Fróðskaparrit - 01.01.1976, Page 7

Seismic Refraction Measurements around the Faeroe Islands 15
minimum on the shelf northwest of the islands (see fig. 2);
the minimum is much deeper than indicated from measurements
on land.
A number of magnetic field measurements in the region
have been published (Avery et al. 1968, Fleischer et al. 1974,
Schrøder 1971, see also Voigt & Avery 1974). It is found that
the basalt areas are characterized by irregular, high frequency
magnetic anomalies, but on the deeper parts of the shelf east
of the Faeroe Islands the frequency is significantly decreased
and evidence from seismic reflection profiles indicates that the
lavas give way for sediments (Bott & Watts 1970 and Korsa-
kov 1972).
NASP travel time for stations on the Faeroes.
The station F6 shows some peculiarities which will be dis-
cussed later, but otherwise the stations on the Faeroe Islands
give rather uniform results. Characteristic graphs of reduced
travel times are given as fig. 3. Some intercept times and least
square velocities can be found as table 3 in the appendix.
All shots in the range up to about 25 km give first arrivals
which are consistent with a direct wave of about 5 km/s
(Pálmason 1965 found 4.9 km/s) but at longer distances the
lines A and C (north) (= line C north of e. g. the station Fl)
seem to differ significantly from the lines B and C (south).
The shots of line A and line C (north) recorded by the
stations on the islands give an apparent velocity of appr. 6
km/s on the shelf but with significant scatter. Line C (north)
does not extend outside the shelf but line A gives an apparent
velocity of appr. 8 km/s in the range above 170 ... 200 km;
for shorter distances the apparent velocity is appr. 7 km/s
outside the shelf.
Stacked seismograms for line A are given as fig. 4. It is
seen that there are late, poorly defined arrivals of large ampli-
tude; these arrivals are probably S-arrivals which can be ex-
pected because the large contrast between the sea water and
the basalt in the sea bed will cause conversion from P-waves