Rit (Vísindafélag Íslendinga) - 01.06.1971, Qupperneq 85

CRUSTAL STRUCTURE OF ICELAND
85
hóp-Eyjafjördur (LIO). In fact, no first arrival with an apparent
velocity corresponding to layer 4 is found for distances up to about
60 km. Beyond that the records are of rather poor quality. At a dis-
tance of 87.4 km the first arrival is probably at about 14.5 sec which
indicates that the P3-wave may still be the first arrival. Although
this evidence is not conclusive, it rather supports the indication that
the depth to layer 4 is considerahly greater in northern Iceland than
in southwestern Iceland.
The above reintérpretation of Báth’s data has brought it into good
agreement with the present work. It has furthermore indicated a
major change in depth to layer 4 on going from W-Iceland to N-
Iceland.
7. AMPLITUDES OF THE REFRACTED WAVES
The amplitude variation with distance of the refracted waves may
provide significant information which may strengthen or weaken
the choice of models f or the refraction interpretation. The trace ampli-
tudes of the refracted waves of P or S type have therefore been mea-
sured for some of the profiles in southem and westem Iceland, includ-
ing the offshore profiles. These amplitudes were corrected for the
effects of charge size, amplification and geometrical spreading, as
has been described earlier. The relative amplitudes of waves from
different layers can he used to distinguish them from one another, if
the head wave coefficients are sufficiently different.
It has been found in some cases that the amplitude-distance graph
shows regular variations which can be related in a qualitative way
to structural variations in the upper boundary of the refracting layer,
as deduced from the delay time variation.
In some cases the reduced amplitude shows a systematic decrease
with distance which indicates that attenuation is taking place. It is
necessary to observe the same wave over an interval of at least 20-30
km of the profile in order to be able to make a meaningful statement
about attenuation, because other factors may strongly affect the
amplitude-distance curve over a short interval.
In the following a brief description will be given of the amplitude
behaviour on those profiles where such studies have been made. These
are mainly profiles which are long enough to give the amplitude of
a particular wave over a considerable distance, and profiles which