PSN-L Email List Message

Subject: Re: Filters
From: "Geoffrey" gmvoeth@...........
Date: Mon, 8 Jan 2007 22:55:02 -0700

One thing you did not mention is phase distortion when you filter ???

I should imagine that phase distortions will
cause the original waveform to be mishapen
when it is read by the A/D
converter and I know of no way to stop this from happening.

It is good that most seismic phase signals stay
inside a relatively narrow range.

----- Original Message ----- 
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? 
> 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. 
> You can learn a lot about filters by downloading "" 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). 
>  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.
> Bob

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