Jökull - 01.12.1976, Síða 54

Some Observations of tbe Heimaey
Deep Drill Hole During the Eruption of 1973
AXEL BJÖRNSSON,
NATIONAL ENERGY AUTHORITY, REYKJAVÍK
leó kristjánsson,
SCIENCE INSTITUTE, UNIVERSITY OF ICELAND, REYKJAVÍK
HLÖÐVER JOHNSEN,
VESTMANNAEYJAR
ABSTRACT
During and after the Heimaey eruption of
1973, measurements of water temperature and
discharge at a 1565 m deep drill hole near the
active fissure xvere made daily. Both reached a
maximum about four months after the beginn-
ing of the eruption, but returned to static
conditions some six months after the end of
eruptive activity. This behaviour appears to be
mostly due to the pressure of volcanic gas on
deep aquifers.
INTRODUCTION
A deep drill liole (1565 m) was sunk by the
National Energy Autliority (N.E.A.) on Heima-
ey island, off the south coast of Iceland, in
1964 in search of fresh water. The water level
in the hole fluctuated during the drilling, but
later settled to 8—10 m depth after the drilling
period. Aquifers with about half the salinity
of sea water were penetrated at 900 m, and
the hole has not been used. Following pumping
tests in 1970, it became blocked at 821 m, and
the water level changed to 5.4 m. The site is
200 m from the local harbour and 18 m above
mean sea level.
The drill liole is cased with 95/s” casing
down to 94 m and 7" down to 197 m, and then
drilled with 6(4" bit to the bottom. Thermal
gradient below 900 m, measured in 1967, was
63° C/km. Further details on the hole and
52 JÖKULL 26. ÁR
local geology are given by Pálmason et al.
(1965), Pálmason (1967) and Tómasson (1967).
Early January 23, 1973, an eruption broke
out on a fissure 1700 m from the drill hole.
Water discharge from the hole probably started
between 22 that day and 03 on January 24.
Temperature measurements in the hole were
started on January 24 by the N.E.A.
A valve and manometer were installed at the
drill ltole in early February, and daily mea-
surements of temperature and rate of discharge
initiated. These were at first made by geologists,
students and technicians from Reykjavík, but
since May 7 by H. Johnsen.
The eruption gradually diminished in inten-
sity after early February, when activity had
become confined to one crater, and the last
outbursts in that crater were noted on June 26.
MEASUREMENTS
Temperature measurements at the water sur-
face of the hole were made daily or twice a
day using a mercury thermometer with an
accuracy of 0.5° C. Fig. 1 shows the results,
which scatter somewhat from day to day, but
a rapid increase in temperature occurs at the
beginning of the eruption reaching a maximum
around February 6. Then the temperature
decreases to a minimum around March 15
and increases to a maximum in late May. A
steady drop is then noted to around 25° C in
the first week of July, wlien the discharge