PSN-L Email List Message

Subject: Re: New Lehman on line (almost)
From: ACole65464@.......
Date: Mon, 30 Sep 2002 22:04:30 EDT


In a message dated 10/01/2002 12:14:06 AM !!!First Boot!!!, 
ChrisAtUpw@....... writes:


> In a message dated 30/09/02, shammon1@............. writes: 
> 
> >> The standard rule is to pull the boom back a few inches and let it go. 
>> The boom 
>> should loose 30% of its motion on each swing past center and come to rest 
>> in 3 1/2 swings.
> 
> Hi Steve, 
> 
>       I am puzzled as to where this *standard rule* is supposed to come 
> from? But using it will give you a quite seriously underdamped system! A 
> critically damped system experiences no oscillation at all. This is 
> inherent in the maths. 
>       This is important if you apply post processing to the recorded signal 
> with the assumption that it was critically damped to start with. It will 
> also give problems with the amplitudes and frequencies calculated in FFT 
> displays and may 'smear' P and S wave recordings. 
>       A procedure to get critical damping could involve deflecting the beam 
> a very small amount (microns) and recording the amplifier output. You 
> progressively increase the damping until the arm just returns to the 
> balance position without having crossed the zero line. If you increase the 
> damping further, the arm will simply take longer to get back to zero. If 
> you use huge deflections like a few inches, you are likely to encounter non 
> linear effects which do not apply to the tiny (hopefully!) signals that we 
> normally record.   
>       It is helpful if the recording displays just what the earth is doing. 
> It is really not helpful if the system adds an oscillating tail to every 
> transient. 
> 
>       Regards, 
> 
>       Chris Chapman 

Steve, 

In support of what Chris has stated, please go to:  
http://www.seismo.com/msop/msop79/inst/inst4.html#aa250  Go to section 4.5 
for a text description, and then click on figure 4.5.1a to see how pendulums 
are supposed to be damped. About Critical is the response you should obtain. 
I hope this helps a little, the diagrams may not make much sense at first but 
it shows how professional instruments (electromagnetic, aka Lehman designs) 
are adjusted.

Regards,

Allan Coleman 
In a message dated 10/01/2002 12:14:06 AM !!!First Boot!!!, ChrisAtUpw@....... writes:


In a message dated 30/09/02, shammon1@............. writes:

The standard rule is to pull the boom back a few inches and let it go. The boom
should loose 30% of its motion on each swing past center and come to rest
in 3 1/2 swings.


Hi Steve,

      I am puzzled as to where this *standard rule* is supposed to come from? But using it will give you a quite seriously underdamped system! A critically damped system experiences no oscillation at all. This is inherent in the maths.
      This is important if you apply post processing to the recorded signal with the assumption that it was critically damped to start with. It will also give problems with the amplitudes and frequencies calculated in FFT displays and may 'smear' P and S wave recordings.
      A procedure to get critical damping could involve deflecting the beam a very small amount (microns) and recording the amplifier output. You progressively increase the damping until the arm just returns to the balance position without having crossed the zero line. If you increase the damping further, the arm will simply take longer to get back to zero. If you use huge deflections like a few inches, you are likely to encounter non linear effects which do not apply to the tiny (hopefully!) signals that we normally record.  
      It is helpful if the recording displays just what the earth is doing. It is really not helpful if the system adds an oscillating tail to every transient.

      Regards,

      Chris Chapman


Steve,

In support of what Chris has stated, please go to:  http://www.seismo.com/msop/msop79/inst/inst4.html#aa250  Go to section 4.5 for a text description, and then click on figure 4.5.1a to see how pendulums are supposed to be damped. About Critical is the response you should obtain. I hope this helps a little, the diagrams may not make much sense at first but it shows how professional instruments (electromagnetic, aka Lehman designs) are adjusted.

Regards,

Allan Coleman

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