Jökull - 01.12.1963, Page 44

Seismic Refraction Investigation of the Basalt Lavas
in Northern and Eastern Iceland
BY
GUDMUNDUR PÁLMASON
DEPARTMENT FOR NATURAL HEAT,
STATE ELECTRICITY AUTHORITY, REYKJAVIK, ICELAND
ABSTRACT.
Results are given of 30 seismic refraction pro-
files rneasured in 1962 and 1963 in northern
and eastern Iceland with tlie object of studying
the structure of the basalt lavas that cover the
whole of Iceland. Profile lengths were 20—35
km in most cases. The area studied embraces
the Neovolcanic zone in northeastern Iceland
and the adjacent Tertiary basalt districts in
northern and eastern Iceland. In southeastern
Iceland three profiles were measured in an area
with prominent basic and acid intrusions. Bot.h
P and S waves were usually recordecl.
Four layers were found ivith tlie following P
velocities: 2.82 ± 0.07, 4.16 ± 0.04, 5.06 ± 0.04
and 6.32 ± 0.04 kmjsec. The S velocities for the
three deeper layers are as follows: 2.32 ± 0.03,
2.74 ± 0.03 and 3.51 ± 0.02 kmjsec. Poisson’s
ratio for the three loiuer layers are: 0.276 ± 0.006,
0.281 ± 0.003 and 0.272 ± 0.004.
The first layer is interpreted as Qiiaternary
volcanic products, the second layer as the upper
part of the Tertiary floocl basalts, the third layer
as Tertiary flood. basalts mixed wi.th. basic and
aci.d instrusions, while the fourth layer is un-
hnown, although there are indications t.hat it is
basaltic or gabbroic in nature.
The depths to the bottom layer are found to
vary from 1.2 to 4.5 km, while the thickness of
this layer is unknown. In one location a clepth
of 0.6 km to this layer is indicated over a re-
stricted area.
INTRODUCTION
Seismic refraction studies o£ the earth’s crust
in Iceland were initiated in 1959 by a Swedish-
Icelandic expedition which measured a 253 km
long profile in western Iceland (Báth, 1960).
This profile gave a surface layer 2.1 km thick
with P velocity 3.69 km/sec overlying a 15.7 km
thick layer with P velocity 6.71 km/sec and a
10.0 km thick layer with velocity 7.38 km/sec. In
the following year these studies were continued
and eight shorter profiles were measured to
study particularly the surface lava layer
(Tryggvason and Báth, 1961). It was found that
this surface layer is usually not a single velocity
layer, but rather is composed of 2 or 3 layers.
These investigations were continued in 1962
and 1963 by the Department for Natural Heat
of the State Electricity Authority, Reykjavik, as
a part of its programme for the study of thermal
activity in Iceland. Thirty refraction profiles of
the order of 30 km length were measured, main-
ly in northern and eastern Iceland. The present
report gives the results of these studies.
The area covered embraces the Neovolcanic
zone in northeastern Iceland and the adjacent
Tertiary basalt districts in northern and eastern
Iceland. Tlie 'main objective was to map the
structure of the basalt lava, and in particular
any difference between the zone of recent vol-
canism and the Tertiary basalt areas. In south-
eastern Iceland three profiles were measured to
study the seismic structure of an area where
prominent basic and acid intrusions occur. The
choice of profile locations within these areas was
largely dictated by accessibility. The profile
locations are shown in Fig. 1.
The refraction instruments used were of the
same kind as described by Báth (1960). The
profiles were usually unreversed, but in a few
cases where identification of layers was difficult
or doubtful, reversed profiles were shot at a
later date (profiles 10, 12 and 13). Four to six
vertical component geophones (6 c/s) were used
JÖKULL 1963
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