In a message dated 13/07/06, nicholas.ward@......... writes:

> Great,
> Thanks for the info.
> N
> 
> On 13 Jul 2006, at 10:10, james fisher wrote:
> 
> > Hi, a piezo disk will pick up the higher frequencies
> > you are interested in. use a 100 gram weight on top of
> > the piezo disk and bury in a 1' deep hole to isolate
> > disc from direct sound pickup. Superglue a strong
> > magnet to disk and stick on a iron rod driven into the
> > ground. Insulate everything well. See past posts on
> > piezo disks. They really work well for picking up
> > seismic waves as well as sound waves.

Hi Nicolas,

       You will need FET input opamp TL071? to match the high input impedance 
of the piezo disk.
       Superglue will not last long; a modified two part acrylic like 
Holdtite ST3295 is much better. It is tough and doesn't crack. 
       Can you stick the head of a 3 mm brass bolt to the centre of the piezo 
disk and screw a 20 gm disk of brass on top? A high weight like 100 gm is OK 
for very low frequency seismic signals, but it lowers the resonance of the 
disk to maybe 200 Hz.

       Regards,

       Chris Chapman 
 
In a me=
ssage dated 13/07/06, nicholas.ward@......... writes:



Great,

Thanks for the info.

N



On 13 Jul 2006, at 10:10, james fisher wrote:



> Hi, a piezo disk will pick up the higher frequencies

> you are interested in. use a 100 gram weight on top of

> the piezo disk and bury in a 1' deep hole to isolate

> disc from direct sound pickup. Superglue a strong

> magnet to disk and stick on a iron rod driven into the

> ground. Insulate everything well. See past posts on

> piezo disks. They really work well for picking up

> seismic waves as well as sound waves.




Hi Nicolas,



       You will need FET input opamp TL071? to=
 match the high input impedance of the piezo disk.

       Superglue will not last long; a modifie=
d two part acrylic like Holdtite ST3295 is much better. It is tough and does=
n't crack. 

       Can you stick the head of a 3 mm brass=20=
bolt to the centre of the piezo disk and screw a 20 gm disk of brass on top?=
 A high weight like 100 gm is OK for very low frequency seismic signals, but=
 it lowers the resonance of the disk to maybe 200 Hz.



       Regards,



       Chris Chapman