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Project IDA (International Deployment of Accelerometers)Project IDA
Measuring Seismic Noise
IRIS Metadata WorkshopJanuary 12, 2012
Peter DavisProject IDA
Scripps Institution of OceanographyUniversity of California, San Diego
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Calculating background noise:
• Remove mean and trend of time series
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Calculating background noise:
• Remove mean and trend of time series
• Apply a taper to reduce spectral leakage
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Types of data tapers (windows):
• Hanning
• Hamming
• Bartlett
• …
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Calculating background noise:
• Remove mean and trend of time series
• Apply a taper to reduce spectral leakage
• Compute the Fourier transform
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Calculating background noise:
• Remove mean and trend of time series
• Apply a taper to reduce spectral leakage
• Compute the Fourier transform
• Remove the instrument response
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Observations from 2002
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Ocean waves ->
Rayleigh waves
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• Dominant period – 20 seconds
• Conversion at coast to Rayleigh-type motion
Ocean waves -> Rayleigh waves
(Hasselmann, 1963)
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Ocean waves ->
P-waves
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Opposing wave motion (Longuet-Higgins, 1952):
• Interaction of standing waves with ocean bottom
• reflection from a coast
• wake of a moving storm
• center of a storm
(Haubrich and McCamy, 1969)
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Temperature variations
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Temperature effects:
• Beyond 250 seconds period, temperature becomes very important
• Temperature dependence varies with the model of seismometer
• Temperature changes of 1e-6 oC are important
Foam thermal insulation at GSN station in the UAE
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Effect of changes in barometric pressure:
• buoyancy of the seismometer mass
• Newtonian attraction of air mass
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STS1 placed under vacuum at GSN station SUR
Vacuum lost!
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(Zuern and Widmer, 1995)
Barometer data used to remove effect of air pressure changes in seismic data:
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Effect of changes in barometric pressure:
• buoyancy of the seismometer mass
• Newtonian attraction of air mass
• loading effect of air mass
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Change of air pressure in a cavity
Symmetric Case
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Change of air pressure in a cavity
Asymmetric Case
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(Sorrels, 1971)
KS-54000 borehole seismometer installed at WRAB GSN station
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Continuous free oscillations
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(Nishida and Kobayashi, 1999)
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Human-induced noise
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Examples of human-induced noise:
• Car and truck traffic
• Repeating machinery (pumps, generators)
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Frequency bands most effected by installation:
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http://www.iris.edu/servlet/quackquery/
IRIS DMC provides tools to analyze data
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Thank you!