Jökull - 01.01.2010, Síða 89

Reviewed research article
Geothermal noise at Ölkelduháls, SW Iceland
Ólafur Guðmundsson1,2 and Bryndís Brandsdóttir3
1Department of Science and Engineering, Reykjavík University, Menntavegi 1, 101 Reykjavík, Iceland
2Department of Earth Sciences, Uppsala University, Villavägen 16, SE-752 36 Uppsala, Sweden
3Institute of Earth Sciences, Science Institute, University of Iceland, Sturlugata 7, 101 Reykjavík, Iceland
ogud@ru.is, bryndis@raunvis.hi.is
Abstract — Seismic noise was recorded at 19 sites along a roughly linear array crossing the geothermal area
at Ölkelduháls, SW Iceland. Noise in the frequency range between 3 and 7 Hz was identified as associated
with the geothermal activity based on amplitude decay with distance and timing of energy in intra-station
cross-correlation. This geothermal tremor is very stable in time, both in its amplitude and correlation relations.
Differences of amplitude-spectral shape from one station to another and between the different components of
motion at each station indicate a complex structure of the wave field, possibly due to a distributed source and
scattering effects. The cross-correlation constrains horizontal group velocity weakly at about 1 km/s. Based on
particle-motion diagrams and three-component polarization analyses the noise appears to be primarily com-
posed of surface waves. The spatial decay of the noise away from the inferred source implies an elastic quality
factor of Q∼10 assuming the geometrical spreading of a surface wave in a laterally homogeneous Earth.
INTRODUCTION
Geothermal activity generates seismic noise. This
has been clearly demonstrated by seismic observa-
tions now at Hengill and Hellisheiði (Figure 1), but
also earlier at Svartsengi (Brandsdóttir et al., 1994)
and at a number of geothermal areas in North Amer-
ica (e.g. Douze and Laster, 1979; Liaw and McEvilly,
1979; Oppenheimer and Iyer, 1980). This geothermal
noise, or tremor, is generally found in the frequency
range between 1 and 10 Hz, but in many cases con-
fined to a narrower frequency range close to 5 Hz.
This suggests a common mechanism and a common
spatial scale for the phenomenon, although the mech-
anism is not known. A possible mechanism is due to
bubble growth or bubble collapse in saturated or sub-
cooled boiling in the geothermal fluid (Leet, 1988).
Kedar et al. (1998) base their tremor model for Old
Faithful on similar concepts and verify their model
with detailed observations around the geyser. How-
ever, the behavior of an erupting geyser may differ
from a steam field such as the one at Ölkelduháls. The
geothermal tremor is episodic at Old Faithful, while it
is steady with time at Ölkelduháls.
Some effort was directed at study of the possible
usefulness of geothermal tremor for geothermal ex-
ploration in North America in the 1960s and 1970s.
A series of dense-array studies at various geothermal
fields resulted in estimates of predominant velocity of
the recorded tremor found to be quite low, in some
cases as low as 500 m/s with a wavelength as short
as 20 m (Liaw and McEvilly, 1979). This suggests
that the wave content of the noise is primarily surface
waves from a shallow source. Therefore, the geother-
mal noise does not reveal much about the underlying
geothermal reservoir.
JÖKULL No. 60 89