Rit (Vísindafélag Íslendinga) - 01.06.1971, Page 42

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GUÐMUNDUR PÁLMASON
except by assuming relative thicknesses of layers 1 and 2. It can,
however, be stated that this depth is between 2.4 and 3.0 km. If an
average velocity of 4.6 km/sec is used for the material between layers
0 and 3, a depth of 2.8 km is found to layer 3.
From the shot point at Graenavatn three profiles have been mea-
sured in different directions. Two of these are Báth’s profiles from
1959, while the third is the Graenavatn-Eyjafjöll profile (L3).
The surface layer at Graenavatn has a P-wave velocity of 3.25
km/sec and is about 1.0 km thick. The delay time for the P3-wave
at Graenavatn can be computed only indirectly from the total delay
time on the various profiles. On the Graenavatn—Eyjafjöll profile
the delay time increases considerably at first and then decreases
again in ölfus. This could be due to an increased thickness of the
overlying layers as one goes east from the shot point in Graenavatn,
but another interpretation is also possible. The profile runs along
the main road (cf. Fig. A7) and it is found that the farther to the
south the seismometer stations are the larger is the delay time. The
irregularities in the delay time between Graenavatn and ölfus could
thus be due to lateral changes in depth to layer 3. This interpretation
is made all the more likely by the offshore profile Thjórsárós—Eldey
(L6), which has a much higher delay time for the P3-wave than ob-
served on the shore.
On profile CP the total delay time for the P3-wave near the shot
point is about 1.00 sec. This is a considerably lower value than is
found on the Graenavatn-Eyjafjöll profile. It is thus apparent that
considerable structural differences exist between these two profiles.
By a comparison of the delay times of these two profiles it is probable
that the delay time at the shot point in Graenavatn lies between 0.50
and 0.55 sec. A value of 0.53 sec will consequently be assumed. This
gives a thickness of layer 1 of 1.6 km and a total depth to layer 3 of
2.6 km, or about 2.5 km below sea level.
Báth’s profiles CP and WP will be discussed in more detail in
section 6.11 and reinterpreted in the light of more recent refrac-
tion data.
As mentioned above an attempt was made to find structural varia-
tions across the volcanic zone on the Beykjanes peninsula by an arc
shooting procedure from a shot point in Graenavatn. The shot-
seismometer distance was between 28 and 36 km, which implies that
the P3-wave is the first arrival on the seismograms.
The total delay time is given in Fig. 11. It shows two conspicuous