Jökull - 31.12.2001, Side 34

Thorsteinsson et al.
(Figure 4). The method cannot be recommended, be-
cause it seriously increases the risk of getting the drill
stuck at the bottom of the drillhole.
Water-pumping experiment
When 85 m depth had been reached, it was decided
to pump the water out of the hole in an attempt to
improve the drilling performance. Figure 6a shows
the result of this experiment. The discharge of wa-
ter was steady at 0.881/s (531/min) during the first
lOminutes and then dropped steadily until the hole
was empty after 28minutes of pumping. A total
amount of (1100±150) liters was pumped out of the
hole in this period. The total volume of the water-
filled part of the hole (35-85 m) was 685 liters, indi-
cating that water was rapidly leaking into the hole dur-
ing pumping. After waiting for 12minutes the pump
was restarted and the hole emptied again by pumping
a total amount of 1401 in 5 minutes (Qmax = 0.441/s).
Following another 12 minute wait, 1 lOliters could be
pumped out in 5 minutes (Qmax=0.44 1/s). Drilling
was then resumed without any significant improve-
ment in productivity, except for a slight decrease in
runtime (Figure 5a, runs 209-220) while the water
level was lower than normal. The water level in the
borehole rose with time in the hours following pump-
ing (Figure 6b). After 3 hrs, the water level had risen
to 40 m and was back at the normal level, 35 m, after
6 hrs.
The density profile shown in Figure 7 indicates
that the water table at 35 m depth is located very
close to the level where the transformation of firn to
glacial ice (p=830kg/m3) is completed. Water thus
permeates through interconnected air spaces in the
firn above 35 m and an excessive amount of water can
freely drain away as runoff. Below 35 m, close-off of
air pockets to form bubbles is completed, and water
can only move through the glacier ice by draining
through a network of very small veins with diameters
of the order of 25 /rm, situated on the crystal bound-
aries (Paterson, 1994, p. 110). The permeability of
this vein network is very low, making it unlikely that
the water that entered the hole during and after pump-
ing originated in the ice below 35 m. Instead, the wa-
ter most likely originated in a thin, permeable layer at
the firn-ice boundary. The volume of firn supplying
the water can be roughly estimated as follows:
The total amount of water that leaked into the hole
during the experiment was (1100-685)+140+110+
685=1350 liters. Just above the fim-ice boundary, the
volume percentage of passages which can conduct
water is approximately 10%. Hence, a fim volume of
13.5 m3 is a sufficient source of the water leaking into
the hole, which, to give an example, would correspond
to a 0.1 m thick layer with an areal extent of 135 m2.
Comparison with the Bárðarbunga drilling
Table 2 presents a comparison of the drilling opera-
tions at Hofsjökull and Bárðarbunga. It can be seen
that average daily core production is similar, but there
were nevertheless important differences between the
two projects, as outlined below:
1. The Bárðarbunga drill was designed to move
through water, allowing a much faster lowering/ hoist-
ing speed and thus decreasing the runtime. The impor-
tance of this factor increases with depth.
2. Surface time was much shorter at Hofsjökull,
2minutes as compared with 15minutes at Bárðar-
bunga, which explains the shorter runtime at lOOm
depth on Hofsjökull.
3. An antifreeze mixture was used to improve the
drilling process at Bárðarbunga. As described by
Arnason et al. (1974), a polyethylene bag containing
typically 180 ml of isopropyl alcohol was fastened in-
side the core barrel prior to each mn. The bag burst
as rotation started and the alcohol mixed with wa-
ter at the bottom of the hole, lowering the freezing
point (Bjömsson, 1973) and thus preventing freezing
onto the cutters, which up to then had seriously ham-
pered the drilling. The marked improvement on core-
drilling speed at Bárðarbunga is obvious (Table 2). A
comparable problem of freezing-on did not seem to
arise on Hofsjökull, where problems with chips trans-
port seemed to be the main obstacle to drilling effi-
ciency and productivity. This interpretation is sup-
ported by the fact that the core drilling speed with-
out the use of antifreeze at Hofsjökull was compa-
rable to the drilling speed using antifreeze at Bárð-
arbunga. The difference can probably be explained
by the different cutter geometry. The cutters on the
32 JÖKULL No. 51