PSN-L Email List Message

Subject: Re: Simple pendulum response
From: Larry Cochrane lcochrane@..............
Date: Wed, 06 Dec 2006 18:39:17 -0800

Hi Pete,

You are correct, it should have read "velocity
sensor will always outperform a position sensor"


Pete Rowe wrote:
> Hi Larry
> thanks for forwarding this good paper. There is a
> confusing typo in the 9th paragraph. Here is the
> sentence: 
> 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"
> Pete
> --- 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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