PSN-L Email List Message
Subject: Re: Sensor noise
From: Brett Nordgren brett3nt@.............
Date: Tue, 29 Dec 2009 08:26:10 -0500
Good try but, unfortunately, I don't think so.
At 09:12 PM 12/28/2009 -0800, you wrote:
>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.
Watch our wiggles
or watch some very very good wiggles
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