Jökull - 01.01.2020, Side 63

Einarsson and Jakobsson
The Surtsey eruptions also coincided with the ad-
vent of the Plate Tectonics Theory and attracted the
interest of the international geoscience community to
Iceland. A group from the Lamont-Doherty Earth Ob-
servatory came to Iceland in 1967 and brought several
portable seismographs with them. The idea was to
trace the plate boundary through Iceland by locating
microearthquakes that were thought to occur more or
less continuously along the boundary. Instead it was
discovered that the microseismicity was mostly con-
fined to geothermal areas (Ward et al., 1969; Ward
and Björnsson, 1971).
The results of the microearthquake studies led to a
major project to monitor the seismicity of the Reykja-
nes Peninsula, that was showing very high activity at
this time (Björnsson et al., 2020). The instrumenta-
tion from Surtsey was modified and augmented to es-
tablish six semi-permanent stations on the peninsula.
In addition, a large field project was launched in the
summers of 1971 and 1972 (Klein et al., 1973, 1977).
Three of the portable instruments, that the
Lamont-group brought, were set up in South Iceland
and "forgotten" over the winters of 1971 and 1972.
This was done to test how well these prototype instru-
ments would perform under difficult circumstances
and in the care of non-technical people. The best lo-
cations were at Skammadalshóll near Katla volcano
and Laugarvatn, in the secondary school there. These
instruments recorded, among other things, the be-
ginning of the eruption in Heimaey in January 1973
(Björnsson and Einarsson 1974).
Landsnet
The positive experience of operating sensitive seismo-
graphs at farms and installations throughout the coun-
try prompted the decision to design and build similar
instruments and establish a seismic network. Small
grants were obtained from the NATO science program
for a three-year project for this purpose. The design
was based on the prototype instruments from Lamont,
but the electronics parts were designed by Marteinn
Sverrisson at the Science Institute, University of Ice-
land. The hardware was made by Karl Benjamínsson
in the machine shop of the institute. The work was
directed by Sveinbjörn Björnsson and Páll Einarsson
(Einarsson and Björnsson, 1987).
The instrument had to be inexpensive, yet robust
and easily serviceable by the local people. It had to be
sensitive in the frequency band 3–30 Hz for the detec-
tion of local earthquakes, yet insensitive in the band of
microseisms (0.1–0.2 Hz) from the swell of the North
Atlantic Ocean, that seriously limits the sensitivity of
island seismographs. The sensor was a geophone of
3 Hz natural frequency. The amplifier had a HP filter
selection (at 0.1 or 1 Hz) that could be adjusted ac-
cording to the level of microseisms activity, also LP
filter adjustable between 15 and 30 Hz. The electron-
ics was arranged on five boards that could be individu-
ally replaced in case of failure, i.e., power board, pre-
amplifier, amplifier, power amplifier, and time signal
receiver (Figure 2). Radio time was put directly on
the seismographic trace, thus avoiding using a clock
and cumbersome clock corrections. Recording was
by pen and ink on paper attached to a rotating drum
(Figure 3). Usually, the paper sheet was changed once
per day. This allowed a time resolution of 90 mm per
minute, one line per 10 minutes on a sheet that is 90
cm long. During days of very high activity the interval
between lines was increased to avoid tangling of the
lines. Then the sheet had to be changed more often.
The seismograph could be modified for a teleme-
tered version. The power amplifier card was then
simply exchanged for a tone modulator and the tone
fed into a hand-held radio link. The tone on the re-
ceiving end was then de-modulated and amplified
in a similar box and fed into the pen-motor of the
recorder. The telemetering system required a line of
sight and often a repeater station to carry the signal
over large distances. This was used for seismic sta-
tions on the Reykjanes Peninsula and in the Hengill
area near Reykjavík. During the Krafla episode in
1975–1984 several stations were also telemetered to a
central recording station in Reynihlíð. A network of
telemetered stations was installed in Central Iceland
in 1985, funded by Landsvirkjun (the National Power
Company), for the monitoring of the active volca-
noes around their main power stations. The signals
were telemetered over repeater stations on Búrfell
and Skarðsmýrarfjall to Reykjavík, where they were
recorded on paper at the Science Institute (Figure 4).
60 JÖKULL No. 70, 2020