Fróðskaparrit - 01.01.1976, Side 15

Seismic Refraction Measurements around the Faeroe Islands 23
were treated by Bott et al. (1976) who found true velocities
of 6.73 + 0-12 and 7.79 + 0.10 km/s and time terms around
SU s and 2.5 s for these refractors below the ridge. The two
refractors were reversed by using DU5 in Iceland and a
station established by the Soviet ship »Lomonosov« in the
middle of line A. The two refractors were identified as corre-
sponding to layer 3 on Iceland (Pálmason & Sæmundsson 1974)
and Moho, respectively.
On line C (south) the apparent velocity of 5 km/s is
poorly reversed by the ship station LOMFS(A) and a time
term analysis of the corresponding data gave the velocity
4.91 + 0.27 km/s. This result is not inconsistent with a
direct wave in basalt lavas to the south of C49 and a wave
in the 6 km/s basement to the north of C49. The velocity
of 4.91 km/s is, however, based on very few observations
and therefore uncertain, but the fit is rather good for the
shots C45 ... C49, when the boundary between 5.90 km/s
basement to the north and the 4.91 km/s area to the south
is assumed to be about 5 km north of C49 (except that the
shots closest to F6 show a somewhat lower velocity).
The similar apparent velocity (5 km/s) of line B cannot
be reversed. A number of ship stations were, certainly, occu-
pied in the Faeroe—Shetland channel, but no true reversal
of the shots in question (B1 . . . B14) was obtained, and the
results point at a complicated structure in the channel. Fíow-
ever, a velocity of 4.9 km/s is unlikely because negative time
terms seem unavoidable for the shots. A more realistic velocity
is probably 5.3 km/s. In the same manner as for line C
(south) the stations on land get negative time terms which are
again explained by assuming a wave in the 6 km/s basement
below the islands, but the fit is rather poor, probably because
the azimuths of the shots are far from constant, so that the
distance travelled in the basement varies from shot to shot.
This varying azimuth also explains the low apparent velocity
found by station F5 (table 3).
It was mentioned earlier that F6 shows some irregularities,