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Subject: Re: Gravimeter used as a vertical motion seismograph?
From: Roger Baker rcbaker@.............
Date: Wed, 16 Feb 2000 13:27:09 -0600

       actually I designed the Hi-Q seismo, referenced in this article, and
I am also a member of this list, although it has been awhile since I last
posted. One of my interests up until about a year ago was in hacking better
seismograph and geophysical instrument designs, and these have resulted in
easily-built designs for a microbariograph, the Hi-Q vertical, a taught
wire suspension Bennioff force-feedback horizontal built with a glass and
silicone frame published in the Society for Amateur Scientist's Bulletin,
and a strong motion sensor now in the possession of Larry Cochrane. I think
these designs are in the PSN archives.

I believe my Hi-Q design is sound and easily built, and there is a chart of
a teleseismic detection on my web page. As always, I invite others to
verify my work cited at

The original concept for my magnetic suspension vertical (Hi-Q) was based
on the fact that the virtue of a force feedback vertical is a function of
its "Q", and inversely proportional to damping losses, which obscure
slow,low amplitude motion. Magnetic spring suspensions using ceramic
magnets take minutes to come to rest, unlike most metal spring
arrangements. And so I used this physical novelty as the instrumental basis
for my compact magnetic force feedback design. It did introduce the
additional complexity that most magnets are pretty temperature sensitive.

The Hi-Q is quite good, in my opinion, as a vertical seismo, but just
passable as a gravimeter because gravimeters must by nature be highly
resistant to slow temperature variations and creep in order to see the slow
and very minor fluctuations in gravity caused by earth tides. Therefore,
the temperaure sensitive parts of the electronics should be placed inside
the isothermal enclosure, and probably the vertical rigidity of the 
insulated styrofoam enclosure should be improved further so that earth
tides are not seen so much as fluctuations on slow background change due to
creep -- unless your central heating system is unusually good.

The basic instrument strategy is that you should add just enough feedback
force to push the optical sensor back into its linear central sensing zone,
and then amplify and slow-filter this till you are satisfied with the
seismic or gravimetric results, whichever you want to study. 

It is obvious from the postings of Meridith Lamb on his Hall effect seismo
that the selection of workable displacement sensors for seismographs is
wider than commonly supposed. It should be obvious from the cantilever
sensors of atomic force microscopes that optical sensors can detect
displacements down to sub-atomic dimensions. So far as I know, capacitance
micrometers are the most ultimately sensitive displacement detectors
hackable by amateurs (see the work by R.V. Jones about 1973 in the British
Journal of Physics, Scientific Instrument series), although optical sensors
are cheap and small and easy to build from Radio Shack parts. I published a
hack of a capacitance micrometer from Radio Shack parts in the SAS bulletin
a few years ago.

(The last issue of SAS described a super-sensitive technique for the
detection of trace organics. My next 
Science Hacker column will be a description of progress in building an
amateur vibrating probe scanning microscope, using tiny mica flakes as
reflective cantilevers).

--Yours, Roger Baker


At 12:07 PM 2/15/00 EST, RLLaney@....... wrote:
>Hello all:
>A friend who is interested in amateur seismology send me a copy of the 
>article "Detecting Extraterrestrial Gravity" from the Amateur Scientist, 
>Scientific American, January 2000.  The device measures the gravitational 
>pull of the Sun and Moon and was built by Roger Baker of Austin, Texas who 
>claims it cost less than $100 to build.  Baker originally designed the
>as a vertical-motion seismograph.
>My friend has two questions:
>1)  Could this article be like the one describing the ADXL05 sensors in SA a 
>couple years ago where the readers were mislead into believing that the 
>sensors would work for detecting distant earthquakes, that is, another
>that doesn't quite measure up as advertised?
>2)  More important, could this device be modified back for use as a 
>reasonable vertical-motion seismograph?
>Would appreciate hearing from anyone who has seen the article and/or is 
>familiar with this device.
>Bob Laney
>Herndon, VA
>Public Seismic Network Mailing List (PSN-L)
>To leave this list email listserver@.............. with the body of the 
>message: leave PSN-L


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Larry Cochrane <cochrane@..............>