PSN-L Email List Message

Subject: Re: Microseism filter
From: "Connie and Jim Lehman" lehmancj@...........
Date: Thu, 24 Feb 2005 10:05:40 -0500

PSN & Davids/2
     Regrets to hear of necessity to tune out microseisms.  I have always
considered them a friend,--they are always there--a quick check of system,
performance-- and really quite entertaining as changes occur
daily--sometimes hourly.  Certainly in a classroom or a Science Center
setting the presense of microseisms adds to the total picture of Earth
activities.  My location inland in Virginia about 100 miles
from the Atlantic puts me in range of a host of activity on the Eastern
Seaboard --based of course on long period performance that I havc used.
Tropical storms show the moment they reach Florida and move northward to New
England or North Atlantic--  and  the normal flow of barometric highs & lows
in the southeast to northeast trending storm tracks through VA.  Then for
whatever reason in summer months, microseismic amplitudes go low for weeks
at a time--an opportunity to crank up the gain a bit.  I try to keep
microseisms about 1/4 in. in amplitude most of the time.
    Microseisms build to obnoxious amplitudes only as hurricanes move up the
east coast usually east of us, and there have been times where I closed down
recording for 10 hours or so.  Perhaps you folks by oceanside locations have
to live with more of this "pounding" a lot of the time.
   I am curious what affect filtering of microseisms would have on an
earthquakes profile--expecially the first arrivals of P-waves that often
don't appear to have periods too far removed from the cluster of 3-7sec
periods of typical microseisms.
----- Original Message -----
From: "David H. Youden" 
Sent: Thursday, February 24, 2005 7:07 AM
Subject: Re: Microseism filter

> Dave,
> 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...
> 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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