Rit (Vísindafélag Íslendinga) - 01.06.1971, Side 93

CRUSTAL STRUCTURE OF ICELAND
93
8. REFLECTED WAVES FROM THE 3-4 BOUNDARY
It is always desirable to check the model used for interpreting the
refraction data by looking for the reflected waves to be expected.
These waves are characterized by an increase in amplitude at distan-
ces beyond the critical one corresponding to total reflection. In the
present case it is difficult to identify reflected waves from the 2-3 in-
terface and other shallower ones because these are too close in time to
refracted waves. A search was made, however, for reflected waves
from the 3-4 interface on selected profiles.
The two pairs of offshore profiles, L1 and L2 in Faxaflói and L4
and L5 off the south coast, were chosen for this purpose. For each pair
of profiles, all arrivals which had arrival times close to the ones ex-
pected on the basis of the refraction interpretation, were plotted on
the same diagram. It tumed out that such arrivals were fairly com-
mon and at the larger distances had relatively large trace amplitudes.
In order to get a clearer picture of the variation of amplitude with
distance, corrections were applied to the measured trace amplitudes
for the effects of geometrical spreading, charge size and amplifica-
tion as described earlier. No corrections were made for transmission
and reflection coefficients for the relatively shallow boundaries be-
tween layers 1, 2 and 3, or for surface effects, as these should be more
or less constant. What then remains is the effect of the reflection coef-
ficient at the 3—4 boundary and possibly an effect of a nonisotropic
radiation intensity pattern of the seismic energy at the shot point.
The arrival times of the assumed reflections are shown in Figs. 33
and 34. They agree fairly well with the expected ones, shown by the
dashed curves. The corrected amplitudes, in decibels, are shown in
Figs. 35 and 36. It is seen that the amplitude decreases at first, and
then increases again at distances somewhat beyond the critical one.
At larger distances a gradual decrease is observed. The amplitude
maxima occur some distance beyond the critical distance. This is in
agreement with theoretical calculations by Cervený (1967) for low
frequencies. It is not clear why the amplitude is relatively strong
at short distances. It is possible that these arrivals are not reflec-
tions, but late-arrival refracted waves. A possible explanation is
also that the seismic P-wave energy is radiated nonisotropically, the
highest radiation intensity being vertically downward (Múller,
1968). It is fairly clear, however, that there is a marked increase in