NORSAR Power Density Spectra

We compute hourly power spectral density (PSD) for each channel using Welch's method:

1. We subdivide the 1-hour time window into 200-sec segments with 100 sec overlap (i.e., 35 segments).
2. For each segment we compute the periodogram (i.e., FFT and squaring of the magnitude).
3. We compute the hourly PSD by averaging the 35 periodograms.

The seismograms are not instrument-corrected and therefore in units of counts. The unit of the PSD is accordingly counts**2/Hz

We display the logarithm (log10) of the PSD in monthly panels (see example for NAO00_SZ above). The x-axis is time in units of days the y-axis is frequency in [Hz] and the PSD is color-coded (blue: low amplitude, red. high amplitude). In order to increase the dynamic resolution of the plots the average PSD (computed from all hourly PSDs in the panel) has been substracted. The average PSD is overlaid on the plot as a red curve. Its scale is indicated with red markers on top and on the bottom of the plot.