PSN-L Email List Message

Subject: Re: Instrumentation Question
From: chrisatupw@.......
Date: Sun, 30 Dec 2012 13:11:22 -0500 (EST)






Hi Dave,=20

****A couple of comments that I forgot to include !

****If you are getting serious problems with 1/f long period noise when try=
ing to look at Earth=20
Eigemodes, why don't you try building a chopper amplifier ? They are IMMUNE=
 to 1/f noise !=20

****The only factor which limits Lehmans like the SEPUK on period is the da=
ily ground tilt drift. If I=20
eliminate this using very long period optically sensed force feedback, I ca=
n set mine up fairly easily=20
to run at 60 seconds period. If you added a capacitative position sensor to=
 drive the force feedback=20
and detect the signals, you should be able to sense stably out to 1,000 sec=
onds. The commercial=20
HS3 used this technique.=20


The basic concepts  for the Lippman circuit and the Robertson circuit are d=
escribed in detail in the "Applications Manual for Computing Amplifiers" by=
 Philbrick Research Laboratories in 1965. This is FAR from new science. The=
 French School of Seismometers may have extended a 4.5 Hz geophone to 20 se=
conds but we don't see them in the field today because it was, and still is=
, impractical because of high noise.  A practical, operationally useful ext=
ended period geophone has limits.

****I suggest that you have a look at some of the French School web sites s=
ometime. There are about 50 of them 'in the field today' ! The seismometer =
response graph is on their web site. You can go to particular schools and d=
isplay the current traces on line. I was a bit sceptical too, so I looked, =
but they don't seem to suffer from excessive long period noise. But I would=
 agree that the French do seem to have pushed the technique as far as it ca=
n go. =20
    I don't have access to the Philbrick manual, but in 1965 we didn't have=
 Integrated Circuit opamps. Do they describe chopper amplifiers ? We were u=
sing them in nuclear applications then. I think that I may still have one t=
ucked away in the loft !=20
    Sure there are practical limits, but it can be fun to see just how far =
you can push them !


The absolute maximum in damping is not necessary or desired. My goal was 0.=
5 Hz not because that is the best that can be done but because that is all =
that is required by local/regional earthquake location and shake map  gener=
ation. If you can extend the range to 20 seconds still have a practical ins=
trument - -  go for it.
The practical, not absolute, limit seems to be about 10 to 1 in frequency.

****Do check out www.lennartz-electronic.de/index.php They show a graph of =
a 20:1 extension and the 2 Hz to 20 seconds seismometer is 40:1. There may =
be operational reasons why a seemingly 'old fashioned' system may be the pr=
eferred choice. Reliability, guaranteed life and repair policies are not al=
ways quoted. I haven't noticed Nanometrics boasting about any of these.=20

    Incidentally, you can make a reasonably good 'school grade' seismometer=
 using the 44mm OD piezo sounder disks.=20

    Have you got two circuit boards for me please ?=20

    Regards,=20

    Chris Chapman=20

=20
=20
=20
=20


Hi Dave,

****A couple of comments that I forgot to include !

****If you are getting serious problems with 1/f long peri= od noise when trying to look at Ea= rth
Eigemodes
, why don't you try building a chop= per amplifier ? They are IMMUNE to 1/f noise !

****The only factor which limits Lehmans like the SEPUK on= period is the daily ground tilt drift. If I
eliminate this using very long period optically sensed for= ce feedback, I can set mine up fai= rly easily
to run at 60 seconds period. If you added a capacitative position sensor to drive the force feedback
and detect the signals, you should be able to sense stably out to 1,000 sec= onds. The commercial
HS3 used this technique.

The basic concepts  for= the Lippman=20 circuit and the Robertson circuit are described in detail in the "Applicati= ons=20 Manual for Computing Amplifiers" by Philbrick Research Laboratories in 1965= ..=20 This is FAR from new science. The French School of Seismometers may have=20 extended a 4.5 Hz geophone to 20 seconds but we don't see them in the = field=20 today because it was, and still is, impractical because of high noise. = ; A=20 practical, operationally useful extended period geophone has limits.=

****I suggest that you have a look at= some of the French School web sites sometime. There are a= bout 50 of them 'in the field today' ! The seismometer response grap= h is on their web site. You can go to particular schools and = display the current traces on line. I was= a bit sceptical too, so I looked, but they don't seem to = suffer from excessive long period noise. But I would agree that the French do seem to have pushed the techni= que as far as it can go. 
    I do= n't have access to the Philbrick manu= al, but in 1965 we didn't have Integrated Circuit opamps. Do they describe = chopper amplifiers ? We were using them in nuclear applications then. I thi= nk that I may still have one tucked away in the loft !
    Sure there are practical limits, but it= can be fun to see just how far you can push them !=

The absolute maximum in damping is not= necessary or=20 desired. My goal was 0.5 Hz not because that is the best that can be done b= ut=20 because that is all that is required by local/regional earthquake location = and=20 shake map  generation. If you can extend the range to 20 seconds still= have=20 a practical instrument - -  go for it.
The practical, not absolute, limit see= ms to be=20 about 10 to 1 in frequency.

****Do check out
www.lennartz-electronic.de/index.php They s= how a graph of a 20:1 extension and the 2 Hz to 20 seconds seismometer is 4= 0:1. There may be operational reasons why a seemingly 'old fashioned' syste= m may be the preferred choice. Reliability, guaranteed life and repair poli= cies are not always quoted. I haven't noticed Nanometrics boasting about an= y of these.

    Incidentally, you = can make a reasonably good 'school grade' seismometer usin= g the 44mm OD piezo sounder disks.

    Have you got two circuit boards for me = please ?

    Regards,

    Chris Chapman


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