Hi Randall,

Just to be clear, though, my comment was directed at our trying to 
extend the long period response a geophone.  Referring to 
force-balance instruments, I would be saying "Almost anything can be 
done with electronics."  It's my contention that in such an 
instrument, the quality of the electronics pretty much entirely 
determines its performance.

The spring-mass in a force-balance vertical does have some effects 
(mostly bad).  Indeed, you must have a mass in order to sense the 
ground acceleration and therefore velocity.  The function of the 
spring is to support the mass near its rest position so that the 
electronics does not need to work too hard to keep it centered.  The 
spring, is a "necessary evil" which acts to slightly reduce the 
effectiveness of the feedback, and it also introduces long-period 
noise as its strength varies with small (microdegree per second) 
temperature variations.  Newly installed springs also exhibit 
frequent "pops" which slowly become less frequent with time.  Baking 
a stressed spring overnight in a kitchen oven causes the pops to 
essentially disappear.

In addition, the atoms of the mass exhibit thermal (Brownian) motion 
which can introduce measurable noise when using masses of a few grams 
or less, which is reduced by the narrowness of the bandwith (Q) of 
the spring-mass sysem.  For a given mass value, the natural frequency 
of the spring-mass defines the stiffness of the spring, which should 
be adequately low so as not to interfere too much with the operation 
of the feedback electronics.

Regards,

Brett


At 09:15 AM 9/26/2012, you wrote:
>I was glad to see here a recent statement with which I heartily 
>agree, "only so much can be done with electronics".




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