PDF Noise Analysis

In addition to the PSD analysis developed by Bob Uhrhammer, the BSL has implemented the Ambient Noise Probability Density Function (PDF) analysis system developed by McNamara and Buland (2004). This system does its noise analysis over all the data of a given time period (week or year), including earthquakes, calibration pulses, and cultural noise. This is in contrast to Bob Uhrhammer's PSD analysis, which looks at only the quietest portion of data within a day or week. Pete Lombard of the BSL extended the McNamara code to cover a larger frequency range and support the many different types of sensors employed by the BSL. Besides the originally supported broadband sensors, our PDF analysis now includes surface and bore-hole accelerometers, strain meters, and electric and magnetic field sensors. These enhancements to the PDF code, plus a number of bug fixes, were provided back to the McNamara team for incorporation in their work. The results of the PDF analysis are presented on the web at http://www.ncedc.org/ncedc/PDF/. One difficulty with using these plots for review of station quality is that it is necessary to look at data from each component separately. To provide an overview, we have developed summary figures for all components in two spectral bands, 30 - 60 s and 0.125 - 0.25 s. Figure 3.26 shows these overviews for 2006. As the processing takes place shortly after the data arrives at the datacenter, data gaps may be present (white areas) in these plots. Most of these gaps have been filled by retrieving data from the stations.

Figure 3.26: Noise overview taken from the PDF analysis for 2006 for BDSN stations. The top row shows daily minimum noise levels for the vertical (left) and horizontal (right) stations in the band 30 - 60 s. The lower row shows corresponding values in the band 0.125 - 0.25 s. White areas indicate times when realtime data are missing. Most such gaps have been filled in the datacenter archive by retrieving the data from the datalogger. The high noise level for KCC in the spring shows when water is running past the seismometers through the tunnel.