Rit (Vísindafélag Íslendinga) - 01.06.1971, Síða 90

90
GUÐMUNDUR PÁLMASON
Fig. 31. Amplitude and delay time of the Pz-wave on profile 60.
Hraunsmúli-Kolbeinsstadir (58). The P3-amplitude remains
more or less constant to a distance of 26 km, when it increases 10—15
dB. Up to about 25 km the P3-wave delay time is increasing, but at
larger distances it levels off. It thus appears that the amphtude is
lower on the down-dip part of the profile than farther away, where
the 2—3 interface is horizontal.
Ólafsvík-Kolbeinsstadir (59). The P3-amplitude appears to be
steadily increasing along this profile, on the average 0.3 dB/km. No
clear variation due to structure is evident. The P4-wave is observed
as a second arrival at 45-60 km and appears to he 0-5 dB stronger
than the P3-wave.
Stykkishólmur-Ólafsvík (60). The reduced amphtude of the P3-
wave is shown in Fig. 31 together with the delay time. At a distance
of about 17 to 25 km the delay time decreases, presumahly because
of an upward structure in the 2-3 interface. In the same way as out-
lined by Dix (1952, p. 267) and shown experimentally by Laster et
al. (1967), one would expect a decrease in amplitude as a result of
diffraction of the head wave before it is refracted into the upper layer.
Such a decrease of about 15 dB is in fact observed on this profile.
Beyond the structure the amplitude reaches a similar value as hefore.
This may be taken as supporting evidence for the refraction inter-