Jökull - 01.12.1964, Page 5

REDUCTION OF THE OBSERVATIONS
The method here used to reduce the observ-
ed gravity values is similar to that used by Bull
and Hardy (1956). After correcting for drift,
the free air and Bouguer corrections were
applied, using a density of 2.6 g/cm3 for all
the material underneath the station.This density
value was chosen in accordance with the value
used by Einarsson (1954) in his gravity survey
for the central highland of Iceland. The Bou-
guer anomaly was then computed using the
International Gravity Formula. In this way
large negative Bouguer anomalies were obtain-
ed, which are largely due to the mass deficiency
of the ice cap in comparison with the density
of 2.6. Variations in the Bouguer anomalies
are then due partly to variations in ice-thick-
ness, and partly to an inhomogeneous mass
distributions in the underlying rock forma-
tions.
Terrain corrections were computed using a
density of 0.9 g/cms. They were usually very
small, less than one milligal. Cylindrical zone
JÖKULL 1964
charts were usecl out to a distance of about
15 km.
RESULTS AND DISCUSSION
A gravity station was established on Grims-
fjall at the triangulation point 2023 near the
hut. The altitude of this station is 1719 m
according to the measurements of the Icelandic
Survey Department. The gravity value at this
station was found to be 981,860.4 mgal referred
to the value of 982,274.5 mgal for the gravity
station at the University of Iceland in Reykja-
vík. The closure error Reykjavík—Grímsfjall
—Reykjavík was 0.8 mgal.
At Jökulheimar, at the western edge of Vatna-
jökull, a gravity station was located at the
flagpole close to the hut of the Icelandic
Glaciological Society. The gravity value at this
station was found to be 982,103.8 mgal. Its
altitude is 674 m.
A Bouguer gravity map of the Grítnsvötn
area has been drawn from the computed Bou-
guer anomalies and is shown in Fig. 2. The
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