Jökull - 01.12.1970, Page 44

gravity survey in 1967 and seismic survey in
1955.
Mynd 3. Þversnið af Tungnárjökli eftir lin-
unni. SNR—NKP reiknað út frá þyngdarmœl-
ingunum 1967 og jarðsveiflumcelingunum 1955.
A, B, C, D in Figs. 1, 2, 3. The calculated
lieight of these ridges is limited (< 50 m). The
method of reduction (assumption of horizontal
interfaces) will lead to smoothing out of the
calculated bedrock surface as compared with
the real one, but rough model calculations seem
to indicate that the resulting decrease of eleva-
tion contrast can at most be in the order of
few tens of meters. The major source of un-
certainty rises from the fact that the gravity
survey lines do not coincide with the seismic
stations and the quoted quality of reflection in
the critically situated seismic station XX is
only “fair”. Thus our estimate of the two-layer
Bouguer anomaly for the area of gravity survey
remains somewhat unsatisfactory. A markedly
42 JÖKULL 20. ÁR
improved picture of the ice-thickness of Tungn-
árjökull can only be obtained with additional
survey work.
It is of interest to note the features of the
ice-surface in 1946 above the points A, B (Fig,
2) keeping in mind that a sudden advance of
Tungnárjökull took place in 1945. The ob-
served ridges at B and C may be an indication
of the fact that the mountain ridge Fögrufjöll
south of Langisjór extends under Tungnár-
jökull. The existence of such a ridge may be
suggested in light of the proposed direction of
water flow under the glacier in connection
with the Skaftárhlaup in 1955 (Fig. 1) (Thor-
arinsson and Rist, 1955).
THE GRÍMSVÖTN PROBLEM
The results from J. P. Martin’s seismic survey
and G. Pálmason’s gravity survey in the Gríms-
vötn area have already been published (Pálma-
son, 1964; Thorarinsson, 1965). Even though
these surveys took place in different years the
conditions were similar in the respect that in
both cases less than a year had elapsed since a
Skeidarárhlaup.
In the area east of the Grímsvötn caldera
the reflection in the seismic survey is quoted
to be fair or good at stations II, XI and XIV
(see Fig. 7 in Tliorarinsson, 1965) and a good
estimate of one-layer Bouguer anomaly (q = 2.6
g/cm3) can be obtained from Pálmason’s data
(see Fig. 2 in Pálmason, 1964). Knowing the
ice-thickness we are able to calculate a two-
layer Bouguer anomaly (qi = 0.9 g/cm3, Q2 =
2.6 g/cm3). The results are shown in Fig. 4.
In the caldera itself we have two stations no.
III and VI where the quality of reflection is
fair or good. The location of these stations is
denoted by a cross in Fig. 4 and the reported
ice-thickness is 500—600 m. Calculating a two-
layer Bouguer anomaly in the same way as be-
fore we obtain approx. the value 0 mgal at
these stations. The proximity of Grímsfjall
makes the assumption of horizontal interfaces
somewhat unsatisfactory and the calculated ano-
maly consequently too low but at most by a
few milligals.
We expect variations in the two-layer Boug-
uer anomaly solely to be due to irregularities
in density in the underlying bedrock or mantle.