Rit (Vísindafélag Íslendinga) - 01.06.1971, Qupperneq 16

16
GUÐMUNDUR PÁLMASON
In order to convert delay times to depth it is necessary to know
the overlying structure sufficiently well. In particular all the Vi and
all thicknesses except hn_i must he known. These are obtained either
from the first part of the profile or from neighbouring profiles.
Strictly speaking the delay time depends on the velocity distribu-
tion between the recording station and the point where the head wave
leaves the refractor. The horizontal distance between these two points
is known as the offset distance.
The total delay times of the P3-wave have been computed for all
profiles, assuming a P3-wave velocity of 6.5 km/sec. These delay
times are given in Figs. 7, 10, 12, 13, 19, 20, 25, 26 and 27. The argu-
ments for choosing this value for the P3-wave velocity are given and
discussed in sect. 5.5.
The total delay times of the P4-wave have been ohtained in an
analogous way, assuming the P4-wave velocity to be 7.2 km/sec. As
this value is found from unreversed profiles only, it is more uncertain
than the value assmned for the P3-wave velocity (see sect. 5.5). The
total delay times are given in Fig. 28.
If reversed overlapping profiles are availahle, it is possible to com-
pute delay times without assuming the value of the refractor velocity,
as long as it is constant. For a station D located on the line between
the shot points A and B, the delay time is:
Td = Vz (Tad + Tbd - Tab) (4.1.4)
where the T terms are the travel times of the head wave between the
respective points. If the station D is not on the straight line between
A and B, it is necessary to apply a correction, which is equivalent to
taking instead of TAB the time corresponding to the distance XAD
+ XBD, which is larger than XAB.
The data from the profiles of Báth (1960) and Tryggvason and
Báth (1961) have been incorporated into the present work and delay
times computed by the methods outlined here.
4.2. Relation of amplitude to charge size.
The variation of amplitude A with charge size Q follows a rela-
tion of the form . _
A oc Qn (4.2.1)
For underwater explosions the exponent n is usually about %
(O’Brien, 1960, 1967b). For explosions in solid rock it is about 1.