PSN-L Email List Message

Subject: Re: Homebrew 3 axis Geophone Torpedo Container
From: Dave Willey davewilley@.............
Date: Wed, 15 Nov 2006 16:46:04 -0800 (PST)


Alright. One more attempt...

  ChrisAtUpw@....... wrote: 
   
  In a message dated 2006/11/15, davewilley@............. writes:



  >>My question for you all is will there be any noticeable magnetic interaction between the
  >>3 units I need to worry about with stacking them (vertically) about .25 - .5 inches 
  >>apart inside the PVC tubing?
  

> Hi Dave Willey,.

>       The geophone units should have no magnetic interactions when stacked together. 
  >However, you do need to connect them fairly accurately at right angles
  >and they should be clamped in place. Can you provide an inner rod / tube of plastic and
  >bore holes in this to mount / align the geophones?


  Yes.  I am planning use a mill with a horizontally mounted index head so as to mill at exact 90 degrees a hole perpendicular in a slightly larger (1.5Ē) PVC pipe, 5 inches long. (What youíre calling an inner tube.)  Iíll be lightly clamping them with set screws to keep the geophones stable. 
   
  The inner tube with the geophones is to be glued into the inside of a 2.5 inch threaded plug that will screw into a matching female connector on the bottom and glued to a short length of 3Ē pipe which acts as the over-all cover. The outer cover has a slip cap on top with a machine screw eye that will come out the top.  Coax cables will come out the bottom of the plug and to the surface. Coax is covered with PVC so I should be able to glue everything together and have it waterproof.
   
  >      The 4.5 Hz geophones do have stray fields at both ends, but the barrel is the mild-
  >steel return path for the axially mounted central magnets. I would suggest a 1 cm spacing.
  >
 >      You can get Roberts' type period extension amplifiers from www.sara.pg.it 
  >These should enable you to receive teleseismic P and S waves, which you won't other-
  >wise see with 4.5 Hz geophones. See the note about 4.5 Hz geophones on Larry's website?

Are you referring to the 3 channel amp boards (SEMP30) and equalizer boards (SEQUA10)? If so, will the eq board work in front of Larryís 4 channel 16-bit preamp board?  (Which I already have.)   I could hang this EQ board on the unused 4th channel for teleseismic work.  That would be a neat setup if they play together.
  
>       When you have remounted the geophones, do make a record of the wire colours 
  >and that they are the correct way around for sensing E, N and UP signals with the
  >correct polarity. Paint / Mark lines on the exterior of the case with the correct directions.


  Yep.  Having been a (semi retired) electronic tech at True Time w/ Keith Paya Iím well aware 
  about watching the polarity as to not be ultimately 90 or 180 degrees out of phase with the direction of the movement.
  
>       Are you going to solder the metal film damping resistors and any RF capacitors across the 
  >geophone terminals? Remember to check the input resistance of your amplifier and use it in the 
  >calculation for the damping resistor. 

  Iím assuming since I bought everything from Larry several years ago as a ďturn-keyĒ system that this has been all done.  Itís been playing fine on the back concrete patio for 1.5 years.  Now itís time to wrap up the test phase of this experiment and get it in the ground away from the house and the roads.
  
>   How about protection diodes? Do you get severe thunder storms in your area? Is the soil 
  >sandy / rocky / dry? It is over many areas in the USA and this can give problems with local 
  >lightning. Your house and system may all be exposed many feet above the local 'Earth' 
  >conducting plane.


  This Iíll have to check on.  I donít think there are any clamping diodes. Iíll put some a silicon version of a 1N914 diode 
  ďnose-to-tailĒ across the cables center conductors and coax shield/case ground.  Good thought.
  
 >      The outer case must be hermetically sealed and the cable end also sealed. Some cable 
  >types allow air to pass through between the wires when under pressure. Also seal the amplifier 
  >end of the cable. I suggest that you add a bag of inert silica gel drying crystals inside the case 
  > to absorb any moisture. 


  Iíll check with Belden Cable about Plenum grade RG-174 and see how impervious to water vapor it is.
   
  Before True Time, I worked at Lemo connectors.  They have several lines of space-rated and deep oil exploration-grade connectors.  They seemed very over-engineered for my needs. So the connectors should stay watertight.
   
  I do like the idea of the silica gel as a last ditch effort to keep things dry inside.
   
