Jökull - 01.01.2010, Page 93

Geothermal noise at Ölkelduháls, SW Iceland
Two main features are seen in Figures 2 and 3.
First, a microseismic peak is clearly visible at the
lowest frequencies (it is clipped, for plotting reasons,
where it exceeds the rest of the spectrum in ampli-
tude). This microseismic peak is small relative to the
remainder of the spectrum at close distance to Öl-
kelduháls (stations LA0 and LB0) and increases in
relative amplitude away from Ölkelduháls. Second,
all the stations exhibit a spectrum that is remarkably
stable with time throughout the 40 hr period, unless
am
pl
itu
de
sp
ec
tr
al
d
en
sit
y
st
ac
k
LB0 ampl = 3.689
LB5 ampl = 0.075 LB7 ampl = 0.342
0 2 4 6 8 10
tim
e [40 hrs increasing dow
nw
ard]
frequency [Hz]
LB9 ampl = 0.193
tim
e [40 hrs increasing dow
nw
ard]
LB2 ampl = 0.768
am
pl
itu
de
sp
ec
tr
al
d
en
sit
y
st
ac
k
LB6 ampl = 0.081 LB8 ampl = 0.054
LB4 ampl = 0.271LB3 ampl = 0.305LB1 ampl = 6.290
0 2 4 6 8 10
0 2 4 6 8 10 0 2 4 6 8 10 0 2 4 6 8 100 2 4 6 8 10
0 2 4 6 8 100 2 4 6 8 100 2 4 6 8 10 0 2 4 6 8 10 0 2 4 6 8 10
0 2 4 6 8 10
frequency [Hz]
0 2 4 6 8 10
frequency [Hz]
0 2 4 6 8 10
0 2 4 6 8 10 0 2 4 6 8 10
0 2 4 6 8 10
0 2 4 6 8 100 2 4 6 8 100 2 4 6 8 10
frequency [Hz]frequency [Hz]
Figure 3. Combined amplitude spectra from seismographs along line B. Each trace is computed from the com-
bined power spectrum of all three components at each site. The spectra are normalized by their maximum in
the frequency range between 3 and 7 Hz. The spectra are labeled according to distance from Ölkelduháls as in
Figure 1. Each spectrum is based on 1 hour of data and are ordered in a temporal sequence from top to bottom.
The observations spanned about 40 hours. Some spectra that are distorted by local events or wind noise have
been dropped. The spectrum at the bottom of each frame is the stack of the hourly spectra. Note the wind
disturbance on stations LB4, LB6 and LB8 in the middle of the observation period. Some sharp spectral peaks
are present, in particular at stations LB3 and LB5 (see also Figure 2) in the beginning and towards the end of
the observation period. – Samantekt útslagsrófa meðfram línu B. Hvert línurit er reiknað út frá öllum þremur
þáttum (Z,N,A) mælinga á hverjum mælistað. Öll rófin í hverjum myndaramma hafa sömu útslagskvörðun sem
gefin er upp efst við hvern ramma. Rófin eru merkt hverri mælistöð eins og á 1. mynd. Hvert línurit er byggt á
einni klst. af mæligögnum og þeim er raðað í tímaröð að ofan og niður. Mælingar stóðu í um 40 klst. Þar sem
vindsuð er mjög mikið þannig að lítið er unnt að lesa út úr rófinu hefur því verið sleppt. Rófið neðst í hverjum
myndaramma er meðaltal rófanna þar fyrir ofan. Takið eftir vindsuði á stöðvum LB4, LB6 og LA8 og skörpum
róftoppum t.d. á stöðvum LB3 og LB5 í byrjun og undir lok mælitímabilsins.
JÖKULL No. 60 93