Jökull - 01.01.2010, Side 90

Ólafur Guðmundsson and Bryndís Brandsdóttir
Geothermal noise has received little attention in
Iceland, where rock types, porosity and other envi-
ronmental factors may differ from other areas, e.g.
in North America. Brandsdóttir et al. (1994) found
a noise source at about 4–5 Hz at the Svartsengi
geothermal field in SW Iceland, which they, based on
amplitude, associated with the surface geothermal ac-
tivity. We have designed and conducted a small survey
at the Ölkelduháls geothermal field to look at this fur-
ther. The experiment consisted of a profile across an
elongate region of geothermal activity near Ölkeldu-
háls which is a part of the Hengill-Hellisheiði geother-
mal fields in SW Iceland. Funding did not allow for
the deployment of a dense 2D array to look at the
noise in detail.
AREA DESCRIPTION
The Ölkelduháls geothermal field lies about 40 km
east from Reykjavík within the Hengill triple-junction
region (Figure 1). The area hosts three volcanic cen-
ters, Grændalur, Hrómundartindur (Ölkelduháls) and
Hengill, in a SE to NW succession from the flank on
the neovolcanic zone to its centre. An elongate region
of geothermal activity extends from Hengill through
Ölkelduháls to Grændalur in the SE (Figure 1). Rocks
in the area are primarily basaltic although some felsic
rocks are found on the SW flank of Hengill. Surface
rocks in the immediate area of field work are primarily
hyaloclastites and pillow basalts and inter-glacial lava.
Hyaloclastite ridges and normal faults have a predom-
inant strike of N30◦E. Topography is significant. The
highest peaks in the area are about 800 m.a.s.l. while
the elevation of the seismographs ranges from about
1 km
Hr
óm
un
da
rt
in
du
r
K!rgil
Klambragil
Ka
tta
tja
rn
ir
LA0 LA1
LA2 LA3
LA4
LA5
LA6 LA7
LA9
LA8
LB0LB1
LB2
LB3
LB4
LB5
LB6
LB7
LB8
LB9 Grændalur
Ölkelduháls
Reykjavík
Hverager!i
10 km
He: Hengill
Hr: Hrómundartindur
Gr: Grændalur
He
Hr
Gr
Figure 1: The configuration of seismometers in the noise experiment at Ölkelduháls is shown by yellow triangles. Each
seismograph is labeled with a three digit code including A or B to distinguish between two deployments. The seismograph
at LA0 = LB0 is common to both deployments. Note that the station separation is about 300 m along line A while it is
half that, 150 m, along line B. Red circles denote locations of surface-geothermal manifestations (adapted from Árnason
et al. 2010). Access roads and tracks are shown in red. The left frame shows the position of the field area relative to the
city of Reykjavik, the town of Hveragerði and the three volcanic centres in the area. – Staðsetningar jarðskjálftamæla við
Ölkelduháls eru sýndar með gulum þríhyrningum. Hver mælir er merktur með þriggja stafa nafni sem byrjar á A eða B
til þess að greina að tvö aðskilin mælitímabil eftir línum A og B. Mælir merktur LA0 = LB0 er einn og sami mælirinn sem
var sameiginlegur báðum mælitímabilum. Fjarlægð á milli mæla var um 300 m eftir línu A en helmingi minni (150 m) eftir
línu B. Rauðir hringir merkja ummerki um jarðhita við yfirborð (sjá grein Knúts Árnasonar o.fl. 2010). Vegir og slóðar eru
merktir með rauðu. Hægri myndin sýnir afstöðu mælingasvæðisins til Reykjavíkur, Hveragerðis og eldfjallakerfa á svæðinu.
90 JÖKULL No. 60