Jökull - 01.12.1963, Síða 45

Fig. 1.
at each station at intervals of 20 meters, ancl on
some of the later profiles one three-component
geophone (Hall-Sears HS-l-LP 3 D, 4.5 c/s) was
also used. The distances between geophone sta-
tions along a profile varied from less than 1 km
near the shot point to more than 4 km towards
the end of the profile. The shots were usually
fired in lakes or rivers or in the sea at 1 to 4
meters depth. The shot instant was transmitted
by radio to the recording station. The shot-de-
tector distances were determined from aerial
photographs in combination with U.S. Army
maps of Iceland in scale 1:50,000. The errors in
the distances are believed to be less than 50—
100 meters in most cases.
Although the use of unreversed profiles, as
here is mainly the case, may be subject to con-
siderable errors due to horizontal structural
changes, it was felt that in a reconnaissance
work of the present kind this was justified.
More accurate measurements would require not
only reversed profiles, but also in many cases
JÖKULL 1963
a detailed mapping of the low-velocity surface
layer along the profile line.
For detailecl information on the geology of
the relevant areas the reader is referred to As-
kelsson et al. (1960) and Thorarinsson et al.
(1959), where further references are found. For
eastern and southeastern Iceland in particular
the reader is referred to Cargill, Hawkes and
Ledeboer (1928), Jónsson (1954) and Walker
(1959, 1960, 1963).'
TREATMENT OF OBSERVATIONAL
DATA
The travel times of all clear phases, both I’
,and S waves, were read from the seismograms
and reduced to the level of the shot point. Least
squares lines were computed for all phases that
were identifiecl. The results are presented in
Table 4 in Appendix. The notation Pt, S; refers
to direct or refracted waves in layer i. The table
41