In a message dated 23/10/05, lehmancj@........... writes:

> Subj:Re: 15 to 20 second free swing
> From:    lehmancj@........... (Connie and Jim Lehman)
> 
> Hello Geoff--The 360 Degree technique sounds good here. A 10:1 ratio no
> doubt is an ideal. I guess when you get "tighter" than that, an event
> readout is compressed in size. It would be neat (for comparison purposes)
> to be able to have different ratios exhibited by the same sensor on the same
> event!!  Thanks--Jim

Hi Geoff,

       There is a basic misunderstanding here. You look at successive half 
swings to define the damping. Say you deflect the boom gently to 10 mm and 
release it. The 10:1 would give you a movement past zero to 1 mm. This should be 
about right. It should then return to zero, but without going through zero for 
another swing!
       If you are getting two or more swings, you are seriously underdamped. 
       If you look at an underdamped response, it is flat for the higher 
frequencies, then rises to a peak and then falls off very fast. Call 
http://www.geospacelp.com/c_hs10.shtml The A curve is underdamped, The B curve is about 
what we are looking for. You usually aim for about 70 to 80% of CRITICAL damping 
- the critical damping response is curve C. 
       If your seismometer is seriously underdamped, sure it will give a 
bigger signal, but this will only relate to it's OWN oscillatory characteristic, 
NOT to the ground motion that you want to measure. The output only starts to 
fall seriously when the seismometer is overdamped.
       See http://www.jclahr.com/science/psn/wbauer/damped/d.htm for a 
seismometer applet that gives a range of characteristics that you can choose. Read 
to the bottom of 
http://www.jclahr.com/science/psn/epics/links.html#Seismometer%20Basics to get typical values.

       Regards,

       Chris Chapman

> ----- Original Message -----
> From: "Geoff" 
> Subject: Re: 15 to 20 second free swing
> 
> > I do not mean to butt in it is just I am following somewhat the things you 
> say here and I want to clarify a point on damping.
> >
> > Until now I have been judging damping in cycles of 360 degrees meaning I 
> judge my damping by looking only at the positive or negative half cycle.
> > Say I let loose with a large swing on the positive half cycle I will then 
> measure >the first and second negative swings and get my damping from those 
> two peak >values.
> >
> > Say it swings ten divisions and then 360 degrees later it swings one 
> division
> > I then say my damping is 10:1.
> >
> > I have been told by a Geophysicist named  Willis Jacobs at Golden Colorado 
> > that a 10:1 damping is desirable to shoot for.
> > Any comments in regard to this?
> >

In a message=20=
dated 23/10/05, lehmancj@........... writes:



Subj:Re: 15 to 20 second=
 free swing

From:    lehmancj@........... (Connie and Jim Lehman)



Hello Geoff--The 360 Degree technique sounds good here. A 10:1 ratio no

doubt is an ideal. I guess when you get "tighter" than that, an event

readout is compressed in size. It would be neat (for comparison purposes=
)

to be able to have different ratios exhibited by the same sensor on the=20=
same

event!!  Thanks--Jim




Hi Geoff,



       There is a basic misunderstanding h=
ere. You look at successive half swings to define the damping. Say you defle=
ct the boom gently to 10 mm and release it. The 10:1 would give you a moveme=
nt past zero to 1 mm. This should be about right. It should then r=
eturn to zero, but without going through zero for another swing!

       If you are getting two or more swin=
gs, you are seriously underdamped.=20

       If you look at an underdamped re=
sponse, it is flat for the higher frequencies, then rises to a peak and then falls off very fast. Call http://www.geospacelp.com/c_hs10.shtml=
 The A curve is underdamped, The B curve is about what we are looking for. Y=
ou usually aim for about 70 to 80% of CRITICAL damping - the critical dampin=
g response is curve C.=20

       If your seismometer is seriously un=
derdamped, sure it will give a bigger signal, but this will only relate to <=
U>it's OWN oscillatory characteristic, NOT to the ground motion that=20=
you want to measure. The output only starts to fall seriously when the s=
eismometer is overdamped.

       See http://www.jclahr.com/science/p=
sn/wbauer/damped/d.htm for a seismometer applet that gives a range of charac=
teristics that you can choose. Read to the bottom of http://www.jclahr.com/s=
cience/psn/epics/links.html#Seismometer%20Basics to get typical values.



       Regards,



       Chris Chapman



----- Original Message -=
----

From: "Geoff" <gmvoeth@...........>

Subject: Re: 15 to 20 second free swing



> I do not mean to butt in it is just I am following somewhat the thi=
ngs you say here and I want to clarify a point on damping.

>

> Until now I have been judging damping in cycles of 360 degrees mean=
ing I judge my damping by looking only at the positive or negative half cycl=
e.

> Say I let loose with a large swing on the positive half cycle I wil=
l then measure >the first and second negative swings and get my damping f=
rom those two peak >values.

>

> Say it swings ten divisions and then 360 degrees later it swings on=
e division

> I then say my damping is 10:1.

>

> I have been told by a Geophysicist named  Willis Jacobs at Gol=
den Colorado=20

> that a 10:1 damping is desirable to shoot for.

> Any comments in regard to this?

> gmvoeth