Brett
Yeah. I also forgot about the phase angle at a particular frequency . Why d=
o you think a low pass rather than a=A0 notch filter, centered at the micro=
seismic frequency zone, is used?. That way you aren't totally eliminating t=
he higher frequencies and potentially the P an maybe S signals. That's the =
beauty of winquake. You can play with the signal later.=20

Regards
Barry
http://www.seismicvault.com

--- On Tue, 12/29/09, Brett Nordgren  wrote:

From: Brett Nordgren 
Subject: Re: Sensor noise
To: psn-l@..............
Date: Tuesday, December 29, 2009, 5:26 AM

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 componen=
t changes and used this as a trigger mechanism for an event. Could one use =
the=A0 ( 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 sa=
me time window so the frequencies would compare. Could it be a simple subtr=
action of the "before" from the "during"? This would assume that the backgr=
ound noise didn't change in the period during an event.

By its very nature, noise *is* changing....randomly.=A0 Its overall spectru=
m may remain constant, but that is created by time-averaging a noise voltag=
e which is constantly varying in an unpredictable way.=A0 A possible workin=
g definition of noise would be 'that part of a signal which can not be pred=
icted and subtracted out'.

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

To really reduce the noise to a minimum you have to have a quiet site and b=
uild a quiet seismometer.

For the noise that remains, filtering eliminates both noise and earthquake =
signals.=A0 Hopefully, the worst noise will be at frequencies that are not =
required to properly see the quakes.=A0 For the FBV we often filter out any=
thing above 0.08 Hz, as that's where you find a lot of the noise, while muc=
h of the motion from distant quakes will be below that frequency.=A0 ANMO i=
n Albuquerque go even further with their Web display and filter at somethin=
g like 0.05 Hz, and they sometimes don't show the higher-frequency quakes v=
ery 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


__________________________________________________________

Public Seismic Network Mailing List (PSN-L)

Brett
Yeah. I also forgot about the phase =
angle at a particular frequency . 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 potenti=
ally the P an maybe S signals. That's the beauty of winquake. You can play =
with the signal later. 

Regards
Barry
http://www.seismicvault.com

--- On Tue, 12/29/09, Brett Nordgren <brett3nt@bnordgr=
en.org> wrote:

From: Brett Nordg=
ren <brett3nt@.............>
Subject: Re: Sensor noise
To: psn-=
l@..............
Date: Tuesday, December 29, 2009, 5:26 AM

Hi Barry,

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

At 09:12 PM 12/28/2009 -0800, you wrote:
> All
&g=
t; I have successfully used a running fft to sense signal frequency compone=
nt 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 th=
e same time window so the frequencies would compare. Could it be a simple s=
ubtraction of the "before" from the "during"? This would assume that the ba=
ckground noise didn't change in the period during an event.

By its v=
ery nature, noise *is* changing....randomly.  Its overall spectrum may=
 remain constant, but that is created by time-averaging a noise voltage whi=
ch 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 tryin=
g to shape a multi pole filter to eliminate the noise. I have found that of=
ten a portion of the event signal is in the same frequency range as the noi=
se.

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, fil=
tering eliminates both noise and earthquake signals.  Hopefully, the w=
orst 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 t=
hat's where you find a lot of the noise, while much of the motion from dist=
ant 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 th=
ey sometimes don't show the higher-frequency quakes very clearly that we ca=
n see fine.

Regards,
Brett

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

or watch some very very g=
ood wiggles
http://aslwww.cr.usgs.gov/Seismic_Da=
ta/telemetry_data/ANMO_24hr.html


___________________________=
_______________________________

Public Seismic Network Mailing List =
(PSN-L)

To leave this list email PSN-=
L-REQUEST@.............. with the body of the message (first line only)=
: unsubscribe
See http://www.seismicnet.com/maillist.html for more inform=

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