Quick Test of Geophone Response

I just wanted to post a quick article about geophones.  Geophones are essentially instruments that allow us to measure the velocity or acceleration of the ground.  Yes, seismometers do this, but generally when we refer to geophones we are talking about single sensor (almost always vertical sensing) devices used for seismic imaging in oil/gas exploration.  I've talked about seismic surveys before (here for example). The "element", or the actual sensor is pictured below.  These sensors have a magnetic element on a spring inside a coil of wire.  Motion of the magnet (resulting from ground motion) generates a small electrical potential in the coil.  If I can find a cheap element/case on eBay I'll do a teardown of one in the future.  The signal generation happens through a process called "electromagnetic induction", described by Michael Faraday in 1831!  Want to know more about induction? Head over to the wikipedia page or shout out and we can put together a demonstration.

Dr. Ammon, whose office is next door, brought over an old element that he wanted to compare with our seismometers in the basement of the building.  Not knowing the output voltage range well, we hooked it up to a Rigol DS1102E oscilloscope on my desk.  I set the trigger of the oscilloscope (when it started collecting data) to just above ground potential so that any appreciable motion will trigger data recording.  We recorded the voltage output of the sensor about 6800 times per second!

The sensor element from a geophone.  (Image: Ebay)

Below is the waveform collected from hitting my desk with moderate force.  Surprisingly these elements put out +/-4 Volts! When shaking the element to it's limits we were seeing voltages of around +/- 10 Volts.  To me this indicates there are many turns in the coil and a very strong, probably rare earth, magnet inside.  Measurement of the coil resistance or a teardown will tell if this is correct! I've also included the power spectral density for those of you interested.  These figures tell us about the frequency response of the instrument.  Depending on how the spring system is setup, the oscillator is very sensitive to some frequencies and not so sensitive to others.  These diagrams help us characterize this response.  

Collected waveform from hitting my desk.
Power Spectral Density

Power Spectral Density: Zoomed in

Sorry for the short post, but I just wanted to share a quick desktop experiment!

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