Jökull - 01.12.1970, Page 45

A jump in the anomaly of the magnitude ob-
tained above between stations inside and out-
side the caldera seems to call for some addi-
tional explanation esp. as values obtained in
other places on Vatnajökull would rather in-
dicate that the two-layer Bouguer anomaly
should be increasing across the caldera from
the stations east of it.
Thorarinsson has pointed out that the exist-
ence of a water-layer under the ice in the
Grímsvötn caldera could cause a misinterpreta-
tion of the seismic reflection data if the re-
flection at the ice-water interface is not detect-
ed (Thorarinsson, 1965). As the velocity of
sound in water (at 0° C) is 1400 m/sec com-
pared with approximately 4000 m/sec in ice
(at a depth > 200 m) (Clark, 1966), an assump-
tion that there is no water present and we
have a reflection at an ice-bedrock interface
rather than a water-bedrock one would result
in a considerable underestimation of the bed-
rock elevation.
Assume instead that the ice-thickness is only
ca. 220 m as Thorarinsson suggests from his
observations of the lake near Mt. Depill. At
stations III and VI we must therefore have an
underlying water-layer of approx. 100 m. Now
calculate a three-layer Bouguer anomaly in an
analogous way to the two-layer one taking
density of ice, water and bedrock to be 0.9,
1.0 and 2.6 g/cm3 resp. The results for the
two stations are 15—20 mgal, i.e. values in much
better accordance with those obtained for the
stations east of the caldera than we had before.
The gravity data thus seem to support Thor-
arinsson’s idea that the elevation of the bed-
rock in the Grímsvötn caldera is considerably
higher than the one previously calculated from
the seismic data.
BOUGUER ANOMALY
IN WESTERN VATNAJÖKULL
T. Einarsson published in 1954 a map of
Bouguer anomaly in Iceland (Einarsson 1954).
His survey was particularly lacking in gravity
stations in S.E. Iceland. The stations on Vatna-
jökull provide valuable additional information.
We collect in Fig. 4 results from all gravity
observations up to 1968 on or near to western
Vatnajökull. The calculated Bouguer anomaly
Fig. 4. Bouguer anomaly in western Vatnajök-
ull according to gravity observations up to 1968.
Myncl4. Bouguer frávik í vestanverðum Vatna-
jökli samkv. þyngdarmœlingum fram til 1968.
for stations on Vatnajökull is the two-layer
Bouguer anomaly with qi — 0.9 g/cm3 and q2
= 2.6 g/cm3 (in accordance with E.inarsson’s
density estimates). Tlie survey c.arried out since
1954 indicates that the central bowl in the
Bouguer anomaly extends more to the south-
east than suggested in Einarsson’s map.
ACKNOWLEDGEMENTS
The idea of the reported survey was initiat-
ed by the late ]. Eythórsson, president of the
Icelandic Glaciological Society. The work was
carried out while the author was employecl at
the Department of Natural Heat of the Na-
tional Energy Authority in Iceland, under the
supervision of G. Pálmason. Elevation values in
Table 1 and figures on location of survey lines
and profile sections were made available to the
JÖKULL 20. ÁR 43