Jökull - 31.12.2001, Page 40

Thorsteinsson et al.
Figure 9a. Tephra layer from the Hekla 1991 eruption at 40.70—40.83 m depth. - Gjóskulag úr Heklugosinu
1991 á 40,70^40,83 m dýpi.
mination of the annual layers. The most promising
method for detecting summer layers is the study of
wind-blown dust, originating in the surrounding areas
and presumably deposited on the ice cap during the
period when the land surface is snow-free (late sum-
mer to early autumn). In addition, the Hofsjökull core
will provide an opportunity to study if the stratigraphy
of meltlayers in a temperate ice cap contains a sea-
sonal signal. Results of these studies and other mea-
surements of the core will be reported elsewhere.

Figure 9b. Tephra layer from the Hekla 1980 eruption
at 69.94 m depth. - Gjóskulag úr Heklugosinu 1980 á
69,94 m dýpi.
CONCLUDING REMARKS
Improving the drilling technique
Based on the experience gained at Hofsjökull and
comparison with the Bárðarbunga drilling some de-
sign and engineering modifications are suggested on
the AWI-type drill, in order to make it more suitable
for drilling in temperate ice:
1. A drill motor built for submersion in water should
be used. Other components, such as the slipring unit
and cable connectors also need to be thoroughly pro-
tected from water.
2. A different chip transporting and storage system
is needed. A 1.0-1.2m long core barrel seems suf-
ficient, because it is unlikely that core lengths above
1 m can be attained below the water table. Above the
core barrel, a narrow shaft should connect the bar-
rel to the coupling mechanism that transforms rota-
tion from the drill motor to the barrel and drill head.
With this arrangement, the chips would be transported
upward along the spirals on the core barrel and float
directly into the space around the shaft where they be-
come collected. The Bárðarbunga drill was designed
in this way (Arnason et al., 1974) and similar so-
lutions have recently been incorporated into designs
of drills used for deep coring on the polar ice sheets
(NGRIP/EPICA drill: S.J. Johnsen and others, private
communication; Dome F drill: Tanaka et al., 1994).
3. A redesign should permit the free flow of water
through or past the different components of the drill
and allow for greater clearance between the outer bar-
rel and the hole wall. The use of a submersible motor
will eliminate the need to operate with a closed motor
section.
4. The drill head does not seem to need modifications.
The cutters appear well-suited for operations in tem-
38 JÖKULLNo. 51