Hi Barry,

Good try but, unfortunately, I don't think so.

At 09:12 PM 12/28/2009 -0800, you wrote:
>All
>I have successfully used a running fft to sense signal frequency component 
>changes and used this as a trigger mechanism for an event. Could one use 
>the  ( I guess you call it ) power spectrum of the signal just before and 
>during the event to remove the noise? I guess you would have to use the 
>same time window so the frequencies would compare. Could it be a simple 
>subtraction of the "before" from the "during"? This would assume that the 
>background noise didn't change in the period during an event.

By its very nature, noise *is* changing....randomly.  Its overall spectrum 
may remain constant, but that is created by time-averaging a noise voltage 
which is constantly varying in an unpredictable way.  A possible working 
definition of noise would be 'that part of a signal which can not be 
predicted and subtracted out'.

>This could be better than trying to shape a multi pole filter to eliminate 
>the noise. I have found that often a portion of the event signal is in the 
>same frequency range as the noise.

To really reduce the noise to a minimum you have to have a quiet site and 
build a quiet seismometer.

For the noise that remains, filtering eliminates both noise and earthquake 
signals.  Hopefully, the worst noise will be at frequencies that are not 
required to properly see the quakes.  For the FBV we often filter out 
anything above 0.08 Hz, as that's where you find a lot of the noise, while 
much of the motion from distant quakes will be below that frequency.  ANMO 
in Albuquerque go even further with their Web display and filter at 
something like 0.05 Hz, and they sometimes don't show the higher-frequency 
quakes very clearly that we can see fine.

Regards,
Brett

Watch our wiggles
http://bnordgren.org/seismo/gif_images.htm

or watch some very very good wiggles
http://aslwww.cr.usgs.gov/Seismic_Data/telemetry_data/ANMO_24hr.html


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