PSN-L Email List Message

Subject: Re: Optical siesmometer
From: Charles R Patton charles.r.patton@........
Date: Wed, 16 Jan 2013 09:55:14 -0800


On 1/16/2013 3:14 AM, chrisatupw@....... wrote:
> From: Larry Cochrane 
> To: psnlist 
> Sent: Wed, 16 Jan 2013 3:21
> Subject: Fwd: Optical siesmometer
>
> Subject:Optical  seismometer
> Date:Tue,15  Jan  2013  15:33:51  -0600
>
> Larry,
> Ithought  you  might  find  this  interesting:
> http://www.ctbto.org/fileadmin/user_upload/SandT_2011/presentations/T3-O5%20J_Berger%20Optical%20Seismometer.pdf
> -Charlie
>
> Hi Charlie,
>      It looks as if Mark & Co have done quite a bit more development work.
> But amateurs would likely to have difficulty measuring optical fringes to
> 1/2 ppm / Root Hz and Michaelson Interferometers are not cheap.
> Amateurs can get about 10 nano metres resolution over 10 Hz using large area
> photocells and a stabilised light source, but this is likely to be adequate -
> unless you can 'lay your hands' on a couple of redundant Streckheisens !
> Regards,
> Chris Chapman

OK, how about this for a thought experiment?

Take a standard $10 or so USBweb cam -- definitely a cheaper one, no 
autofocus, but rather one where you can unscrew the lens easily.It will 
have a 640 x 480 or better resolution at a 60 Hz sample rate.Use it in 
an optical lever arrangement on the seismometer and project a laser beam 
spot on the face of the sensor.So how sensitive could it be?

Let's assume a 20" pendulum and a 20" optical lever length.We're 
interested in duplicating the interferometer capability in the 
Zumberge/Berger paper -- about:

3e-7 * 1e-6 = 3e-13m=1.18e-12"(see pg 8).

Although I have a problem with this number.They describe a 16 bit 
conversion so the number can't be much better than:

3e-7 / 65536 = 4.6e-11m=1. 8e-9"

So, assuming the typical 1/3" sensor in the webcam.Therefore

0.33" / 640 = 5.21e-4"640 pixel spacing

5.21e-4" / 256= 2.03e-6"due to interpolation from the 8-bit analog 
digitization

As the optical lever length is assumed equal to the pendulum length, 
then for small movements, the projected laser dot displacement will 
equal the pendulum movement.

So the optical sensor is still a factor of 1000 away from the 
interferometer.Averaging the sample rate from 60 Hz/16 down to approx 
4Hz, could add another 4x resolution improvement or about:

2.03e-6" / 4 = 0.5e-6"

Not really close enough.No joy there.

I can't think of another major improvement to the resolution except:

1) Maybe project the beam through a cylindrical lens that would increase 
the deviation, but also spread the beam so probably a wash.

2) An optical lever distance of 1000 x 20" = 2e4"= 1667'.I don't think 
so, unless we did it with a set of parallel mirrors spaced perhaps 2' 
apart and where we allow the laser beam to enter at an almost 
perpendicular angle to bounce back and forth 800 times before 
exiting.Mirror loss per bounce of 1% would attenuate the beam by 8.That 
shouldn't be a problem -- just the quality of the mirrors would be tough.

Any other ideas?

Regards,

Charles R. Patton


  
    
  
  
    
On 1/16/2013 3:14 AM, chrisatupw@....... wrote:
From: Larry Cochrane <lcochrane@..............>
To: psnlist <psnlist@..............>
Sent: Wed, 16 Jan 2013 3:21
Subject: Fwd: Optical siesmometer

Subject: Optical seismometer
Date: Tue, 15 Jan 2013 15:33:51 -0600

Larry,
I thought you might find this interesting:
http://www.ctbto.org/fileadmin/user_upload/SandT_2011/presentations/T3-O5%20J_Berger%20Optical%20Seismometer.pdf
-Charlie

Hi Charlie, 
    It looks as if Mark & Co have done quite a bit more development work. 
But amateurs would likely to have difficulty measuring optical fringes to 
1/2 ppm / Root Hz and Michaelson Interferometers are not cheap. 
Amateurs can get about 10 nano metres resolution over 10 Hz using large area 
photocells and a stabilised light source, but this is likely to be adequate - 
unless you can 'lay your hands' on a couple of redundant Streckheisens !
Regards,
Chris Chapman

OK, how about this for a thought experiment?

Take a standard $10 or so USB  web cam -- definitely a cheaper one, no autofocus, but rather one where you can unscrew the lens easily.  It will have a 640 x 480 or better resolution at a 60 Hz sample rate.   Use it in an optical lever arrangement on the seismometer and project a laser beam spot on the face of the sensor.  So how sensitive could it be?

 

Let's assume a 20" pendulum and a 20" optical lever length.   We're interested in duplicating the interferometer capability in the Zumberge/Berger paper -- about:

3e-7 * 1e-6 = 3e-13m=1.18e-12"   (see pg 8). 

Although I have a problem with this number.  They describe a 16 bit conversion so the number can't be much better than:

3e-7 / 65536 = 4.6e-11m=1. 8e-9" 

 

So, assuming the typical 1/3" sensor in the webcam.  Therefore

0.33" / 640 = 5.21e-4"     640 pixel spacing

5.21e-4" / 256  = 2.03e-6"  due to interpolation from the 8-bit analog digitization

 

As the optical lever length is assumed equal to the pendulum length, then for small movements, the projected laser dot displacement will equal the pendulum movement.

 

So the optical sensor is still a factor of 1000 away from the interferometer.  Averaging the sample rate from 60 Hz/16 down to approx 4Hz, could add another 4x resolution improvement or about:

2.03e-6" / 4 = 0.5e-6" 

Not really close enough.  No joy there. 

 

I can't think of another major improvement to the resolution except:

1) Maybe project the beam through a cylindrical lens that would increase the deviation, but also spread the beam so probably a wash. 

2) An optical lever distance of 1000 x 20" = 2e4"= 1667'.  I don't think so, unless we did it with a set of parallel mirrors spaced perhaps 2' apart and where we allow the laser beam to enter at an almost perpendicular angle to bounce back and forth 800 times before exiting.  Mirror loss per bounce of 1% would attenuate the beam by 8.  That shouldn't be a problem -- just the quality of the mirrors would be tough. 

 

Any other ideas? 

 

Regards,

Charles R. Patton


[ Top ] [ Back ] [ Home Page ]