Jökull - 01.12.1989, Blaðsíða 12

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Fig. 6. a) East-west cross-section through the main caldera in Grímsvötn. Vertical exaggeration 2:1. The
interface giving rise to reflection ’c’ in 5a is shown. Above, the magnetic field strength in nanoteslas.
Mynd 6. a) Þversnið frá vestri til austurs gegnum meginöskju Grímsvatna. Fyrir ofan er segulsviðið eftir
sniðinu.
forming the step are magnetic.
It is therefore proposed here that the higher areas
represent a lava flow, or flows, covering a portion of
the southem and westem caldera floor. The steps
are believed to be the edges of these lavas. In the
lower areas in the northem and eastem parts of the
lake, the lakefloor is believed to be covered with
sediments, at least in the areas where reflections
from horizons beneath the lakefloor are present.
DEEPER REFLECTIONS
Reflections from interfaces below the lakefloor
are believed to be present in the northem half of line
2 (Fig. 5b). In the area 4.9-5.6 km, the water bottom
reflection (’b’) is at about 250 ms. At around
290 ms, the first deeper event can be seen (’c’). A
second event (’d’) is present at 310-320 ms and a
third reflection (’e’) can be seen at 390-400 ms. The
possibility that these three deeper events are
multiples can be ruled out as their arrival times are
very different from those for the double reflections
that might be present. Events, not as clear as the
other three, are present between 'd’ and ’e’ (Figs. 5b
andöb).
In the central part of line 1, a reflection (’c’) is
present at 280-300 ms. It is not a strong event on
most traces, but it is continuous and its arrival time
is not consistent with that of a multiple. It is also in
an area of a low lying lake bottom, considered to be
covered with sediments. This event must therefore
be considered to be a genuine reflection. The lake
bottom in line 3 between 2.7 and 3.7 km is con-
sidered to be sedimentary in nature. There are indi-
cations of weak deeper reflections in places between
250-350 ms (’c’, ’d’ and ’e’), but individual re-
flections are continuous for only 200-300 m.
It is unclear whether the same deeper horizons are
seen on all lines, but the limited extent of each
10 JÖKULL, No. 39, 1989