Rit (Vísindafélag Íslendinga) - 01.06.1971, Page 11

CRUSTAL STRUCTURE OF XCELAND
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ments has been used for the field work reported here. For each seismo-
meter station a separate shot has therefore been necessary.
In the first year of the work some experiments were made with a
reflection equipment that was purchased together with the long-
distance refraction equipment. Records at shorter distances were
made with this equipment while the refraction equipment was used
at larger distances. No clear reflections were found and as the re-
fraction technique appeared more promising, the reflection technique
was abandoned for the time being.
When a survey of the “lava layer” had been made over most of
the country, the measurements were extended to the offshore shelf
and some of the land profiles were increased in length to obtain in-
formation on the deeper structure of the crust. The offshore measure-
ments in 1966 and 1967 were made possible through the cooperation
of the Icelandic Coast Guard, from whose vessels “Alhert”, “Thór”
and “Ódinn” explosive charges were fired along eight offshore lines
with a maximum length of 140 km. For each offshore profile the re-
cording station was kept in a fixed position on the shore. The land
profiles were extended to a distance of some 100 km. Three of these
profiles were in southem Iceland and one in northern Iceland.
The approximate location of the short refraction profiles is shown
in Fig. 1, and of the long profiles, together with Báth’s profiles WP
and CP, in Fig. 2. The shot point (recording station of offshore pro-
files) is shown as a dot. The detailed location of shot points and indi-
vidual seismometer stations for the land profiles and one of the off-
shore profiles (L9) is shown in Appendix A. Some data on the profiles
are given in Tables 5.1 and 5.2 and by Pálmason (1963).
Shot-detector distances were determined from maps in scale
1:50,000 for profiles on land. The error in distance on these profiles
is probably less than 50 meters in most cases. On the offshore profiles
two methods of determination of distance were used. For the profiles
off the west coast and for profile L9 the coordinates of the shot points
were determined by measuring angles between directions to known
points on the shore with a sextant, and supplemented with radar
distance measurements to coastal cliffs. The accuracy of this method
was tested on the profile Kjalames—Faxaflói (48), where a direct
water wave was detected by a hydrophone close to the recording
station for distances up to about 30 km. The distances computed from
the sextant measurements agreed to within 200 meters with those
obtained from the water wave. The accuracy of location of the off-