INFILTEC: How to Build an Inexpensive Seismometer - make seismograph kit
Geophone | Hackaday
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ASTM D7400-08 and ASTM D4428 Borehole 3D Geophone Downhole 3c Geophone Crosshole Geophones for Gea 24 Seismograph - China Borehole Geophone, Land Geophone | Made-in-China.com