PSN-L Email List Message

Subject: Re: Microseism filter
From: "David H. Youden" dyouden@.............
Date: Thu, 24 Feb 2005 07:07:13 -0500


Tell me more about your filter. I live on the East coast, and am 
troubled by the level of microseisms. I have messed a bit with switched 
capacitor circuits in the past, but for different purposes. Could you 
send a circuit diagram directly to me? Or point me to the chip you used 
so I can puzzle out my own filter.


Dave Nelson wrote:

> I have recently added a Russian ASMET -1V MET broadband seismometer to 
> my setup. As I live near the coast the microseism level was dominating 
> the noise backgrond to the extent that earthquakes of interest were 
> being lost in the " noise". The microseism spectrum is very distinct 
> and I felt  its elimination  would not seriously effect the earthquake 
> detection capability .
>  I have developed a simple filter based on switched capacitor 
> technology that virtualy eliminates the microseism background. The 
> filter has no critical componants or  capacitors and is tuned by a 
> clock frequency. The Q is 0.5 which gives roughly an octave bandwidth 
> centered at 0.015 hz or 6.6 seconds . Since the center frequncy is 
> clock dependant it can be shifted arbitarily but 6.6 seconds seems to 
> work very well. The filter is placed directly ahead of the A/D at the 
> highest voltage swing  level in the system to avoid adding noise.  
> Just two wires -- in and out (plus power of course).
> The results are very good, the harmonic like signal from microseism is 
> gone and the effective signal to noise ratio of eathquakes is 
> significantly better. The spectrum has a deep null at 6.6 seconds 
> instead of a  dominant peak .
> I know this kind of spectrum shaping can be easily done in software 
> but that is usually beyond the scope of an amateur or computer dum- 
> dum like me. The whole thing is  two IC's and a 15 hz clock generator. 
> ( 100 times the center frequency)
> The Russian seismometer is truely amazing. Virtually plug and play -- 
> four wires ,no adjustments or critical setup. The respose is 
> advertised to 20 seconds but seems to respond well to over 100 seconds 
> on the low end and too well to be useful ,given the urban background 
> noise ,at the high frequency end. I have another swiched capacitor low 
> pass filter switchable to 5, 2.5 and 1.25 hz. to control the high end 
> cutoff.
> My setup also includes 3 axes of short period based on HS-10-1  1 hz 
> seismometers that I found in a surplus store. I am using Amaseis and 
> experimenting with Seismowin for analysis and display.
> I am a retired aerospace engineer who spent too much of my career in 
> management and I am having a ball doing the seismology thing.Good to 
> be back doing some development and putting the old brain to work. I 
> intend to do some seismometer development as my next project -- I have 
> some ideas for some different ,but not outrageous, concepts which 
> may ( or may not ) work.
> I would be glad to correspond with anyone interested in or commenting 
> on what I am doing.
> Dave Nelson   ( California not New Zealand)


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