PSN-L Email List Message

Subject: Re: Sensor noise
From: Brett Nordgren brett3nt@.............
Date: Tue, 29 Dec 2009 10:32:49 -0500

Hi Barry

At 05:54 AM 12/29/2009 -0800, you wrote:
>Yeah. I also forgot about the phase angle at a particular frequency .

Also instantaneous amplitude.  It is point-by-point data that you would 
have to be subtracting if you are looking for a point-by-point result, that 
is to say, a waveform.

>Why do you think a low pass rather than a  notch filter, centered at the 
>microseismic frequency zone, is used?. That way you aren't totally 
>eliminating the higher frequencies and potentially the P an maybe S 
>signals. That's the beauty of winquake. You can play with the signal later.

Yes, the ability to post process the signal is very useful.

There are several reasons why I don't generally use a notch filter.  First 
the microseism zones are bands of frequencies and the notch(es) would have 
to be fairly wide.  In addition to microseisms, generally at the higher 
frequencies, is all the human-caused noise, so-called cultural noise, which 
has pretty much unpredictable frequencies.  By the time I have notched out 
all the noise, there's not much left and it is simpler to just use a 
low-pass.  And even with the 12.5 second low pass filter, the larger quakes 
will clearly show some of the principal body phases.

If I come across a future quake that I want to look at in more detail which 
has too much noise at a particular frequency, I may try and see what notch 
filtering can do.  That might be particularly useful with small local quakes.


Watch our wiggles

or watch some very very good wiggles


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