Hi gang,
  Ad in Nutz & Volts:
  Geophone, 1.6" hi, 1.2" dia., made by Geosource, $7.95.  The resonant
freq. is not mentioned.
 www.gatewayelex.com
   Their web site offers (in Gadgets) only a kit (which incls. the
geophone) for $9.95.  Maybe a 'phone call to 1-800-669-5810 will get the
geophone by itself.
Bob
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