Rit (Vísindafélag Íslendinga) - 01.06.1971, Síða 29
CKUSTAL STRUCTURE OF ICELAND
29
first three determinations are from the Snaefellsnes peninsula
(sect. 6.5).
(i) Ólafsvík-Hraunsmúli.................V3 — 6.52 km/sec
(ii) Hraunsmúli—Hofsstadir...................6.74 „
(iii) Hofsstadir-Kolbeinsstadir................6.50 „
(iv) Hrútafjördur, 2 values (6.45; 6.57), average 6.51 „
(v) Vatnsdalirr.............................6.55 „
(vi) L1 and L2, composite travel time diagram . . 6.47 „
(vii) L4 and L5, „ „ „ „ 6.46 „
The last two velocities are not determined from reversed pro-
files, hut are nevertheless more reliable than apparent velocities
from single unreversed profiles.
From the above results it is clear that values close to 6.50 km/sec
are obtained where good velocity determinations have been
made. Deviations from this value do occur, however, as shown
by case (ii) above.
The above arguments are considered sufficiently conclusive to
]ustify the use of the value 6.50 km/sec as the most probable P-wave
velocity in layer 3. This value has therefore been used to calculate
delay times of the P3-wave and depths to layer 3 (sect. 6). If devia-
tions are known or discovered later it is easy to make the appropriate
corrections in the delay times.
It should be remarked here that the apparent geographical distri-
bution of higher (6.5km/sec) andlower (6.2km/sec) apparent velo-
cities in layer 3 (Pálmason, 1967a) can in many cases he shown to
be due to dips in the 2-3 boundary. This emerges much clearer, when
the delay time method is used for depth calculation, than when struc-
ture is computed on the basis of assumed horizontal boundaries.
The true velocity in layer 4 is less well known than in layer 3.
Apparent velocities are considerably affected liy dips in the 3—4
boundary. In particular this applies to profiles L4, L5, and prohably
Ll. Profiles L3, L6, L7, L8 and L9 are probably least affected by
dips as far as can be judged from available data. The average value
of the P4 -wave velocity on these five profiles is 7.19 km/sec. A value
°f 7.20 km/sec has been used to compute delay times and depths to
this layer.