## PSN-L Email List Message

Subject: Re: serious pendulum misconceptions
From: ChrisAtUpw@.......
Date: Tue, 19 Feb 2008 19:52:31 EST

```In a message dated 2008/02/19, PETERS_RD@.......... writes:

> Just because a physical pendulum has a long period does not mean it is
> useful as a seismometer! For two decades I have been using such an instrument to
> study internal friction. The period of such a pendulum approaches very long
> values (easily beyond 20 s), by causing the center of mass toget ever closer
> to the axis of rotation.  The way this is done, of course, is to put mass
> above the axis as well as below it as in the simple pendulum  The long periods of
> oscillation are possible
> only for a structure that is very rigid, having a large quality factor in
> the absence of externally imposed damping.
>      The reason this long period pendulum is not a useful seismometer is
> very easy to understand from the physics of extended bodies.  When you apply a
> force to an extended body, as opposed to a point mass, the acceleration that
> results involves both translation and rotation.  If the force acts exactly
> through the center of mass, the result is strictly translation; i.e., rotation
> is not possible because the moment arm responsible for torque has vanished.
>     In the case of the pendulum, the acceleration of the case is equivalent
> (in terms of response) to a force in the opposite direction to the
> acceleration acting directly on the pendulum through the center of mass. As the center
> of mass approaches the axis, there is no torque with which to produce
> rotation.  Without rotation there is no response.  Thus the instrument is not a
> viable seismometer, even though it is a wonderfully useful tool for studying the
> influence of defect structures.

Hi Randall,

Understood.

>     What this demonstrates is just one more example of the critical need to
> understand conceptually the physics involved, if one is to build a useful
> instrument.  That physics continues to be 'clouded', even by the 'analogy' you
> mention Chris--about pucks on a table.  The demonstrations that you observed
> were cases
> (as appropriate to the discussion of seismic behavior) in which the
> frequency character of disturbance was much higher than the natural frequency of the
> analogous seismometer (puck/spring arrangement).  The unit was therefore
> functioning as the 'vibrometer' that I mentioned earlier.

My fault. I had not picked up that you were referring to a seismometer
when the excitation frequency was below resonance, but were calling it a
vibrometer when the excitation frequency was above resonance.

The vibrometer works on the basis > of the fact (low eignfrequency of the
> instrument compared to frequency of acceleration disturbance) that there is
> insignifcant motion of the mass (puck) over the time intervals of external (case)
> disturbance.  This is not the regime for which are trying so hard to improve
> instrument performance.

OK

> That regime is at the opposite end of the excitation frequency spectrum.
> When the case of a seismometer is accelerated at very low frequencies of the
> earth's motion, there is absolutely no way one can think of the inertial mass
> remaining at rest! The mass is connected to something (whether spring or
> pendulum rod) that serves as
> a device to keep it centered in the case and which is responsible for the
> seismometer being a mechanical oscillator.  If it were not an oscillator, then
> there would be no reason to provide eddy current dampers using rare earth
> magnets.  Simply stated, the inertial mass MUST be part of an oscillator IF it
> is to be a seismometer.  If it oscillates, then the mass cannot remain at
> rest, and there is a repeat interval of time associated with the motion, called
> the period of oscillation.  The finite value of this period is what in turn
> causes an upper limit on the sensitivity that is governed by the square of the
> period of oscillation. The reason there is a limit to the amount of relative
> motion between mass and case (instrument sensitivity) is the FACT that the
> inertial mass DOES move. There is ABSOLUTELY no way it CANNOT!

Agreed.

>     On a different subject:

> Chris you mention what I believe to be indeed true - publishable results
> out to (and beyond) 2000 s could change a great deal in the world of
> seismology. It is a virtually unexplored regime. Let me give you an example. I was just
> yesterday looking at the differences between the N-S and E-W channels of my
> VolksMeter here in Macon. Because the concrete cylinder that is part of the
> monolithic pier goes 20 ft into the ground, there is a significant reduction
> in the thermoelastic tilt that is otherwise seen for instruments sitting on a
> slab on top of the earth.  What is really interesting about the two channel
> records, for 1 sample per minute over 24 h is the following.  Sometimes the
> two channels are almost completely correlated.  Whatever is tilting the pier is
> the same in both N-S and E-W direction.
