PSN-L Email List Message
Subject: Pendulum Q
From: Larry Cochrane lcochrane@..............
Date: Thu, 18 Jan 2007 21:48:09 -0800
Dr. Randall Peters has ask me to forward the following to the list.
I have noticed that many of you have great interest in detecting small teleseismic
earthquakes, but probably don't have a great deal of interest in carefully studying
the temporal features of what gets recorded. Thus I have a recommendation for those
of you who are using long-period instruments, such as the Lehman or a folded
pendulum. If your instrument's natural period is in the vicinity of about 20 s, then
try the following simple experiment--remove all external damping from your
instrument. In other words, take off the magnets, or remove the oil pot. I have
already learned to wear the label comfortably, but before you view me as a heretic
(which some will do, since anything other than near-critical damping is viewed as
ridiculous) consider the following.
Your undamped pendulum's acceleration responsively at 20 s will be increased by the
value of Q. My guess is that many of you will thus see at least a 10- to 50-fold
increase in your ability to then detect small, distant earthquakes. No doubt the
objection to be raised by some to my recommendation will be--BUT the transient
response of the pendulum will 'mess up' everything! Yes, you will have immeasurably
complicated things if your interest is traditional. This traditional viewpoint is
one that I believe will become for some observers increasingly antiquated and
irrelevant. In the days before computers, such a mode of operation was indeed
ridiculous for those who wanted to see all the intricacies of arrival time features
governed by phase, etc. But now with the computer we have the ability to graph what
is probably the single-most-important feature (to one with a single instrument as
opposed to an array of them) of what the seismometer is telling us about the earth's
motion--the POWER. By using the computer to compensate for the transfer function of
the pendulum, one can generate a reasonably good power spectral density no matter
whether the instrument is damped or undamped. I will provide details on this
important calculation for those who are interested.
What is the biggest problem with the traditional approach which I'm suggesting
that some of you abandon?. Consider an analogy from the electromagnetic case. How
does everybody detect radio (TV or countless other types of similar) waves? The
answer--by 'TUNING' their receiver with a relatively high Q. Of course, if you want
to see a whole lot of different regions of the E-M spectrum you must be able to
adjust the bandpass of your receiver. In the case of the earth, a whole lot of
different natural period pendulums would be necessary to see everything of interest.
Think about the following: If communication systems were to operate in the manner
of seismology, we would just pick up everything in the electromagnetic spectrum with
a broadband antenna and try to sort things out with sophisticated electronics!
I have already demonstrated the viability of this method in the case of
microseisms with periods around 4 s; c.f. the paper "Compound Pendulum to Monitor
Hurricanes and Tropical Storms"
http://physics.mercer.edu/hpage/compound/compound.html There's no
reason the same thing can't be done for the longer period region of teleseismic
earthquakes (around 20 s). I predict you will be AMAZED at what starts to show up in
I hope that some of you will try this surprisingly simple experiment and tell me of
your excitement after a few days watching the output from your seismograph.
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