Rit (Vísindafélag Íslendinga) - 01.06.1971, Page 19

CRUSTAL STRUCTURE OF ICELAND
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100 per cent. Actual amplification was commonly 20-100 per cent.
If the relationship (4.2.1) is valid with n = % and if furthermore
the geometrical spreading factor is r~2 (cf. sect. 4.3) it will he seen
that the normalized charge size required for a constant amplitude
would be proportional to r3 (r is distance between shot and seismo-
meter stations). The solid line in Fig. 4 shows that this is approxi-
mately the case and it can therefore be used for predicting the re-
quired charge size at various distances. Fig. 4 shows furthermore that
much smaller charges were required in the offshore work, where the
explosions were usually made at a depth of about 20 meters, than on
the land profiles, where the explosions were usually made at a depth
of 1-4 meters in lakes and rivers.
4.3. Amplitude data.
The amplitudes of the various phases on a seismogram can aid
substantially in correctly identifying them. The amplitudes depend
on a number of factors such as charge size, geometrical spreading,
attenuation, ground-seismometer coupling and recording equipment.
Some of these factors are of a rather irregular nature and difficult to
predict, such as the ground-seismometer couphng. They have not
been considered separately and appear as a noise in the amplitude-
distance diagram.
The amplitudes have been read from the seismograms in such a
way that the sum of the two largest excursions of each arrival has
heen used. In the case of first arrivals this usually means taking the
first trough and the second peak and adding them together. The
first peak was usually much smaller in comparison with the noise
level, and therefore less reliable. In most cases average values of two
or more channel traces were used.
The effect of charge size on amplitude was discussed in the pre-
vious section. The geometrical spreading factor will be discussed
next. In a two-layered model the particle displacement in the head
wave varies in the following way (Heelan, 1953; Zvolinsky, 1958;
Brekhovskikh, 1960):
A = H • F(t) • (r • L3)-% (4.3.1)
where H = head wave coefficient
F(t) = displacement potential of the incident pulse
r = distance from shot to receiver
L = distance traveled by wave in high velocity layer.