Seismograph Hole Plug?

[muerte33]

Can someone tell me if this is old school seismograph hole plugs (geological survey crew holes)?

I have found 3 of these buried on family land so far.

 

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[duggap]

Soil testing perhaps.

http://www.google.com/url?sa=t&rct=...Co66LyhVHAbEgc5GA&sig2=aE6St-hTbyJfE_5GsGUmcA

 

[muerte33]

Note the hole in the rim of the cap (middle picture) so that maybe they can insert their fuse?
The newer ones are plastic now from what I see on the internet, but these are rusty and old school.
The holes are pretty deep. I have not measured one yet though (probably would hit water).

 

[relic lover]

They drill holes like that around my area to look for coal. never saw a cap on one they just let them open around here