Rit (Vísindafélag Íslendinga) - 01.06.1971, Page 63

CRUSTAL STRUCTURE OF ICELAND
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most probably due to differences in depth to layer 3 along the two
profiles.
The Vesturhóp-Skagafjördur profile has a fairly constant total
delay time up to a distance of about 55 km, when it enters the Skaga-
fjördur valley. Here a relatively abrupt increase in delay time occurs
which is caused by the structural changes in Skagafjördur (see later
in this section). On the Vesturhóp-Vídidalur profile on the other
hand the delay time is 0.20—0.25 sec lower over most of the profile
with a minimum close to Borgarvirki. There is some uncertainty in
converting these delay times to absolute depth because of the rather
irregular conditions near the shot point and the consequent uncer-
tainty in the delay time at the shot point. The profile Vesturhóp-
Skagaf jördur indicates a shot point delay time of about 0.42 sec while
the Vesturhóp-Vídidalur profile indicates that it may be somewhat
lower. For depth estimates it will be assumed that the shot point de-
lay time is 0.40 sec. This gives an estimated depth of 2.8 km to layer
3 close to the shot point. Along most of the Vesturhóp—Skagafjördur
profile the depth would remain about 3 km until in Skagafjördur
where it increases. On the Vesturhóp-Vídidalur profile a delay time
of about 0.15 sec is inferred near Borgarvirki, increasing to about
0.28 sec farther south along the profile. This corresponds to a depth
of about 0.9 to 1.1 km near Borgarvirki increasing to 1.6 to 1.9 km
farther to the south.
In Vatnsdalur profile 2 was measured in 1962 from a shot point
at the mouth of river Vatnsdalsá. A travel time anomaly was observed
near the farm Hvammur, which was interpreted as an upward bulge
of the 2—3 boundary (Pálmason, 1963). This anomaly was later
studied closer by means of 3 shorter profiles, 7 to 15 km long, with
seismometer stations at intervals of about 500 meters (cf. Fig. A25).
Two north-south running gravity traverses were also made, one on
each side of the valley.
The more detailed refraction work confirmed the relatively shal-
low depth of layer 3 in the Hvammur area, but puts its depth some-
what greater than previously thought. The gravity profiles also re-
vealed a positive gravity anomaly of about 10 milligals which appears
to be superimposed on a regional north-south gradient of about 0.71
mgal/km. The Bouguer gravity anomaly is shown in Fig. 22.
In the interpretation of the gravity data it is assumed that the
structure is two-dimensional and that the profiles are at right angles
to it. It has not been possible to test the validity of this assumption,