Jökull - 01.12.1989, Side 14

■103nTesla
Fig. 6. c) Cross-section along seismic line 3. The broad ridge in the bedrock at 4 km is the divide between
the main caldera and the north caldera.
Mynd 6. c) Þversnið frá suðri til norðurs gegnum megin- og norðuröskjuna eftir endurkastsmælilínu 3.
1000 m. One successful sounding was made on the
ice shelf in 1951 by the French-Icelandic expedition
and there were four successful soundings during the
Icelandic-French expedition in 1955. Details of the
latter survey have never been published but the
results were summarized by Þórarinsson (1965).
These surveys were successful, in the sense that a
reflection from the lakefloor was observed. How-
ever, no reflection from the ice-water interface was
detected. The fact that only one reflection was
observed led the French seismologists to interpret
their data as if no water layer were present. They
therefore calculated the depth to bedrock using ice
velocities (Vrn=3550 m/s, Joset and Holtzscherer,
1954). The elevation of the caldera floor which
results from these calculations is 800-900 m a.s.l.,
contrasting with 1060-1100 m a.s.l. calculated from
the present survey.
The existence of the subglacial lake over the past
decades is well documented and beyond dispute
(Wadell, 1920; Þórarinsson and Sigurðsson, 1947;
Þórarinsson, 1953, 1965, 1974; Bjömsson, 1974,
1988). It can therefore be taken as a fact that a water
layer was present at the time of the previous sur-
veys. The failure to detect the ice-water interface
was probably caused by the field methods used. The
long offsets result in a relatively short time interval
between the arrival of the P-wave and that of a shal-
low reflection. Furthermore, a surface dynamite
source appears to produce a high amplitude P-wave
with a duration of about 100 ms (5-6 complete
cycles, Joset and Holtszcherer, 1954, p. 20, Fig. 18)
which masks any other arrivals which might occur
in that time interval.
It is possible to reinterpret the results of the ear-
lier surveys converting the ice thickness values
given by Joset and Holtzscherer (1954) and Þórar-
insson (1965) back to traveltime. To do this, infor-
mation given by Joset and Holtzscherer (1954, p.
20) on the method of calculation is used.
12 JÖKULL, No. 39, 1989