Fróðskaparrit - 01.01.1976, Side 3

Seismic Refraction Measurements around the Faeroe Islands 11
institutions. All these stations were equipped with MARS-in-
strumentation (Berckhemer 1970); except for F2 and F3 they
all had 3 vertical geophones, about a hundred metres apart;
the approximate characteristics of the geophones were: reson-
ance frequency 2Hz, and a low pass filter 20 Hz. The stations
F2 and F3 had a horizontal geophone besides vertical geo-
phones; for more details see Casten (1974).
The station DU4 was a 3-component station run by Durham
University (description of instrumentation: Long 1974). The
stations outside the Faeroe Islands were of different types and
run by different organizations but these travel times and di-
stances, when used here, are taken from the thesis of Smith
(1974). The arrivals at the Danish stations on the Faeroe Is-
lands were picked or repicked, and travel times for first arri-
vals observed on the Faeroe Islands can be found as table 2 in
the appendix. The positions are given in table 1 in the appen-
dix, and the positions of the shots were usually found by the
use of Decca and radar fixes, and the error of a position is
then rather small. Especially on line A Decca could not be
used, instead Loran was used giving a larger error of the posi-
tions, but the error will probably not exceed 1. . 2 km. The
positions of the geophones are all well defined but the connec-
tion between the geophone positions and the channels on the
registrations is not always clear for the stations F1 . . . F6;
further, the geophone sites are very inhomogeneous so that
changes of distance with a hundred metres gave no systematic
change of travel time (except F6 which was placed on the
rather homogeneous basalt of the lower series). The positions
of the stations F1 . . . F6 are therefore »mean values« giving
an error of about a hundred metres.
Geophysical structure of the Faeroe Islands.
The surface rocks of the Faeroe Islands consist almost exclus-
ively of Tertiary basalt lavas, including some basalt intrusions.
The basement of these lavas is found nowhere on the islands