PSN-L Email List Message

Subject: Re: Instrumentation Question
From: Geoff gmvoeth@...........
Date: Sun, 30 Dec 2012 23:14:09 +0000


On 12/30/2012 8:46 PM, Dave Nelson wrote:
> Hi Chris ,
> I believe the Lennartz 20 second instrument is based on a 1 second 
> geophone. It has a self noise level  at least 20 db above the low 
> noise model at 20 seconds. My standard  of comparison for  an 
> expensive vertical seismometer is below the low noise model at 20 
> seconds . Even my own homemade verticals are below the low noise model 
> to 35 seconds . I just don't buy it.
> always try chopper op amps in all my systems to see if they help with 
> long period noise -- usually the AD 706 is best. I have  tried many op 
> amps and made noise tests
>
> *From:* chrisatupw@....... 
> *Sent:* Sunday, December 30, 2012 6:11 PM
> *To:* psnlist@.............. 
> *Subject:* Re: Instrumentation Question
>
> Hi Dave,
>
> ****A couple of comments that I forgot to include !
>
> ****If you are gettingserious problems with 1/f long period noise when 
> trying to look at Earth
> Eigemodes, why don't you try building a chopper 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 force feedback, 
> I can set mine up fairly 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 seconds. The commercial
> HS3 used this technique.
>
> The basic concepts  for the Lippman circuit and the Robertson circuit 
> are described 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 seconds 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 extended period geophone has 
> limits.
>
> ****I suggest that you have a look at some of the French School web 
> sites sometime. 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 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 technique as far as it can go.
> 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 using them in nuclear applications then. I think 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 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  generation. If you can extend the range to 20 seconds still 
> have a practical instrument - -  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 preferred choice. Reliability, guaranteed life and 
> repair policies are not always quoted. I haven't noticed Nanometrics 
> boasting about any of these.
>
> Incidentally, you can make a reasonably good 'school grade' 
> seismometer using the 44mm OD piezo sounder disks.
>
>     Have you got two circuit boards for me please ?
>
>     Regards,
>
> Chris Chapman

sorry to butt in like this,

There must be some kind of strain gauge where you simply hang a weight 
like a pendulum
and any swing however slight would create a signal.

????

Then the signal would be omnidirectional which is what I am looking for.

A single plane oriented sensor which produces an omnidirectional signal.

A hoovering mass like the globes you can buy
suspended within a magnetic field


????

A gyroscope of light where you measure phase differences between input 
and output
A laser beam traveling through a fiber optic coil.
?????


A foam pad which changes resistance with compression
so you just put a mass like a gallon of water on it
to measure vertical forces

???

Isn't there something novel and cheap and easy to try ???

Something which requires X10,000 or less in gain.


regards,
geoff

  
    
  
  
    
On 12/30/2012 8:46 PM, Dave Nelson wrote:
Hi Chris ,
 
I believe the Lennartz 20 second instrument is based on a 1 second geophone. It has a self noise level  at least 20 db above the low noise model at 20 seconds. My standard  of comparison for  an expensive vertical seismometer is below the low noise model at 20 seconds . Even my own homemade verticals are below the low noise model to 35 seconds . I just don't buy it.  
 
always try chopper op amps in all my systems to see if they help with long period noise -- usually the AD 706 is best. I have  tried many op amps and made noise tests 
 

Sent: Sunday, December 30, 2012 6:11 PM
Subject: Re: Instrumentation Question

Hi Dave,

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

****If you are getting serious problems with 1/f long period noise when trying to look at Earth
Eigemodes
, why don't you try building a chopper 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 force feedback, I can set mine up fairly 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 seconds. The commercial
HS3 used this technique.

The basic concepts  for the Lippman circuit and the Robertson circuit are described 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 seconds 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 extended period geophone has limits.

****I suggest that you have a look at some of the French School web sites sometime. 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 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 technique as far as it can go. 
    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 using them in nuclear applications then. I think 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 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  generation. If you can extend the range to 20 seconds still have a practical instrument - -  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 preferred choice. Reliability, guaranteed life and repair policies are not always quoted. I haven't noticed Nanometrics boasting about any of these.

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

    Have you got two circuit boards for me please ?

    Regards,

    Chris Chapman


sorry to butt in like this,

There must be some kind of strain gauge where you simply hang a weight like a pendulum
and any swing however slight would create a signal.

????

Then the signal would be omnidirectional which is what I am looking for.

A single plane oriented sensor which produces an omnidirectional signal.

A hoovering mass like the globes you can buy
suspended within a magnetic field


????

A gyroscope of light where you measure phase differences between input and output
A laser beam traveling through a fiber optic coil.
?????


A foam pad which changes resistance with compression
so you just put a mass like a gallon of water on it
to measure vertical forces

???

Isn't there something novel and cheap and easy to try ???

Something which requires X10,000 or less in gain.


regards,
geoff

[ Top ] [ Back ] [ Home Page ]