Fróðskaparrit - 01.01.1976, Blaðsíða 14

22 Seismic Refraction Measurements around the Faeroe Islands
The determination of true velocities will here be based on
time term analyses (Bery & West 1966 a, 1966 b), which are
least square analyses, where it is assumed that the travel times
can be written
t(A,B) = a + a + d(A, B) / v
A B r
Here aA and aB are the socalled »time terms« of the obser-
vation points A and B, and d (A, B) is the distance between
the two points A and B. The refractor velocity is vr, and if
v (h) is the wave velocity as function of the depth h then the
time terms can be written
(v(h ) / v )2 )2 / v(h)
r
A time term analysis including all shots in the 6 km/s
area gave a velocity of 5.90+ 0.04 km/s and time terms
between 0.25 s and 0.54 s for the land stations (Bott et al.
1976), and the experiment of 1970 (Casten 1973) fits nicely
into this analysis. The boundaries of the area have not been
accurately determined but are probably not far from the shots
A7, B3 and C49. With the possible exception of A88, A89,
C58 and C59, which give small or even negative time terms,
the whole area thus covered seems underlain by the same re-
fractor. However, different ways of using the data for shots
on the northwestern shelf only give possibly significantly
higher velocities in the range 6.1 ... 6.2 km/s in agreement
with previous results (Casten 1974, Bott et al. 1974).
Shot A3 is close to the centre of the deep gravity minimum
northv/est of the islands (fig. 2) and the time term of A3 is
about 0.3 s larger than neighbouring time terms, also A4 seems
to have an enlarged time term, but these delays seem to be the
only effect connected with the gravity low.
The other apparent velocities of 7 and 8 km/s of line A
dh (1 -