  
>       Regards,

 >     Chris Chapman


   
  One again Chris you (and the others on this list) have been a great resource of ideas.  
   
   
   
  Thanks,
    
   
   
  Dave Willey
   
Alright. One more attempt...

ChrisAtUpw@....... wrote:
 
In a message dated 2006/11/15, davewilley@............. writes:


>>My question for you all is will there be any noticeable magnetic interaction between the
>>3 units I need to worry about with stacking them (vertically) about .25 - .5 inches
>>apart inside the PVC tubing?


> Hi Dave Willey,.

>       The geophone units should have no magnetic interactions when stacked together.
>However, you do need to connect them fairly accurately at right angles
>and they should be clamped in place. Can you provide an inner rod / tube of plastic and
>bore holes in this to mount / align the geophones?

Yes.  I am planning use a mill with a horizontally mounted index head so as to mill at exact 90 degrees a hole perpendicular in a slightly larger (1.5Ē) PVC pipe, 5 inches long. (What youíre calling an inner tube.)  Iíll be lightly clamping them with set screws to keep the geophones stable.
 
The inner tube with the geophones is to be glued into the inside of a 2.5 inch threaded plug that will screw into a matching female connector on the bottom and glued to a short length of 3Ē pipe which acts as the over-all cover. The outer cover has a slip cap on top with a machine screw eye that will come out the top.  Coax cables will come out the bottom of the plug and to the surface. Coax is covered with PVC so I should be able to glue everything together and have it waterproof.
 
>      The 4.5 Hz geophones do have stray fields at both ends, but the barrel is the mild-
>steel return path for the axially mounted central magnets. I would suggest a 1 cm spacing.
>
 >      You can get Roberts' type period extension amplifiers from www.sara.pg.it
>These should enable you to receive teleseismic P and S waves, which you won't other-
>wise see with 4.5 Hz geophones. See the note about 4.5 Hz geophones on Larry's website?

Are you referring to the 3 channel amp boards (SEMP30) and equalizer boards (SEQUA10)? If so, will the eq board work in front of Larryís 4 channel 16-bit preamp board?  (Which I already have.)   I could hang this EQ board on the unused 4th channel for teleseismic work.  That would be a neat setup if they play together.

>       When you have remounted the geophones, do make a record of the wire colours
>and that they are the correct way around for sensing E, N and UP signals with the
>correct polarity. Paint / Mark lines on the exterior of the case with the correct directions.

Yep.  Having been a (semi retired) electronic tech at True Time w/ Keith Paya Iím well aware
about watching the polarity as to not be ultimately 90 or 180 degrees out of phase with the direction of the movement.

>       Are you going to solder the metal film damping resistors and any RF capacitors across the
>geophone terminals? Remember to check the input resistance of your amplifier and use it in the
>calculation for the damping resistor.
Iím assuming since I bought everything from Larry several years ago as a ďturn-keyĒ system that this has been all done.  Itís been playing fine on the back concrete patio for 1.5 years.  Now itís time to wrap up the test phase of this experiment and get it in the ground away from the house and the roads.

>   How about protection diodes? Do you get severe thunder storms in your area? Is the soil
>sandy / rocky / dry? It is over many areas in the USA and this can give problems with local
>lightning. Your house and system may all be exposed many feet above the local 'Earth'
>conducting plane.

This Iíll have to check on.  I donít think there are any clamping diodes. Iíll put some a silicon version of a 1N914 diode
ďnose-to-tailĒ across the cables center conductors and coax shield/case ground.  Good thought.

 >      The outer case must be hermetically sealed and the cable end also sealed. Some cable
>types allow air to pass through between the wires when under pressure. Also seal the amplifier
>end of the cable. I suggest that you add a bag of inert silica gel drying crystals inside the case
> to absorb any moisture.

Iíll check with Belden Cable about Plenum grade RG-174 and see how impervious to water vapor it is.
 
Before True Time, I worked at Lemo connectors.  They have several lines of space-rated and deep oil exploration-grade connectors.  They seemed very over-engineered for my needs. So the connectors should stay watertight.
 
I do like the idea of the silica gel as a last ditch effort to keep things dry inside.
 

>       Regards,

 >     Chris Chapman

 
One again Chris you (and the others on this list) have been a great resource of ideas. 
 
 
 
Thanks,
 
 
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