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The USGS Low-Noise Model

The USGS New Low Noise Model [Peterson 1993] summarizes the lowest observed vertical seismic noise levels throughout the seismic frequency band. It is extremely useful as a reference for assessing the quality seismic stations, for predicting the detectability of small signals, and for the design of seismic sensors. Fig. 18 is one of several possible representations of this model. Stationary seismic noise is normally measured in terms of acceleration power density. This density must be integrated over the passband of a low-pass or band-pass filter to obtain the power (the mean square of the amplitude) at the output of the filter. The square-root of the power is then the rms (or effective) amplitude. To determine absolute signal levels from a diagram of noise power density is thus a little inconvenient; also, power densities cannot directly be compared to the levels of transient signals such as earthquakes, so earthquake signals cannot be included in a diagram of power density. We have therefore chosen a different representation of the NLNM.

Fig. 18 displays the minimum vertical seismic noise directly as rms amplitudes in a bandwidth of one-sixth decade, between 82.5% and 121% of the central frequency. By coincidence, these amplitudes may also be interpreted as average peak amplitudes in a bandwidth of one-third octave (a standard bandwidth in acoustics). An example: the minimum vertical ground noise between the periods of 10 and 20 sec is at -180 dB relative to ${\rm 1 m/s^2}$, thus ${\rm 10^{-180/20}\;m/s^2 = 1\;nm/s^2}$ average peak in one-third octave. Since the bandwidth considered is a full octave, the total average peak amplitude in this band is ${\rm\sqrt{3}\;nm/s^2}$.


  
Figure 18: The USGS New Low Noise Model, here expressed as rms amplitudes of ground acceleration in a constant relative bandwidth of one-sixth decade
\includegraphics[width=\textwidth]{Fig/nlnm.eps}

By definition of the model, almost all sites have a noise level above the NLNM, some of them by a large factor. At short periods, a noise level no more than 20 dB (a factor of 10 in amplitude) above the NLNM may be considered as very good in most areas. The marine microseismic noise between 2 and 20 sec has large seasonal variations and may be 50 dB above the NLNM in wintertime. At longer periods, the vertical ground noise is often within 10 or 20 dB of the NLNM even at otherwise noisy stations. The horizontal long-period noise may nevertheless be horrible at the same station due to tilt-gravity coupling (see section 3.3), and a station can be considered good when the horizontal noise at 100 to 300 sec is within 20 dB above the vertical noise.


next up previous contents
Next: Site selection Up: Seismic noise. Site selection. Previous: Seismic noise. Site selection.
Erhard Wielandt
2002-11-08