Rit (Vísindafélag Íslendinga) - 01.06.1971, Page 84

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GUÐMUNDUR PÁLMASON
Akranes-Hvalfjördur), while the station 63 (CP) is in the relatively
high delay time area to the east of lake Thingvallavatn. The dif-
ference in delay times hetween stations 63 (CP) and 64 is to be ex-
pected from the difference in the structure overlying layer 3 under
the two stations. The arrival times of the P3-wave (Báth’s P2-wave)
at stations 12, 34 and 63 give an apparent P3-wave velocity of 6.52
km/sec.
It may safely be assumed that all the more distant stations on
profiles CP and WP record the P4-wave as the first arrival. Varia-
tions in the P4-wave delay time are then probably due to variations
in depth to layer 4 or to variations in the overlying structure.
The delay time of the P4-wave at the shot point in Graenavatn is
somewhat uncertain. The profile Graenavatn-Eyjafjöll indicates a
value of ahout 1.09 sec, while profiles CP and WP indicate a value
below 1.0 sec. If the delay time is assumed to be 1.0 sec, the depth to
layer 4 at the shot point will be about 8.7 km. This is in good agree-
ment with other results in SW-Iceland and it will therefore be as-
sumed that the P4~wave delay time at Graenavatn is 1.0 sec. The
following depths to layer 4 are then found under the profile WP,
taking into account the overlying structure.
Table 6.4.
Station Distance km t - x/7.2 sec Delay time sec Depth km
Shot point 0 1.00 8.7
102 101.72 1.96 0.96 9.5
165 164.60 2.37 1.37 15.6
206 206.21 2.24 1.24 13.4
253 253.44 2.12 1.12 14.0
The relatively sharp increase in depth to layer 4 between stations
102 and 165 is noteworthy. It occurs in an area where the depth to
layer 3 is relatively great. Since there is still some uncertainty in the
true value of the P4-wave velocity, there will he a corresponding un-
certainty in absolute depths computed on the basis of delay times
from unreversed profiles (cf. sect. 5.6). The main features of the
depth variation will, however, not be changed when a more accurate
value of P4-wave velocity has been found.
The relatively great depth to layer 4, which is indicated ahove for
the distant part of profile WP, should show up on the profile Vestur-