PSN-L Email List Message

Subject: Re: Simple pendulum response
From: Pete Rowe ptrowe@.........
Date: Wed, 6 Dec 2006 07:51:07 -0800 (PST)

Hi Larry
thanks for forwarding this good paper. There is a
confusing typo in the 9th paragraph. Here is the
For frequencies above the 
> natural frequency of the pendulum, a velocity sensor
> will always outperform a 
> velocity sensor. 

I believe it should say "a velocity sensor will always
outperform a position sensor"


--- Larry Cochrane  wrote:

> Hi Everyone,
> Dr. Randall Peters asked me to forward the following
> message to the list.
> Regards,
> Larry Cochrane
> Redwood City, PSN
> I've been following with interest the discussions
> concerning instrument 
> characteristics.  Now that my schedule is easing
> somewhat, I felt that I should get 
> involved.  Should it happen that any of you respond
> to these comments and don't hear 
> back from me for a while, it's because I will be
> away for about a week to the Amer. 
> Geophys. Union Fall Conference in San Francisco
> (starting 11 Dec.). There I will give 
> a 15 minute oral presentation titled "State of the
> art Digital Seismograph" .  The 
> abstract is posted at
> The instrument which will be described (and also
> demonstrated at one of the booths) 
> uses a "simple" compound pendulum with a natural
> frequency of 0.92 Hz.  It employs my 
> fully differential capacitive detector as a
> displacement sensor (array form), with 
> electronics based in Analog Devices' new award
> winning capacitance to digital 
> converter integrated circuit (AD7745).  Kudo's to
> our own Larry Cochrane as the 
> brains behind all of (i) the electronics hardware
> necessary to do the I2C logic 
> operations required of the chip, and (ii) the
> software operating system in the form 
> of WinSDR and WinQuake.
>      For those of you who have been monitoring
> Larry’s instruments at
>   you may have noticed two real-time helicord
> records generated by the 
> single-pendulum instrument (N-S orientation) that he
> placed online.  The 
> raw-data-train is lctst.gif, which has been
> high-pass filtered (corner frequency of 
> 10 mHz) before display.  The unfiltered waveform is
> available via download upon 
> request from Larry.  This lctst is best suited to
> the real-time display of 
> earthquakes local to the Redwood City, CA site.
>       For registering teleseismic earthquakes
> real-time, Larry has also provided 
> lctst1.gif, which is the numerical integration of
> lctst after first doing a high-pass 
> filter.  This operation on the VolksMeter’s output
> provides a display similar to what 
> is provided by ‘bandwidth extension’ using
> electronic means in other instruments such 
> as geophones.
>        I was pleased to see John Lahr provide links
> on his webpage describing (i) 
> transfer function differences between velocity and
> position sensing, and (ii) 
> discussion of the zero-length spring that was
> invented by physicist Lucien LaCoste in 
> the early part of last century.
>       There are some things that need seriously to
> be clarified concerning theory of 
> seismometers, since there is so much confusion; not
> only among amateur seismologists, 
> but also even many professional geoscientists. 
> Ultimately, the ONLY source of 
> seismograph excitation (no matter the instrument
> design) is ENERGY.  Additionally, 
> the ONLY thing that delivers energy to the
> seismometer is Earth’s ACCELERATION at the 
> site of the instrument.  This is true not only for
> the instrument’s response to 
> earthquake waves whose periods are shorter than
> about 300 s, but also for earth ‘hum’ 
> in which the instrument responds mainly to tilt,
> when the periods are greater than 
> about 300 to 1000 s.
> Keep in mind that it is very difficult to see a 300
> to 1000 s periodic signal with a 
> velocity sensor.  It is equivalent to trying to look
> at a very low frequency signal 
> with an oscilloscope using a.c. coupling.  Only d.c.
> coupling (position sensing) is 
> appropriate in this case.
>         There is a dramatic difference between the
> forcing functions of tilt as 
> contrasted with horizontal ground acceleration. The
> tilt response is independent of 
> frequency, whereas the response to earthquakes
> (horizontal acceleration devoid of 
> significant eigenmode oscillatory components) is the
> classic response given by John 
> Lahr at the following website:
>     If you look at John’s six transfer function
> plots provided at 
> it is the right-most pair (response to acceleration)
> that ‘summarize the physics’ of 
> how a seismometer operates.  Yes, one can configure
> an instrument to plot data 
> according to any one of the six possibilities John
> has indicated, but the response to 
> acceleration is what ‘tells the story’ of
> performance.  For frequencies above the 
> natural frequency of the pendulum, a velocity sensor
> will always outperform a 
> velocity sensor.  On the other hand, for frequencies
> below the natural frequency, a 
> position sensor will always outperform a velocity
> sensor (all things otherwise 
> identical).
>       I don’t know about you, but I’m not
> particularly interested in frequencies 
> above 1 Hz.  Our Volksmeter easily picks up dynamite
> blasts and other local 
> disturbances that are nearly always manmade. 
> Because the earth is so large, motions 
> it exhibits in response to dynamic changes
> (earthquakes, tidal forces, ….) are at low 
> frequencies (not high).
>        At low frequencies where everybody seems
> increasingly interested in going 
> (reason for bandwidth extension) there is no
> question of the superiority of position 
> sensing over velocity sensing.  Why this obvious
> fact is so muddled in the minds of 
> so many is a great mystery to me.  Maybe it’s
> because even classical physics is 
> difficult for most everybody to understand.
>       I have placed a paper on my webpage which
> speaks to this matter, titled 
> ‘Seismometer design based on a simple theory of
> instrument-generated noise equivalent 
> power:
>        For those of you who want to ‘escape the rut’
> of velocity detection that has 
> held folks captive for way too long—Larry and my
> other business partner, Les LaZar 
> are positioned to provide you with reasonably-priced
> essential components to build 
> your own version of the VolksMeter.  Probably most
> of you will prefer to do this 
> rather than pay the present $1000 ‘turnkey’ price
> for our single-pendulum instrument.
>      I want to point out something that is the
> result of recently discovered 
> physics—why small-mass instruments don’t perform
> well.  Although conventional wisdom 
> says that it’s because of Brownian motion (larger
> for smaller masses), this is not 
> really the culprit.  The performance limitation is
> really the result of internal 
> friction problems that science is only beginning to
> understand.  The smaller the 
> seismic mass, the smaller the spring that supports
> it.  The smaller the spring, the 
> more significant is the internal friction associated
> with the ‘snap, crackle, pop’ of 
> defect structural changes in the spring (processes
> that operate at the mesoscale). 
> For decades we’ve recognized the all-important
> properties of defects in 
> semiconductors (basis for p and n material of which
> devices are made), but until 
> recently very little was understood concerning the
> importance of defects to internal 
> friction that regulates the low-frequency
> performance of seismometers.
>       The influence of defects is worse in
> instruments with springs than in those 
> that use a pendulum, which is more inherently
> stable.  Until better electronics came 
> along, we were stuck with trying to improve
> low-frequency performance by going to 
> lower natural frequencies of the mechanical
> oscillator.  That is no longer the only 
> viable solution.  Although the pendulum lost favor
> years 
=== message truncated ===

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