PSN-L Email List Message

Subject: dithering
From: Randall Peters PETERS_RD@..........
Date: Fri, 08 Feb 2008 17:00:28 -0500

    I had started to correspond with Brett as he indicated.  Copied below is what I

sent to him.
About your question--I would need to give some serious thought to your idea before
commenting.  As engineers, you and Brett probably have more experience with some of

these issues than myself.
    At any rate, as noted below--I don't think dithering of force-balance velocity
sensing instruments is the way to go.  What is needed at low frequencies is a whole

different approach that I mention.  Incidently, I have proven its viability
(soft-feedback with a long time constant integrator) with my modified Sprengnether
(LaCoste vertical).

   I'm not saying that the feedback systems are not good.  The STS-1 ( 'crown
jewel') is a marvel of technology when it comes to earthquake detection most
anywhere. What I am trying to communicate is the very thing you alluded to in
one of your points--as frequency goes toward zero (where the community wants to
operate for studying 'earth hum'), the ability to see important motions with a
velocity sensor also goes toward zero.  The sad part is that the seismo-pro's
don't seem to undertand how this can be so.
    I am not at all opposed to feedback; what I'm opposed to is the insistence
that the only way to do it is by forcing the system into a 'flat-to-velocity'
response by means of force balance.  For a long time now I've been advocating
what is the obvious solution to the low frequency conundrum; i.e., a soft-force
feedback (not force balance) that keeps the system from 'going to the rails' as
the period is lengthened to give the kind of mechanical sensitivity that is
needed for good teleseismic response using a displacement sensor.  Without
feedback in the case of a vertical seismometer, the sensitivity can never be
great enough to yield outstanding performance--because of the effects of (i)
buoyancy change due to atmospheric pressure variations, and (ii) spring
constant change with temperature because of the thermal coefficient of the
modulus.  Without some type of compensation, the latter is a killer when one
tries to obtain a natural period of 20 s.
     I didn't intend to suggest that force balance is not a great technological
achievement (since clearly it is).  It is not, however, the cure-all for every
sensing regime that many folks seem to believe.
    About state of the art--one has to be careful what is meant by the term.
There are those who believe we must somehow continue with force-balance
improvement in order to study the long-period features of the earth.  This is
not the case, and I think that force balance has gone as far as it can toward
low frequencies.  The VolksMeter, at about one-tenth of the price of the force
balance instruments will outperform the very best of them when it comes to
measurements with periods longer than a few thousand seconds.  On the other
hand, as compared to their telesismic sensitivity, the VolksMeter is severely
limited.  Bottom line--there is not (and probably never will be) a single
instrument that can cover the whole spectrum of interest to seismologists.


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