Rit (Vísindafélag Íslendinga) - 01.06.1971, Side 82

82
GUÐMUNDUR PÁLMASON
tions the amplitudes fall below the threshold of detection which is
determined by the noise level on the seismograms. Doubling of the
charge size did not alter the situation. On two stations which are in
the middle of the valley, a signal of normal strength was observed,
but it was delayed by 0.45 to 0.60 seconds relative to the expected
arrival time. This delay is so great that a 1.0 to 1.5 km thick pile of
low-velocity sediments would he required to account for it.
It seems clear from the above results that the Breidamerkurdjúp
submarine valley is more likely to he a deep sediment-filled valley in
the underlying Tertiary rocks than a shallow erosional feature with
a fairly homogeneous rock or sediments beneath. The observed ampli-
tude variation is indicative of a major structural irregularity under
the valley. A closer study of this could probably be made by mapping
the sediment thickness by reflection profiling.
6.11. Refraction lines of M. Báth (1960).
Two refraction profiles were measured in 1959 from a common
shot point in Graenavatn (Báth, 1960). Their location is shown in
Fig. 2. The line CP runs in a northeast direction to a distance of
109.7 km and the line WP runs approximately north to a distance
of 253.4 km.
Since Báth’s interpretation of the travel times on these two pro-
files is at some variance with the structure of SW-Iceland as deduced
from the present work, it is worth while to look at the travel time data
again to try to find what causes the discrepancy. By taking into ac-
count details of the seismic structm'e such as the variable thickness
of layer 0 and the variable depth to layer 3, it is found to be possible
to hring BAth’s data into a very good agreement with the results of
the present work. The apparent discrepancy between the travel times
on profiles CP and WP is also explained.
The travel time diagram for these two profiles (Fig. 29) was
interpreted by BAth as giving a three-layered crust with P-wave velo-
cities of 3.69, 6.71 and 7.38 km/sec. The first layer has later been
shown to be more complicated, consisting normally of 2-3 layers.
The seeond one corresponds to layer 3 and the third one to layer 4 in
the present work. In the light of presently availahle information on
the seismic structure in the vicinity of these two profiles, the follow-
ing comments to Fig. 29 can be made.
The P3-wave (BAth’s P2) is usually the first arrival from a dis-