Dick Webb,

>  I agree that I 
>  could filter or band pass most of the noise.  My interest in combining the 
>  seismometer outputs arose mainly because I have the 3 seismometers and I 
>  wanted to get the most from them. If it is possible to combine the 
>  outputs after signal processing, why would it not be possible to do it 
with 
>  the raw signals.... ?

    Sure you can average the voltages OK. No problem if all the instruments 
have roughly the same response and are only a few hundred feet apart. You 
need to check that no one is much noisier than any other, or it's noise will 
dominate the output and also that they all respond to a distant quake with 
the same polarity signal. If the noise is random you might get a reduction of 
1/root3 i.e. ~ 42% reduction. I suggested filtering / recording environmental 
noise as this can give a much greater reduction and may allow you to identify 
peaks in the 'earthquake' recording caused by environmental noise which might 
otherwise be more difficult to interpret. The quick way to find out is to 
hitch up another OP-AMP..... but don't expect to see a large difference.

    Regards,

    Chris Chapman
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