Jökull - 01.01.2010, Side 101

Geothermal noise at Ölkelduháls, SW Iceland
composed of surface waves, but it is complex. These
overall conclusions agree well with findings at other
geothermal fields (e.g. Douze and Laster, 1979; Liaw
and McEvilly, 1979; Oppenheimer and Iyer, 1980).
The dominant frequency is similar and the wave con-
tent is primarily surface waves. However, more work
is required to characterize the wave field in more de-
tail. A detailed map of the distribution of this noise
over a geothermal area would be helpful. A sum-
mary of noise at existing past observational sites may
even be useful, although in light of the rate of de-
cay from the source evident in Figure 7 would ren-
der that aliased where station spacing exceeds a km
or two. Observations with a dense array (station spac-
ing tens of meters depending on velocity) would al-
low for a more detailed characterization of the wave
field in terms of orientation, that can constrain the
source, and velocity, which can distinguish wave type
together with polarization analyses. Understanding
the body-wave content of the noise, as small as it may
be in relative terms, is crucial for understanding the
usefulness of this geothermal tremor for prospecting.
We note that the geothermal tremor is high in ampli-
tude and detectable over a large dynamic range (5–6
orders of magnitude of power in this case). We also
note that it is highly stable in time and a single hour
of observation may suffice if noise conditions are oth-
erwise good (cultural, wind, seismicity). Finally, we
note that the amplitude decay rate in Figure 7 would
suggest a Q value of about 10 if we assume surface
wave propagation. This is very low, i.e. indicates very
high attenuation. For body waves the inferred attenu-
ation would be less, i.e. the Q value bigger.
ACKNOWLEDGEMENTS
This work was partially funded with a grant from the
Reykjavík Energy Research Fund. Instruments were
provided by the Loki instrument pool operated by the
Icelandic Meteorological Office. We thank Magn-
ús Pálsson and Rögnvaldur Magnússon for their help
with field work.
ÁGRIP
Jarðsuð var mælt á 19 mælistöðum eftir línu sem
liggur yfir jarðhitasvæðið við Ölkelduháls á Hengils-
svæðinu. Á tíðnibilinu 3–7 Hz mátti tengja jarðsuðið
jarðhitavirkninni út frá útslagsdeyfingu með fjarlægð
frá jarðhitasvæðinu og út frá tímasamhengi suðsins
eins og það kemur fram í víxlfylgni suðsins frá einni
mælistöð til annarrar. Þessi jarðhitahristingur er afar
stöðugur yfir tíma, bæði útslagið og tímasamhengið.
Breytanleiki kemur fram í suðrófinu frá einni stöð til
annarrar og frá einum þætti hreyfingarinnar til ann-
ars. Þessi breytanleiki getur stafað af flókinni sam-
setningu suðsins úr bylgjum frá dreifðum uppsprett-
um og tvístrunaráhrifum flókinnar berggerðar efsta
hluta jarðskorpunnar eða yfirborðs. Víxlfylgnigrein-
ing suðsins gefur til kynna að láréttur grúppuhraði
við ofangreindar tíðnir sé nálægt 1 km/s. Jarðhita-
suðið virðist að mestu samsett úr yfirborðsbylgjum ef
marka má hreyfigröf þess og greiningu þeirra. Deyf-
ing útslags jarðhitasuðsins með fjarlægð frá jarðhita-
svæðinu á Ölkelduhálsi bendir til að deyfingarstuðull
á svæðinu sé um Q∼10 við ofangreindar tíðnir ef gert
er ráð fyrir útbreiðsluhraða yfirborðsbylgna.
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