Data Services Newsletter

Volume 17 : No 2 : Winter 2015

The IRIS DMC Noise Toolkit Data Product

Spectral characterization of ambient seismic waves at a particular station is often used to image Earth’s interior and qualitatively understand the effect of environmental processes on the background noise. Frequency band, smoothing interval and the time window are among the main processing parameters that are often tuned for such analysis. The IRIS DMC is now offering 3 sets of highly configureable open-source Python bundles under its Noise Toolkit (NTK) data product to facilitates customizable ambient seismic noise processing:

  • PDF/PSD bundle — The scripts in this bundle compute Power Spectral Density (PSD) of station waveform data using customized parameters. With this bundle users can:
    • request waveforms and response data for given station/channels using the ObsPy FDSN client OR read their local waveform data files (in SAC, MSEED, CSS, etc. format) and only request response information
    • compute and populate an hourly file-based PSD database
    • extract PSDs and PDFs from the PSD database
  • Microseism energy bundle — The scripts in this bundle compute microseism energy (ME) from PSDs. These microseism energy plots could reveal coherent variability of microseism noise at the stations as well as those caused by major storms. These scripts are capable of:
    • calculating power from the existing PSDs over selected period bands (bins)
    • calculating median power for a given time window from the computed PSD powers
    • plotting temporal variation of median power
  • Frequency dependent polarization analysis bundle — The standalone scripts in this bundle perform polarization analysis of seismic noise recorded by three-component seismometers:
    • request waveforms and response data for given station/channels using the ObsPy FDSN client OR read user’s waveform data files (in SAC, MSEED, CSS, etc. format) and only request response information
    • compute and populate an hourly file-based polarization attributes database
    • extract hourly and daily polarization PDFs from the polarization database

The above bundles could be used in many applications such as: station quality control, background noise characterization, generating weather maps of seismic noises, creating near real-time microseism index database, and creating plots as well as monthly summaries for data mining. The following figure shows an example of combined application of the PDF/PSD and ME bundles to look at the microseism energy variations at the GSN station Davao, Philippines (IU.DAV) during the passage of super typhoon Haiyan in November of 2013.

2013 Haiyan
Map shows the super typhoon Haiyan’s path and its wind speeds (formed on 2013-11-03 and dissipated on 2013-11-11) in blue based on data from US Naval Research Laboratory, Marine Meteorology. Location of the seismic station IU.DAV is marked in red. The left inset plots are generated using the NTK PSD bundle to compute the PDF of PSDs for the BHZ, BH1 and BH2 channels of IU.DAV station during the passage of Haiyan. For the right inset plot, the NTK ME bundle is used to calculate the median microseismic power values of the PSDs (BHZ, BH1 and BH2 channels) at period band of 5s to 10s (secondary microseism band, SM). The resulting microseismic power values are smoothed using a moving 6 hour median window.

by Manoch Bahavar and Chad Trabant (IRIS Data Management Center)