I just discovered geophones by watching the video.
Check it out, Geophones, it's the latest. You can make one by hanging a magnet in a rubber band, and let it dangle in a coil. Then magnify the output from the coil. I think.
https://www.youtube.com/watch?v=8NF1129yjwY
Mass/spring resonators
Your magnet is a mass, the rubber band is a spring. You just described a mechanical resonator. The ratio of springiness to mass determines the specific frequency of the resonator. Don’t ask me or the formula. I just sound clever, rather than be so.
If your home made geophone is to work, make sure to tune your resonator to the frequency of the sounds you’re hoping to pick up. Probably in the same range the human ear can hear. Probably the lower end of that range.
Or hire a Native American to put his ear to the floor and have him tell you what’ going on down there…
WOW they are sensitive! with
WOW they are sensitive! with these you could make an electronic drum set for YDM i think. 
also with leds attached they look very sensitive http://www.youtube.com/watch?v=jwLBzCzLcpA&feature=related Imaging to trigger a digital camera, and shoots pictures of a water drop falling down for example. that will look amazing I think.
Good Vibrations
Thanks, might help me locate the rats burrowing under the chicken pen.
Hmmm, now how to hook it up to the RaTenator-2000…
Check the Josephson effect
Check the Josephson effect and SQUID’s, I’m sure they’ll show you ET’s barried in the earth also 
