Jökull - 31.12.2001, Page 36

Thorsteinsson et al.
Table 2. Comparison of the Hofsjökull and Bárðarbunga core drilling projects. Numbers for the Bárðarbunga
hole are derived from information presented by Árnason et al. (1974) and Theodórsson (1976) and should be
taken as approximate. The figures for average runtime at Bárðarbunga refer to cases when isopropyl alcohol
was used. Without it, the average runtime was typically twice as long - Samanburður á borunum á Hofsjökli
og Bárðarbungu. Gögn varðandi Bárðarbunguborun eru að nokkru leyti áœtluð útfrá upplýsingum sem birtar
eru í greinum um hana. Tölur um meðalfærutíma á Bárðarbungu eiga við tilvik þegar ísóprópanól var notað til að auðvelda borun. An þess var færutími að meðaltali tvöfalt lengri.
HOFSJÖKULL 2001 BÁRÐARBUNGA 1972
Length of ice core Lengd ískjama 100 m 415 m
Total drilling time Heildarbortími 9 days 35 days
Average daily production Meðalborun á dag 11.1 m/day 11.9 m/day (0-415 m) 14.6 m/day (298^115 m)
Average runtime Meðalfœrutími 50 m: 12 minutes 100 m: 20 minutes 100 m: ~30 min 200 m: ~37 min 300 m: ~45 min 400 m: ~53 min
Lowering/hoisting speed (below water table) Ferðahraði bors í holu (neðan vatnsborðs) 0.1-0.2 m/s 0.33-0.50 m/s
Core drilling speed (below water table) Hraði kjarnaborunar [neðan vatnsborðs) 0.1-0.2 m/min ~0.02 m/min (without isopropyl alcohol) ~0.2 m/min (with isopropyl alcohol)
Average core length (below water table) Meðal-kjarnalengd (neðan vatnsborðs) 0.27 m 0.7-0.8 m
Bárðarbunga drill were quite massive with a flat un-
derside, whereas the AWI-drill used on Hofsjökull
has small, round-shaped cutters with a smaller con-
tact area against the ice. The cutters were frequently
checked as the drill was pulled from the hole and signs
of freezing-on were not observed.
4. The average core length was much greater at Bárð-
arbunga. It is not clear if the better performance of
that drill is due to the positive effects of the antifreeze
mixture on the drilling process, or if it simply reflects
the fact that the transport of chips into the storage
chamber above the core barrel functioned properly, in
contrast to the situation at Hofsjökull.
5. At Bárðarbunga, the drill frequently became stuck
at hole bottorn for several minutes and a few times up
to an hour, probably due to clogging of chips between
the outer core barrel and the hole wall (Theódórsson,
1976). This problem never occurred on Hofsjökull,
and only on one occasion was it necessary to pull the
cable twice, in order to free the drill from hole bottom.
CORE STRATIGRAPHY AND DENSITY
Preliminary observations of the core stratigraphy were
carried out by visual inspection at the drilling site.
The appearance of the cores changes gradually in the
uppermost 40 m, as the snow is transformed into ice.
Ice layers (meltlayers) formed by melting-refreezing
processes in the snow near the surface are clearly ob-
served in the snow/firn part, but more careful inspec-
tion is needed to detect them as the ice becomes more
transparent. The meltlayers are partially or totally de-
void of bubbles, whereas the ice that has formed by
metamorphosis of firn is rich in bubbles, which typi-
cally are 2-3 mm in diameter.
The density of each core piece was measured in
the field using conventional methods. Detailed core
processing took place at the Alfred Wegener Institute
in October 2001, and the density was then measured
with a resolution of 5 mm using a gamma-ray absorp-
tion technique. The densiometer consists of a gamma-
34 JÖKULLNo. 51