Richard,

I don't know much about the inner workings of automobile charging 
circuits, but I can hazard a guess.

First, your coil is designed to be a magnetic field sensor.  It is 
very sensitive to changing magnetic fields, so it's a good guess that 
the pulses you are seeing are magnetic.  Perhaps as the engine is 
coasting to a stop or starting up, for a few seconds the alternator 
is turning too slowly to put out adequate voltage, so its regulator 
will try unsuccessfully to turn up its voltage up much as it can.  It 
could be that in that situation the alternator, for a moment, 
broadcasts a relatively strong, pulsing magnetic field.  However 
that's all just speculation.

You could try taking a coil similar to your sensor and hold it near 
the alternator while turning off the engine to confirm that's where 
the problem is coming from.

One way to shield the coil would be to surround it with a box of a 
magnetic material, like steel sheet metal (not aluminum), which would 
of course need to have holes for parts of your instrument to go 
through.  You could also use copper if its joints were well soldered 
and it surrounded the coil fairly completely, but steel would 
probably be simpler.  If you want the very best you could use some 
magnetic shielding alloy like mu metal, though that's hard to find, 
hard to fabricate, and is quite expensive, and besides it might not 
be happy being close to strong magnets.

Lead wouldn't do much except to block a bit of the normal background 
radiation--definitely not your problem.

Or you could try locating the seismo farther away from the car engine.

Good luck,

Brett

At 06:11 PM 1/5/2011, you wrote:

>Situation:  High amplitude spikes when starting and again on 
>shutdown of engine from automobile.
>
>  Sensor is about 3 feet from car.
>
>EMF appears to come from auto alternator since I can see it upon 
>engine shutdown.
>
>Any suggestions would be appreciated to mitigate the EMF from the 
>sensor pickup coil without relocating it.
>
>  How about a lead shield?  The kind the dentist uses for X-ray's.


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