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Power Spectral Density Monitor Explanation
A near real-time quality control procedure is applied on the continuous
waveform data of the VEBSN stations, to monitor changes in the behaviour
of the seismic stations caused by malfunctioning of the seismic
instrumentation, changes in response information or (sudden) changes in local site conditions.
The procedure uses a Power Spectral Density (PSD) estimation of the ground acceleration,
taken from data recordings of 30 minutes. Only BHZ and HHZ channels are currently used.
Basically the following steps are involved:
- Extraction of raw data from the pool of continuous VEBSN data into segments of 30 minutes.
Segments less than 28 minutes are not processed and will show up as gaps in the plots below.
- For each segment the PSD is estimated using periodogram averaging
(Welch, 1967). The averaging is done over 50% overlapping (and tapered) windows of length of 819.2 sec, resulting
in a reliable PSD estimation between about 0.007 Hz and the Nyquist frequency.
Only positive frequencies are taken into account (one-sided PSD) to compare
the PSD with the USGS's High Noise Model (NHNM) and Low Noise Model (NLNM)
(Peterson, 1996). PSD values are sligthly smoothed by taking the average of the PSD values in a
constant relative bandwidth of 1/10 decade. (So for low frequencies the averaging is over only
a few points, for high frequencies the averaging is over much more points).
- The PSD is deconvolved with the instrument response to convert the PSD from digital counts
[counts^2/Hz] into ground acceleration [m^2/s^4/Hz]. The instrument response is extracted from
the appropriate dataless SEED volume.
All poles and zeros in SEED blockettes 53 and the (overall) sensitivity in
SEED blockette 58 are used in the deconvolution.
Digital filters (FIR, IIR) are not considered (yet).
- For selected frequencies [0.01 Hz, 0.05 Hz, 0.5 Hz and 2.0 Hz] the PSD is stored as
function of time. These PSD levels are displayed below for all VEBSN stations in 2004.
- The PSD is integrated in frequency bands [20 - 10 sec] and [10 - 1 sec] to get the energy
for these frequency bands. The energy in those two bands is also stored as function of time and
displayed as thumbnails to get a quick overview of the overall VEBSN. (Red: 0.05 - 0.1 Hz; Blue: 0.1 - 1.0 Hz).
- The lower envelope of the PSD is being tracked and updated after the processing of a segment of
30 minutes of data. This envelope shows the lowest, measured noise at each frequency. This
envelope could be misinterpreted when there is no sensor connected to the digitizer, because
then only the digitizer noise is measured. To eliminate this data we have put the following criteria
on the PSD data: PSD values from a segment may only be used to update the minimum PSD level if
the PSD @ 0.2 Hz is above -150 dB.
All above results may be biased when the shape of the system response or the normalization
factors are defined incorrectly. In order to detect inconsistencies in the response information,
we display the lowest, measured noise
(a) after applying full response correction (including sensitivity, poles and zeros, and normalization constants
as given) [in green],
This should give the same result in the frequency band for which the system has a flat velocity response.
(b) after applying full response correction (including sensitivity, poles and zeros, and recalculated normalization constants) [in blue],
(c) after sensitivity correction only (without taking into account the poles and zeros) [in red].