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


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