A near real-time quality control procedure is applied on the continuous waveform data of the seismic 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 effects. The procedure, adapted for the case of the SISMIC servers, uses a Power Spectral Density (PSD) estimation of the ground acceleration, taken from data recordings of 60 minutes. Basically the following steps are involved:

1. Extraction of raw data from the pool of continuous into segments of 60 minutes. Segments less than 28 minutes are not processed and will show up as gaps in the plots below.
2. For each segment the PSD is estimated using periodogram averaging (Welsh, 1978). Only positive frequencies are taken into account (one-sided PSD) to compare the PSD with the USGS's High Noise Model (LNM) and Low Noise Model (LNM) (Peterson, 1996). PSD values are smoothed slightly by taking the average of 5 subsequent PSD values.
3. 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 standard SAC2000 PAZ files. All poles and zeros in SEED blockettes 53 and the (overall) sensitivity in SEED blockette 58 are used in the deconvolution.
4. For selected frequencies the PSD is stored as function of time. These PSD levels are displayed for frequencies  0.01 Hz, 0.05 Hz, 0.5 Hz and 2.0 Hz.
5. The PSD is integrated in several frequency bands to get the energy for these frequency bands. The energy in those two bands is also stored as function of time and the bands [0.01 - 0.2 Hz], [0.1 - 1.0 Hz] and [1.0 - 4.0 Hz] are displayed.
6. The bottom envelope of PSD is being tracked and updated after the processing of a segment of 60 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 @ 0.14 Hz is above -155 dB. for each frequency PSD is integrated in several frequency bands to get the energy for these frequency bands. The energy in the frequency bands [0.01 - 0.2 Hz], [0.1 - 1.0 Hz] and [1.0 - 4.0 Hz] are displayed.