Geoff,


>The best instruments use moving-plate capacitor sensors to detect 
>any microscopic movement of the mass and a coil-magnet arrangement 
>to push the mass back towards its center position.

You started me thinking about how good a capacitor position-sensor 
ought to be in order to reliably see distant quakes, and was a little 
startled by what I found.  What determines its sensitivity is its 
self-generated noise.  You can easily add gain to make a sensor more 
sensitive, up to the point that its own noise becomes 
important.  That's what will define its maximum sensitivity.

On very quiet days, there will sometimes be periods of time where our 
instrument output is moving around by only one count.  With the A/D 
setup we use, that represents an apparent velocity variation of 10 nm 
/ second.  We usually use a filter centered at .04 or .05 Hz with a 
bandwidth of .06 Hz, when making those observations.  To convert 
velocity to displacement you divide the velocity by (2 Pi * 
frequency), which says that the peak displacement noise is less than 
roughly 40 nm in the band centered at 0.04 Hz.  We can't tell whether 
that is still mostly ground noise or is instrument noise, but we can 
say that the "peak" sensor displacement noise over that frequency 
band must be 40 nm or less.

40 nm is about 1/10 the wavelength of violet light.

I need to keep reminding myself how incredibly small the earth 
motions are that we are observing.

Regards,
Brett



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