>     But there are days in which this is not at all true.  A correlation plot
> shows fascinating loop-the-loops that seem to cycle over a period of several
> days.  Is this something local to middle Georgia, or does it have global
> features?  The answer to this important question can only be provided by
> networked sensors.  What I find remarkable is that a simple pendulum has the
> potential to do some experiments which cry out for data collection.  Anybody who
> believes that science is in the
> process of just 'tying up loose ends' to mature understand of nature
> (whether in physics or any other discipline) is hopelessly naive.  It is mind
> boggling to me the extent to which seismology has only 'scratched the surface' with
> regard to a true
> understanding of earth's complex motions. One of the reasons so little
> understanding has been gained derives from the 'delta function' mentality
> concerning sensor type for instruments.

Interesting.

Which was why I suggested siting an instrument at Eskdalemuir 55.3N
3.2W in the UK, about 81 degrees E from Macon 32.85N -83.68W, or at Walferdange
49.7N 6.2E in Luxembourgh at about 89 degrees E? Eskdalemuir is a good bit
further N than Macon.

But we still have to motivate the seismologists and get the long
period equipment to them. STS-1s are in short supply and STS-2s and Guralp CMG 3T
can be ordered in 360 sec version, but the Trillium is limited to 240 seconds.
They are all a bit short on period for Earth Eigenmodes. I haven't checked the
noise levels. The Scripps Institute seems to be junking their STS-1s??

I suspect that Volksmeters could well make excellent Tsunami
detectors, particularly off the west coast of the Americas where the major faults are
less than 500 miles offshore and the warning response times need to be just a
few minutes.They could pick up the tilt signals from vertical ocean floor
changes directly.  I am not sure about the likely deflection amplitudes required?
I have just been watching a TV program about the Cascadia fault off
Canada + USA. They were suggesting movement on the coast of up to a foot and a M
9 quake about 600 miles long. The last quake was in 1700 and the previous one
was about 300 years before that. This sounds too close for comfort.
In a me=
ssage dated 2008/02/19, PETERS_RD@.......... writes:

Just because a physical pendulu=
m has a long period does not mean it is useful as a seismometer! For two dec=
ades I have been using such an instrument to study internal friction. The pe=
riod of such a pendulum approaches very long values (easily beyond 20 s), by=
causing the center of mass toget ever closer to the axis of rotation. =
The way this is done, of course, is to put mass above the axis as well as b=
elow it as in the simple pendulum  The long periods of oscillation are=20=
possible
only for a structure that is very rigid, having a large quality factor in th=
e absence of externally imposed damping.
The reason this long period pendulum is not a usefu=
l seismometer is very easy to understand from the physics of extended bodies=
..  When you apply a force to an extended body, as opposed to a point ma=
ss, the acceleration that results involves both translation and rotation.&nb=
sp; If the force acts exactly through the center of mass, the result is stri=
ctly translation; i.e., rotation is not possible because the moment arm resp=
onsible for torque has vanished.
In the case of the pendulum, the acceleration of the case=
is equivalent (in terms of response) to a force in the opposite direction t=
o the acceleration acting directly on the pendulum through the center of mas=
s. As the center of mass approaches the axis, there is no torque with which=20=
to produce rotation.  Without rotation there is no response.  Thus=
the instrument is not a viable seismometer, even though it is a wonderfully=
useful tool for studying the influence of defect structures.

Hi Randall,

Understood.

What this de=
monstrates is just one more example of the critical need to understand conce=
ptually the physics involved, if one is to build a useful instrument. =20=
That physics continues to be 'clouded', even by the 'analogy' you mention Ch=
ris--about pucks on a table.  The demonstrations that you observed were=
cases
(as appropriate to the discussion of seismic behavior) in which the frequenc=
y character of disturbance was much higher than the natural frequency of the=
analogous seismometer (puck/spring arrangement).  The unit was therefo=
re
functioning as the 'vibrometer' that I mentioned earlier.

