In a message dated 2006/11/15, allane@............. writes:

> Has any one on the list experimented with a blumbob as a seismo and
> have had any success with it?
> Thanks in advance , Allan Egleston kf6uxj

Hi Allan,

       Only with a ~comparable type of system. You would probably need to use 
two sets of foils at right angles for the suspension to prevent the bob from 
rotating. You would need to damp the swing in two directions at right angles 
and provide two sensitive detectors which do not interact significantly. These 
might be tungsten bulb + lens + 2x Si differential photodiodes type? Maybe two 
coils on the bob with magnets on the baseplate? Maybe capacitor plates?
       I would say possible, but not an easy task and that two pendulum 
systems with rigid suspensions at right angles would likely give better + easier 
results. The period of a pendulum is proportional to the square root of the 
length. A 1m pendulum is ~ 2 sec. which is OK for P and S waves. To get 20 sec 
with a simple pendulum you would need 100 metres = 328 ft, which is likely to 
present practical constructional problems (even if you have access to a disused 
mineshaft). My 120 ft well would only give me about 12 secs and I don't fancy 
the climb. 
       Long period seismic pendulums are usually of the garden gate, or of 
the folded type. It is relatively easy to get periods of 20 sec or longer, to 
allow you to detect the larger long period Love waves as well as the P & S 
waves. It is also possible to apply electronic feedback to a 'simple' 1 sec. 
pendulum to greatly extend the period - see Shackleford-Gundersen references on psn. 
It is also possible with a precision capacitance measuring system and a 24 
bit ADC to sample and then electronically integrate to greatly extend the 
period.  

       Regards,

       Chris Chapman
In a me=
ssage dated 2006/11/15, allane@............. writes:



Has any one on the list experim=
ented with a blumbob as a seismo and

have had any success with it?

Thanks in advance , Allan Egleston kf6uxj




Hi Allan,



       Only with a ~comparable type of system.=
 You would probably need to use two sets of foils at right angles for the su=
spension to prevent the bob from rotating. You would need to damp the swing=20=
in two directions at right angles and provide two sensitive detectors which=20=
do not interact significantly. These might be tungsten bulb + lens + 2x Si d=
ifferential photodiodes type? Maybe two coils on the bob with magnets on the=
 baseplate? Maybe capacitor plates?

       I would say possible, but not an easy t=
ask and that two pendulum systems with rigid suspensions at right angles wou=
ld likely give better + easier results. The period of a pendulum is proporti=
onal to the square root of the length. A 1m pendulum is ~ 2 sec. which is OK=
 for P and S waves. To get 20 sec with a simple pendulum you would need 100=20=
metres =3D 328 ft, which is likely to present practical constructional probl=
ems (even if you have access to a disused mineshaft). My 120 ft well would o=
nly give me about 12 secs and I don't fancy the climb. 

       Long period seismic pendulums are usual=
ly of the garden gate, or of the folded type. It is relatively easy to get p=
eriods of 20 sec or longer, to allow you to detect the larger long period Lo=
ve waves as well as the P & S waves. It is also possible to apply electr=
onic feedback to a 'simple' 1 sec. pendulum to greatly extend the period - s=
ee Shackleford-Gundersen references on psn. It is also possible with a preci=
sion capacitance measuring system and a 24 bit ADC to sample and then electr=
onically integrate to greatly extend the period.  



       Regards,



       Chris Chapman