The traffic lights use a buried loop of cable at the intersection.  This 
inductive loop
changes its inductance value when the large metal body/chassis mass of the
vehicle is placed  over it.
   That change of inductance changes the oscillator freq which is 
detected.  We
(the company I work for) uses the same system for detecting cars passing 
through
  the lanes of fast food drivethroughs.

not really sure how you could apply it to seismology in an easy way.  Its 
not overly
compact

Dave




At 02:24 AM 14/04/2011, you wrote:

>Could someone point me in the right direction to view the wiring and 
>schematic diagrams of the FM or AM seismic sensors? I am curious to see 
>how they operate. Keep in mind the following: Automatic traffic signals 
>are using similar concepts to detect presence when a car waits at an 
>intersection. Notice the wires lines on the pavement, they lead to a box 
>at the intersection which times the traffic signals.
>
>
>
>Jim
>
>

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