## PSN-L Email List Message

Subject: Re: plum bob seismometer
From: ian ian@...........
Date: Thu, 16 Nov 2006 21:28:55 +0000

```Hi,

firstly, I'm not a seismologist/geologist so I could be talking
total...  I'm assuming that any seismic activity will cause a localised
tilt of the ground.  So the plum bob isn't being used as a pendulum but
as a tilt meter.

Referring back to some emails from Sean-Thomas on the psn email archive,
circa 2000, he provided a formula for the relationship between
telesiemic quake magnitudes and ground tilt.  In the example discussed,
I think an Ms 6.2 produced a local ground tilt of 100 micro radians.
Doing a quick "back of an envelope" calculation, a 1 meter long plum bob
would move by about 100 nano meters at the tip.

You could get one of those glass columns (from somewhere!) to house it
in and seal it.  You might employ some form of galvanic spot meter type
optics to magnify the movement.

It all starts to sound like more effort than a standard Lehman...

Cheers

Ian

ChrisAtUpw@....... wrote:

> 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

Hi,

firstly, I'm not a seismologist/geologist so I could be talking
total...  I'm assuming that any seismic activity will cause a localised
tilt of the ground.  So the plum bob isn't being used as a pendulum but
as a tilt meter.

Referring back to some emails from Sean-Thomas on the psn email
archive, circa 2000, he provided a formula for the relationship between
telesiemic quake magnitudes and ground tilt.  In the example discussed,
I think an Ms 6.2 produced a local ground tilt of 100 micro radians.
Doing a quick "back of an envelope" calculation, a 1 meter long plum
bob would move by about 100 nano meters at the tip.

You could get one of those glass columns (from somewhere!) to house it
in and seal it.  You might employ some form of galvanic spot meter type
optics to magnify the movement.

It all starts to sound like more effort than a standard Lehman...

Cheers

Ian

ChrisAtUpw@....... wrote:
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

```