Jökull - 01.12.1963, Qupperneq 53

lake Ljósavatn to the north along Kaldakinn.
Profiles 12 and 12a run in opposite directions
along the Reykjadalur-Adaldalur valley, about
8 km east of the Bárdardalur line.
Profile 10 running west along Ljósavatns-
skard gives only P velocities of layers 1 and 2,
and depth to layer 2. Profile 11 along Kalda-
kinn does not either give reliable P3 waves
except at the last station, but the S3 waves are
recorded at several stations along the profile,
and these have been used to compute depth to
layer 3 here. In Ljósavatnsskard, where profiles
10 and lOa are located it is known frorn gravity
measurements (Schleusener, 1943) that the low-
velocity surface layer of glacial drift has a very
uneven thickness, and this may explain the dif-
ferences in Pj velocities found on profiles 10
and lOa, and also the rather high P2 velocity
on profile lOa.
The main structural change in the area of
these profiles is an increase in depth to layer 2
from Fnjóskadalur and Eyjafjördur, where it is
about 0.5 km, to Ljósavatnsskard ancl farther
east, where it is about 1 km. This change ap-
parently occurs near the western end of Ljósa-
vatnsskard. No major change in thickness of
layer 1 is found across the Bárdardalur line.
Profile 10 which runs across the Bárdardalur
line gives a somewhat smaller thickness of layer
2 and depth to layer 3 than neighbouring pro-
files do. As this result is based on only one un-
reversed profile with a low-velocity surface layer
of uneven thickness, it must be considered rather
uncertain. No anomalous behaviour of the travel
time graph for profile lOa can be seen across
the Bárdardalur line. Profiles 11, 12 and 12a
all give similar structure with a depth of about
3 km to layer 3.
13.—13a. This is a reversed profile across
Ivelduhverfi in the Neovolcanic zone. The area
is characterized by Postglacial lava flows with a
number of open fissures running in a north-
south direction. The surface low-velocity layer
which probably consists of Quaternary volcanic
material is relatively thick here, 0.42 km at the
western end of the profile, and 0.73 km at the
eastern end. At the eastern end it is apparently
composed of two layers with velocities 2.33 and
2.74 km/sec, whereas at the western end there
is apparently a single velocity of 2.65 km/sec.
The computed depth to layer 3 is similar for
botli profile directions.
14. This profile runs in an east-west direction
across Axarfjardarheidi just east of profile 13—
13a. It is also in the Neovolcanic zone. The
surface layer is here 0.26 km thick which is
about 0.5 km thinner than in the neighbouring
Kelduhverfi. Layer 1 is also very thin, 0.34 km,
but the depth to layer 3, 3.21 km below sea
level, is similar to what was found in Keldu-
hverfi. It thus appears that there is a major
change in relative thicknesses of layers 0, 1
and 2 between Kelduhverfi and Axarfjardar-
heidi, but that the total thickness or depth to
layer 3 is similar in both cases.
An attempt was made to measure a profile
with a north-south direction between Keldu-
hverfi and Axarfjardarheidi, but the recording
conditions were very poor and the arrivals weak
and no attempt was made to interprete the re-
sults. This may have been due to the horizontal
structural changes found in this area.
15. —16. These profiles are in the Mývatn
area, which is characterized by recent volcanism.
The shot points were in lake Sandvatn and
lake Mývatn respectively, and the direction of
profile 15 was roughly north, but of profile 16
east. Both profiles give a surface layer about
0.65 km thick which is probably Quaternary
volcanic material. The depths to layer 3 were
found to be similar, about 3.5 km below sea
level, on both profiles.
17.—18. These two profiles run in opposite
directions from a common shot point in river
Sudurá near the western boundary of the Neo-
volcanic zone. They are located just east of the
southern part of the Bárdardalur line. The sur-
face rocks are of Quaternary age and on profile
18 mainly recent lava flows. The surface layer
is, however, very thin, probably less than 100
meters. The computed structure is similar on
both these profiles and the depth to layer 3
about 3.5 km below sea level.
19.-21. These three profiles are all located
in tlie central part of the Neovolcanic zone
along the river Jökulsá á Fjöllum. The first two
have a common shot point in a small lake near
the farrn Grímsstadir and run in opposite di-
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