PSN-L Email List Message

Subject: Re: Homade Geophone
From: John Popelish jpopelish@........
Date: Sat, 06 Aug 2005 00:15:40 -0400

Geoff wrote:
> 1. It seems to me that a cylindrical magnet inserted into the center a coil
>   dangling freely might be the best way to go 
>   but it needs to be a very powerful magnet and have no more
>   then 1mm of slop in any direction.

The cylindrical magnets in geophones have a voice coil type winding 
around each end, , one wound each way, and connected in series.  Then 
the coils are surrounded by an iron pipe that carries the flux 
passing radially out through one coil over to pass radially in through 
the other coil.  The magnet and pipe are connected outside the ends of 
the coils.  The coils are supported by spring structures at both ends. 
  In a pendulum version, the pendulum structure replaces the support 

> The question is what dampening 
> device to use is best
>   I have found silicon oil of about 50WT should do nicely
>   but trying to get your hands on silicon oil is like impossible
>   in small quantities.

Such oil is used in copiers and laser printers as fuser oil.  You can 
find that in many office supply places and on Ebay.

> 2. I have found most noise sources do not look anything like a seismic 
> signal
>   if the noise is infrequent you can filter out such things
>   with your own human judgment.
>   This is saying though that people are not out there
>   intentionally trying to induce an EQ signal using noise
>   into your particular system.
> 3.  I am interested in knowing the cheapest way possible to
>    build your own Geophone that consistently receives
>    teleseismic signals over a long period of time
>    The Geophone companies want like $3000 for a single
>    vertical 1Hz Geophone...surely us amateurs can build a simple one
>    for a Lab setting < opposed to a field setting > for
>    much less like $20.

The magnets in my geophone type coil structure cost me about $50 on 
Ebay (4 of 1" diameter by 1/2 inch long and 16 of 1/2 inch diameter by 
1/2 inch long neodymium iron boron), and the iron pole pieces that act 
as the the iron sleeve (but designed to contain most of the flux at 
the ends of the structure) cost me $100 to have machined.  But the 
flux is fierce.  I have to use screw jacks to assemble the magnet 
structure around the coils.  This thing bits!

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