Ed,

I agree with the suggestion that the leveling feet of your seis have
to be free of the carpet. Recall that a horizontal sensor is very
sensitive to tilt, so good contact with a solid foundation is essential.
Stresses in the carpet would tend to drag the seismometer around.
If you cut out a square piece of the carpet, be sure to save it in
case it has to be replaced later.

Large seismometers often use a rounded contact point for the seismometer
feet: it is usually used in conjunction with small glass plate placed 
under it on the pier. The glass minimizes lateral torqueing as the screw 
is turned, and also allows the contact point to slide freely as the base 
expands with temperature.  Disadvantage: a careless move or nasty quake 
can slide the foot of the seis off of the glass plate. Smooth >Glossy< 
glazed ceramic tiles will also work. I use them for the top surface of 
small piers. They can be grouted or thinly epoxied in place, or just
simply "ground in" by rotating them under pressure to make a consistent
contact with the concrete if the surface is smooth enough.

And remember to orient your horizontal seis with the mass movement 
parallel to the adjacent foundation wall, (ie. the boom at a right angle
to the wall) so as to minimize tilting from loading and warping of the
basement floor.

Regards,
Sean-Thomas
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