PSN-L Email List Message

Subject: Microseism filter
From: "Dave Nelson" davefnelson@.......
Date: Wed, 23 Feb 2005 23:53:13 -0800


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.=20

I would be glad to correspond with anyone interested in or commenting on =
what I am doing.









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

[ Top ] [ Back ] [ Home Page ]