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 "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).
> 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.
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