(5834), 111–114 (doi: 10.1126/science.1141758)
Zagorodnov V, Thompson LG, Ginot P and Mikhalenko V (2005)
Intermediate-depth ice coring of high-altitude and polar glaciers
with a lightweight drilling system. J. Glaciol., 51(174), 491–501
(doi: 10.3189/172756505781829269)