My fault. I had not picked up that you were referring to=20=
a seismometer when the excitation frequency was below resonance, but were ca=
lling it a vibrometer when the excitation frequency was above resonance.

The vibrometer works on the basis of the fact (low eignfrequency of the instrument compared to frequency o=
f acceleration disturbance) that there is insignifcant motion of the mass (p=
uck) over the time intervals of external (case) disturbance.  This is n=
ot the regime for which are trying so hard to improve instrument performance=
..

OK

That regime is at the opposite=
end of the excitation frequency spectrum.  When the case of a seismome=
ter is accelerated at very low frequencies of the earth's motion, there is a=
bsolutely no way one can think of the inertial mass remaining at rest! The m=
ass is connected to something (whether spring or pendulum rod) that serves a=
s
a device to keep it centered in the case and which is responsible for the se=
ismometer being a mechanical oscillator.  If it were not an oscillator,=
then there would be no reason to provide eddy current dampers using rare ea=
rth magnets.  Simply stated, the inertial mass MUST be part of an oscil=
lator IF it is to be a seismometer.  If it oscillates, then the mass ca=
nnot remain at rest, and there is a repeat interval of time associated with=20=
the motion, called the period of oscillation.  The finite value of this=
period is what in turn causes an upper limit on the sensitivity that is gov=
erned by the square of the period of oscillation. The reason there is a limi=
t to the amount of relative motion between mass and case (instrument sensiti=
vity) is the FACT that the inertial mass DOES move. There is ABSOLUTELY no w=
ay it CANNOT!

Agreed.

On a differe=
nt subject:

Chris you mention what I believ=
e to be indeed true - publishable results out to (and beyond) 2000 s could c=
hange a great deal in the world of seismology. It is a virtually unexplored=20=
regime. Let me give you an example. I was just yesterday looking at the diff=
erences between the N-S and E-W channels of my VolksMeter here in Macon. Bec=
ause the concrete cylinder that is part of the monolithic pier goes 20 ft in=
to the ground, there is a significant reduction in the thermoelastic tilt th=
at is otherwise seen for instruments sitting on a slab on top of the earth.&=
nbsp; What is really interesting about the two channel records, for 1 sample=
per minute over 24 h is the following.  Sometimes the two channels are=
almost completely correlated.  Whatever is tilting the pier is the sam=
e in both N-S and E-W direction.
But there are days in which this is not at all true. =
; A correlation plot shows fascinating loop-the-loops that seem to cycle ove=
r a period of several days.  Is this something local to middle Georgia,=
or does it have global features?  The answer to this important questio=
n can only be provided by networked sensors.  What I find remarkable is=
that a simple pendulum has the potential to do some experiments which cry o=
ut for data collection.  Anybody who believes that science is in the
process of just 'tying up loose ends' to mature understand of nature (whethe=
r in physics or any other discipline) is hopelessly naive.  It is mind=20=
boggling to me the extent to which seismology has only 'scratched the surfac=
e' with regard to a true
understanding of earth's complex motions. One of the reasons so little under=
standing has been gained derives from the 'delta function' mentality concern=
ing sensor type for instruments.

Interesting.

Which was why I suggested siting an ins=
trument at Eskdalemuir 55.3N 3.2W in the UK, about 81 degrees E from Macon 3=
2.85N -83.68W, or at Walferdange 49.7N 6.2E in Luxembourgh at about 89 degre=
es E? Eskdalemuir is a good bit further N than Macon.

But we still have to motivate the seism=
ologists and get the long period equipment to them. STS-1s are in short supp=
ly and STS-2s and Guralp CMG 3T can be ordered in 360 sec version, but the T=
rillium is limited to 240 seconds. They are all a bit short on period for Ea=
rth Eigenmodes. I haven't checked the noise levels. The Scripps Institute se=
ems to be junking their STS-1s??

I suspect that Volksmeters could well m=
ake excellent Tsunami detectors, particularly off the west coast of the Amer=
icas where the major faults are less than 500 miles offshore and the warning=
response times need to be just a few minutes.They could pick up the tilt si=
gnals from vertical ocean floor changes directly.  I am not sure about=20=
the likely deflection amplitudes required?
I have just been watching a TV program=20=