Brett,
Your equivalent displacemen=
t threshold resolution of 40 nm is quite impressive, since you are measurin=
g the motion of a large inertial mass. But this is probably not a lim=
it imposed by your capacitive sensor. In general, seismometer l=
imitations are the result of one or the other of (i) electronics, or (ii) t=
he =E2=80=98spring=E2=80=99. The latter is a consequence of the fact =
that there are no perfect macro-scale springs; i.e., ones that obey Hooke=
=E2=80=99s law, which we routinely teach to physics students as though they=
describe the real world. My friend and colleague, Tom Erber, who rec=
ently retired from Illinois Tech University=E2=80=94demonstrated experiment=
ally that only with =E2=80=98spring displacements=E2=80=99 approaching atom=
ic dimensions, could a Hooke=E2=80=99s law approximation be possibly achiev=
ed. In other words, by looking at incredibly small motions using an a=
tomic force microscope. So a seismometer responds not only to changes=
(accelerations) of its external environment, but ALSO (which so few folks =
really appreciate) to changes that happen in the spring itself. These=
are especially troublesome when one wants to look at earth motions that ar=
e =E2=80=9Clow and slow=E2=80=9D.
A capacitive sensor wi=
th a large electric field between the plates (the parameter that ultimately=
determines its threshold sensitivity) can measure down to 0.1 nm, even at =
room temperature. This was the displacement threshold of the capaciti=
ve sensor that I used as a student while at Oak Ridge National Laboratory i=
n the late 1960=E2=80=99s. The sensor was one that operated on gap sp=
acing change of parallel plates that had been polished optically flat to be=
tter than a half-wavelength of visible light, and spaced nominally about 5 =
micro-meters (microns) apart. Motion of one plate, relative to the ot=
her, was due to 30 and 60 MHz longitudinally excited (pulse echo) ultrasoun=
d waves in a single crystal. By measuring the 2
nd ha=
rmonic (relative to the fundamental), I could thus determine as a function =
of temperature the =E2=80=9Celastic anharmonicity=E2=80=9D of copper (due t=
o the 3
rd order elastic constants) along each of the three princ=
ipal axes. In recent years I have recognized the importance of =
=E2=80=9Cdamping anharmonicity=E2=80=9D as well as the elastic anharmonicit=
y that I studied as a student. I was greatly satisfied to have finall=
y published something significant on this topic in the 10
th edit=
ion of the McGraw Hill Encyclopedia of Science and Technology. It has=
gained acceptance by at least the chemistry community, as seen by th=
e =E2=80=98Chem-wiki=E2=80=99 page online at
http://chemwiki.ucdavis.edu/Phys=
ical_Chemistry/Quantum_Mechanics/Quantum_Theory/Trapped_Particles/Anharmoni=
c_Oscillator If you type into Goog=
le the following keywords (without the tick marks causing a literal search)=
:
=E2=80=98anharmonic oscillator access science=E2=80=
=9D
You should then be able to read the ful=
l article by clicking on the 2nd link of the first page that get=
s displayed.
Even though my ultraso=
nic system sensitivity was impressive at 0.1 nm, some Russians were able wi=
th cryogenic electronics to do a hundred times better, approaching the size=
of atomic nuclei. So thus I am confident that the ultimate resolutio=
n of a seismograph does not derive from the limitations of a capacitive sen=
sor. But don=E2=80=99t take my word for it. The great physicist=
R. V. Jones (who died in the late 1990=E2=80=99s) is still quoted by the m=
ost knowledgeable modern =E2=80=98artisans=E2=80=99 as the ultimate authori=
ty on sensors of the type used in modern seismology. He noted that tw=
o sensing types will (for reason of the physics used) outperform all others=
; they are (i) capacitive, and (ii) optical. The power of the latter =
is finally beginning to be used by a small number of university research ty=
pes, at least in California. Their optics approach is the same as use=
d by the Laser Interferometer Gravitational Wave Observatory (LIGO) people;=
i.e., measure the displacement (not the velocity) by means of a Michelson =
interferometer. Optical fibers allow such instruments to =
be placed way below ground with minimal difficulties of the type otherwise =
encountered.
I have done enough with cap=
acitive sensors over the last four decades to learn a few things through th=
e =E2=80=98college of hard knocks=E2=80=99, in contrast with the work of my=
degree from the University of Tennessee (involving the aforementioned ORNL=
experience). It should come as no surprise to anybody with very much=
electronics experience, that considerable benefit is gained (when possible=
) by operating capacitive sensors in a differential mode. The improve=
ment in SNR, especially because of better common mode electronic rejection =
can be dramatic. Also, if the electronics can be configured to involv=
e phase sensitive detection, still more significant gains are possible.&nbs=
p; My research that has impacted seismology most significantly involves the=
fully differential capacitive sensor that I patented. I remain amaze=
d at the level of resistance I experienced in trying to publish papers rela=
ted to this patent. One of my papers was actually reviewed by R. V. J=
ones (as was told me by editor Tom Braid of the Review of Scientific Instru=
ments). Of my sensor, Jones stated that =E2=80=9Cthe device was twice=
as sensitive as the conventional (half) differential capacitive types=E2=
=80=9D (for equivalent sized electrode areas total), and he liked its symme=
try, even though he was at that time =E2=80=9Ctoo old to take the time to d=
o a detailed theoretical analysis of it=E2=80=9D.
I have wr=
itten all this to hopefully encourage more folks to =E2=80=98think outside =
the box=E2=80=99 of conventional wisdom. One person who has done so w=
ith some successes is Allan Coleman. A paper of his from several year=
s ago is posted on my webpage at
The=
capacitive sensor, unlike the Faraday Law (coil/magnet) velocity sensor of=
a previous generation (World Wide Standard Seismograph Network--WWSSN) is =
fundamentally a displacement sensor. I encourage you to look in the w=
ritings of one of the world=E2=80=99s most highly respected seismologists (=
Erhard Wielandt). He notes that the =E2=80=9Cmodern=E2=80=9D force ba=
lance instrument (of type perfected by his =E2=80=98sidekick=E2=80=99 Gunar=
Streckeisen, maker of the legendary STS instruments) was made to function =
by means of force-feedback so as to behave like the earlier instruments (su=
ch as the original form of the Sprengnether vertical that I own). I m=
odified my Sprengnether to function more naturally as a displacement-measur=
ing instrument, using my patented sensor. For those of you who are in=
terested, there is a paper that I wrote about six years ago, titled =E2=80=
=9CImproving seismometer performance at low frequencies using newly discove=
red physics=E2=80=9D. It is online at
Anyone interested in follow=
ing Allan Coleman in the use of my sensor for seismic purposes, may want to=
look first at a pedagogical description of how it works. This is loc=
ated on the Tel-Atomic webpage at
http://www.telatomic.com/mechanics/sensor=
..htmlThe Cavendish balance takes advantage, not only of e=
lectronics common mode rejection, but also =E2=80=98mechanical common mode =
rejection=E2=80=99 that eliminates the =E2=80=98curse worthy to students=E2=
=80=99 pendulous swinging modes that made this classic experiment much more=
difficult in the past. It may also be of interest to note that the h=
eart of the electronics is the same AD7745 capacitance to digital converter=
(Analog Devices) that is used by the VolksMeter that I created.
Randall