Geoff,

The guys who build most of the seismographs have found that 
capacitance sensors generally worked the best.  However, in order to 
make them sufficiently sensitive, the spacing between the plates 
needs to be very small.  In his latest FBV design Dave is using 
0.015" plate spacing.  But, doing that requires you to have some sort 
of active feedback mechanism to keep the plates from moving.

What you do is take the output of the capacitance sensor circuit, 
amplify it a whole lot and feed it back to a coil/magnet which pushes 
in the right direction to force the boom to stay centered.  Doing 
that means that the boom will barely move when a quake comes along, 
so the plate gap hardly changes at all.

Then, you take the electical signal that the loop generated to resist 
the boom motion, modify it a bit to get it to represent ground 
velocity, maybe amplify some more and presto, you have a very 
sensitive velocity-responding instrument.

In practice, amplifying the signals is easy, the hard part is to 
design the electronics and mechanism to have extremely low 
internally-generated noise which is really the limiting factor when 
considering sensitivity.

Regarding the optical lever.  To be equivalent to what the FBV's see, 
which generally is < M 5.0, if the quakes are pointed in the right 
direction, you need 10 pixels per 8nm of ground displacement.  Using 
a 3Mp 1/3" camera chip (~4.9E-6 m/row), you'd need a ratio of about 
6220.  But by *far*!! the biggest problem will be keeping the light 
beam from wandering off the chip.  Also the instrument response would 
probably be flat to displacement, which tends to accentuate the high 
frequencies (mostly noise) at the expense of the slower waves you are 
looking for.

Regards,
Brett

At 12:08 PM 3/28/2013, you wrote:
>Let us say we are looking for EQ at M5 or above from anywhere in the world
>and not the 4.5 of a Richter device.
>How much mechanical advantage must we have to detect this
>motion using a simple lever arm and lets say bouncing a laser light 
>between two
>surface silvered mirrors ?
>I think you might get a mile of mechanical advantage this way.
>Talking about increase of motion and not the force of a lever.
>The ratio between the two ends of the fulcrum is of
>what I speak for mechanical advantage of motion.
>All the amplification happens, with this ratio.
>Two opposed mirrors are a decent way to keep things relatively small.
>Like sitting at the Barber shop looking at your reflection
>fading into the distance many times over.
>I just did this very same thing with my web camera
>pointing into the monitor.

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