## PSN-L Email List Message

Subject: Re: plum bob seismometer
From: ian ian@...........
Date: Thu, 16 Nov 2006 09:23:34 +0000

```Hi,

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.

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.

Ian

ChrisAtUpw@....... wrote:

>
>        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
> mineshaft). My 120 ft well would only give me about 12 secs and I
> don't fancy the climb.

Hi,

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.

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.

Ian

ChrisAtUpw@....... wrote:

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.

```