Fróðskaparrit - 01.01.1976, Page 16

24 Seismic Refraction Measurements around the Faeroe Islands
and observations at F6 of line A have been omitted from the
analysis so far. The trouble is that signals from A1 ... A7
are late at F6, later than expected from other stations and
arrivals from line C (north). A speculative explanation could
be that there is an area of lower velocities to the west of the
islands between F6 and line A.
The boundaries of the gravity high to the south of the Faeroe
Islands are close to LOMFS(A) and C36 according to the map
(fig. 2). Signals from this area are very weak or absent on the
islands but the ship station LOMFS(A) gives a well defined
apparent velocity of 6.60 km/s. The stations on the islands,
except F6, have either no arrivals in this region or Moho-
arrivals, and MIR(B) south of the gravity high gives only two
weak arrivals from the region. This means that an inter-
pretation must rely on LOMFS(A) from which only travel
times and distances are available. However, the assumption of
a basement with a velocity of 6.60 km/s below the gravity
high and with time terms of about 2/s s gives a satisfying fit.
The only remaining refractor seems to be Moho. The Moho-
arrivals from line A are reversed by use of observations from
Iceland and the Faeroe Islands, and a time term analysis using
DU5 (Iceland) and a station occupied by.Soviet ship »Lomono-
sov« in the middle of line A gave the true velocity 7.79 + 0.10
km/s and time terms close to 2.5 s for line A (Bott et al.
1976). A time term analysis of all Moho arrivals at the Faeroese
stations gave a true velocity of 7.61 + 0-22 km/s (Casten &
Nielsen 1975).
A number of time term analyses has been made using NASP-
data from Scotland (IGSl), Shetland (UKAEA, DUl) and
Iceland (DU5) together with Faeroese data, and the velocities
were found to vary between 7.8 and 8.3 km/s, with lower
values for line A and higher values for line B. Except for line
A the fit of the time term models is, however, rather poor,
probably because lateral inhomogeneities of the crust make the
time terms dependent on the directions; this explains possibly
the low value 7.61 km/s found previously for the Faeroe
Block itself.