Information on long-term noise levels at seismic stations is important for characterizing the current and past performance of seismic stations and detecting potential operational and/or instrumental station problems. Starting in 2004, data received by the IRIS DMC's near-real time data collection systems (BUD and USArray) have been analyzed by its automated Quality Analysis Control Kit (QUACK) system. For its noise analysis, QUACK utilizes a version of the McNamara and Boaz software to gather noise information on every broadband station that transmits its data to the DMC in real time.

In their noise processing scheme, McNamara and Boaz calculate Power Spectral Density (PSD) for each 1-hour time series segment. Then the raw frequency distributions are constructed by gathering individual PSDs into frequency distribution bins. A histogram of PSD bins results in PDF (Power Density Function) plots in which transient noises map into a low-level background probability while ambient noise conditions appear as high probability occurrences. The web-based interface of QUACK enables users to customize and display PDFs for time interval of their choosing.

Access to the Power Spectral Density data

For those users interested in the raw PSD data (bin) files, IRIS DMC has implemented an automated system that accepts BREQ_FAST requests and provides user with the corresponding PSD data files (daily bin files). To obtain the PSD data files from this DMC service, users should email their BREQ_FAST requests to: pdfpsd@iris.washington.edu.

The following fields must be included in the BREQ_FAST request:

In response, the PSD Bulk Data Delivery system will bundle the selected PSD files into a gzipped tar file and notifies the user via email of their availability for download.

The structure of the delivered bundle is as follows:

Directory Structure

 PDFPSD/network.station.location/channel/Yyyyy/Djjj.bin

 where

 PDFPSF - is the root directory of the PSD data files
 network - is the network code
 station - is the station code
 location - is the location ID
 yyyy - is the year of the data file
 jjj - is the julian day of the data file

Djjj.bin File Format

Djjj.bin is a cumulative "bin" file containing the hour long PSD measurements for an entire day, where jjj is the julian day for any given file. The file type is ASCII with data presented in 3 columns. A MATLAB script is available (download) to plot PSDFs from these PSDs.