Erich,
 
>> Any pulse you record by droping water or beads records too much of the 
environment around the geophone.

    Are you saying that a hydraulic shaker table can't be used to determine 
the overall response of a geophone?

    I wasn't proposing to drop water or beads. Injecting one ml of water into 
a li-lo cushion would move a geophone placed on top upwards by about 10 
microns. It can easily be made a lot less.  

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