PSN-L Email List Message

Subject: Volksmeter information
From: Larry Cochrane lcochrane@..............
Date: Wed, 06 Dec 2006 23:23:46 -0800


With Randall's permission I placed his Power Point presentation on my web site here:

Using the Power Point file I created this web page:

But this web page may not work with all browsers.

Larry Cochrane
Redwood City, PSN

Jerry Payton wrote:
> Myself, being new to the hobby, I would like more information about this 
> "Volksmeter" devise described in this letter.  Where may I find more, 
> photos and etc.
> Regards,
> Jerry
>     ----- Original Message -----
>     *From:* Larry Cochrane 
>     *To:* psn-l@.............. 
>     *Sent:* Wednesday, December 06, 2006 3:50 AM
>     *Subject:* Simple pendulum response
>     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 ago, it is
>     making a
>     comeback.  The success of the Shackleford-Gunderson approach should
>     have been a cue
>     to many that the pendulum needed to be revisited.  With the digital
>     electronics of
>     the AD7745 there are some advantages that did not exist when the S-G
>     instrument was
>     developed around its analog circuitry.  For example, the noise of
>     the Volksmeter
>     electronics does not increase as rapidly with frequency-decrease as
>     is true of analog
>     circuitry (commercial standard in seismometry being synchronous
>     detection).
>     Dr. Randall Peters
>     __________________________________________________________
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