In a message dated 2006/11/16, ian@........... writes:

    > I'm not convinced that you need to match the period of a plum bob to 
> the seismic motion. A plum bob will follow the tilt even if it takes 50 seconds 
> to reach its maximum. At the end of the 50 seconds, it will still be 
> pointing to the centre of the Earth. On top of that you will have the natural period 
> oscillation but this can be filtered out to get the mean value. 

Hi Ian,

       Are you talking about tilt responses which tend to be dominant for 
periods over ~300 sec? At periods below this the tilt signal of a seismic wave is 
minor. 
       Otherwise you have a pendulum system, which if correctly damped will 
give an output with velocity above the resonant period, but falling as f^2 
below it.

       > The real value of having a long plum bob is that the amount of 
> displacement of the end weight is much larger and much easier to measure. It also 
> helps to reduce the significance of mechanical imperfections of the mount at 
> the other end.

       The problem with large pieces of equipment is keeping the air column 
stable. You could evacuate a tubular vertical column, but it would be costly 
and the column could still flex with temperature.

       There need not be a sensor sensitivity problem. Photocells can resolve 
about 15 nano metres, which is likely to be below the background noise. You 
can get down below 1 nm with variable capacitance methods

       Regards,

    Chris Chapman
In a me=
ssage dated 2006/11/16, ian@........... writes:



    
I'm not conv=
inced that you need to match the period of a plum bob to the seismic motion.=
 A plum bob will follow the tilt even if it takes 50 seconds to reach its ma=
ximum. At the end of the 50 seconds, it will still be pointing to the centre=
 of the Earth. On top of that you will have the natural period oscillation b=
ut this can be filtered out to get the mean value. 




Hi Ian,



       Are you talking about tilt responses wh=
ich tend to be dominant for periods over ~300 sec? At periods below this the=
 tilt signal of a seismic wave is minor. 

       Otherwise you have a pendulum system, w=
hich if correctly damped will give an output with velocity above the resonan=
t period, but falling as f^2 below it.



       
The real value of having a long plum bob is that the amount of displac=
ement of the end weight is much larger and much easier to measure. It also h=
elps to reduce the significance of mechanical imperfections of the mount at=20=
the other end
.



       The problem with large pieces of equip=
ment is keeping the air column stable. You could evacuate a tubular vertical=
 column, but it would be costly and the column could still flex with tempera=
ture.



       There need not be a sensor sensitivity=20=
problem. Photocells can resolve about 15 nano metres, which is likely to be=20=
below the background noise. You can get down below 1 nm with variable capaci=
tance methods



       Regards,



    Chris Chapman