Hi Folks,  Just wanted to thank everyone for their most excellent help =
on this subject. Ted
  ----- Original Message -----=20
  From: Bobhelenmcclure@..........
  To: psn-l@.................
  Sent: Monday, January 08, 2007 7:04 PM
  Subject: Re: Filters


  Ted (tchannel) wrote:
  Hi Folks,  One issue where I seem to need help, is how to filter.  I =
have kept notes on the Low and High Pass filter values used by others. I =
also know that the filter values will change with all the variables. Let =
us assume, using a vertical similar to the AS1, what would be good =
starting points for Local, Regional, and Teleseimetic events?   Also =
could someone explain what the poles numbers do?=20

  Hi Ted,

    The object of seismic filtering is to reproduce the signal with as =
little distortion as possible and attenuate the noise. In the case of =
seismic signals, the spectrum containing the signal is dependent on =
event distance, and the noise is primarily from microseisms and cultural =
noise, including traffic, other human activity, and wind. Microseisms =
range widely in intensity and frequency. Their period ranges primarily =
from 2 seconds to 7 seconds. Cultural noise, in my experience, is =
generally short period and can be easily rejected except for local =
events, which also have a short period. Wind, by exerting pressure on =
trees and structures, imparts long period motion to the ground, and can =
for the most part be excluded by long period high pass filtering. The =
same applies to tilt induced by walking anywhere near a horizontal =
sensor. Usually, the sensor itself is the high pass filter. Amplifier =
bias drift and 1/f noise are also long period effects. Atmospheric =
pressure and temperature changes cause the pendulum to drift.

    For intermediate distance and long distance events, microseisms are =
right in the frequency range of interest, and you probably should just =
suffer them.

    My advice on filtering is to let experience be your guide, but use =
the best filters you can, and do as little filtering as you can get away =
with. In my opinion, the lowpass and highpass filters in WinQuake stink, =
and I have developed my own filters. They are implemented in my =
WQFilter.exe utility program, and anyone can download and try them out =
on WinQuake files. The approach I use is to broadbrand the signal first =
with my period extending filter (especially necessary with the AS1), and =
then restrict the high and low ends of the frequency range with =
Butterworth filters. You should restrict the number of poles used. The =
number of poles determines the steepness of the rolloff at the corner =
frequency. However, more poles means more ringing in the filter impulse =
response, which you do not want.=20

  You can learn a lot about filters by downloading "seismic_dataq.zip" =
from John Lahr's web site. Extract "WQFilter.exe", "WQFilterHlp.txt", =
and "Impulse.psn" from the zip. Use both WinQuake and WQFilter to filter =
"Impulse.psn" with the various filters of each program. Using WinQuake, =
examine the resulting filtered response (the impulse response). The FFT =
of the impulse response will give you the passband spectrum of the =
filter. The impulse response itself will tell you about the group time =
delay and ringing response of the filter. My filters have zero time =
delay and phase distortion. This is achieved by filtering the data =
forward in time, and then backward in time (impossible to do with an =
analog filter).=20

    This is the Digital Age. Analog is dead! Do not listen to anyone =
about analog filter circuits. Analog filtering has no place anymore =
except at the front end of the A/D converter, and even that need is =
minimized by sampling at a high rate and subsequently downsampling with =
maybe some digital filtering thrown in.








Hi Folks,  Just wanted to thank everyone for their most =
excellent help=20
on this subject. Ted

  
----- Original Message ----- 
  From:=20
  Bobhelenmcclure@....... =

  
To: psn-l@.............. 
  
Sent: Monday, January 08, 2007 =
7:04=20
  PM
  
Subject: Re: Filters
  


  
Ted (tchannel) wrote:
Hi Folks,  One issue where I seem =
to need=20
  help, is how to filter.  I have kept notes on the Low and High =
Pass=20
  filter values used by others. I also know that the filter values will =
change=20
  with all the variables. Let us assume, using a vertical similar to the =
AS1,=20
  what would be good starting points for Local, Regional, and =
Teleseimetic=20
  events?   Also could someone explain what the poles numbers =
do?=20
  
  
 
  
Hi Ted,
  
 
  
  The object of seismic filtering is to reproduce the signal =
with as=20
  little distortion as possible and attenuate the noise. In the case of =
seismic=20
  signals, the spectrum containing the signal is dependent on event =
distance,=20
  and the noise is primarily from microseisms and cultural noise, =
including=20
  traffic, other human activity, and wind. Microseisms range widely in =
intensity=20
  and frequency. Their period ranges primarily from 2 seconds to 7 =
seconds.=20
  Cultural noise, in my experience, is generally short period and can be =
easily=20
  rejected except for local events, which also have a short period. =
Wind, by=20
  exerting pressure on trees and structures, imparts long period motion =
to the=20
  ground, and can for the most part be excluded by long period high pass =

  filtering. The same applies to tilt induced by walking anywhere near a =

  horizontal sensor. Usually, the sensor itself is the high pass filter. =

  Amplifier bias drift and 1/f noise are also long period effects. =
Atmospheric=20
  pressure and temperature changes cause the pendulum to drift.
  
 
  
  For intermediate distance and long distance events, =
microseisms=20
  are right in the frequency range of interest, and you probably should =
just=20
  suffer them.
  
 
  
  My advice on filtering is to let experience be your guide, =
but use=20
  the best filters you can, and do as little filtering as you can get =
away with.=20
  In my opinion, the lowpass and highpass filters in WinQuake stink, and =
I have=20
  developed my own filters. They are implemented in my WQFilter.exe =
utility=20
  program, and anyone can download and try them out on WinQuake files. =
The=20
  approach I use is to broadbrand the signal first with my period =
extending=20
  filter (especially necessary with the AS1), and then restrict the high =
and low=20
  ends of the frequency range with Butterworth filters. You should =
restrict the=20
  number of poles used. The number of poles determines the steepness of =
the=20
  rolloff at the corner frequency. However, more poles means more =
ringing in the=20
  filter impulse response, which you do not want. 
  
 
  
You can learn a lot about filters by downloading =
"seismic_dataq.zip" from=20
  John Lahr's web site. Extract "WQFilter.exe", "WQFilterHlp.txt", and=20
  "Impulse.psn" from the zip. Use both WinQuake and WQFilter to filter=20
  "Impulse.psn" with the various filters of each program. Using =
WinQuake,=20
  examine the resulting filtered response (the impulse response). The =
FFT of the=20
  impulse response will give you the passband spectrum of the filter. =
The=20
  impulse response itself will tell you about the group time delay and =
ringing=20
  response of the filter. My filters have zero time delay and phase =
distortion.=20
  This is achieved by filtering the data forward in time, and then =
backward in=20
  time (impossible to do with an analog filter). 
  
 
  
  This is the Digital Age. Analog is dead! Do not listen to =
anyone=20
  about analog filter circuits. Analog filtering has no place anymore =
except at=20
  the front end of the A/D converter, and even that need is minimized by =

  sampling at a high rate and subsequently downsampling with maybe some =
digital=20
  filtering thrown in.
  
 
  
Bob