Very high =
performance force=20
balance vertical seismometers are well within the capabilities of the =
amateur=20
seismology community. Instruments with extremely low noise at long =
periods with=20
bandwidths which cover 30 Hz down to .02 Hz ( 50 seconds period ) are =
operating=20
at a number of amateur sites. The genius of Wielandt and =
Strekeisen and=20
Guralp made the force balance system practical and =
ultimately made=20
it the ONLY technique in general use for high sensitivity long =
period=20
seismic instrumentation. It does not take genius to understand their =
techniques=20
and apply them to an amateur instrument with excellent =
performance. =20
The technique =
is not some=20
mysterious magic beyond the scope of any but the most sophisticated=20
professional. Amateur Force balance vertical instruments =
were=20
initially built and demonstrated by Sean Thomas Morrissey and Alan =
Coleman. Understanding the electronics =
and the=20
theory behind the feedback system are well within the abilities of =
a=20
typical electronics professional and many electronics hobbyists. =20
Mathematical modeling of a new design will be essential, trial and error =
does=20
not work well on feedback systems. A =20
general purpose modeling program for force balance systems is available. =
I have been developing force balance vertical =
instruments=20
in collaboration with Brett Nordgren. Their performance rivals the =
best=20
professional instruments when operating at a typical site. These =
instruments=20
normally observe several teleseismic events per day -- not just an =
occasional=20
response resulting from a very large magnitude event. =20
These =
instruments will=20
operate over a large (50C) temperature range without re-centering. =
They=20
are very stable and have noise spectra at long periods which is =
dominated by=20
site noise not instrument noise. Note that the long period corner =
frequency of=20
the amateur instrument was chosen to be 50 seconds while the best =
professional=20
instruments have responses to 120, 240 or 360 seconds. In order to use =
commonly=20
available spring material and still have a wide operating temperature =
range, the=20
long period corner was limited to 50 seconds. Even with this limitation =
earth=20
eigenmodes at periods of 300 seconds have been seen clearly in the =
trace=20
following large seismic events.
There are no special challenges in =
accomplishing=20
this performance--in fact making non feedback instrument is in many ways =
more=20
difficult.
They are so sensitive they MUST be operated =
in =20
pressure sealed case with a rigid base (I use a 2 inch granite =
surface=20
plate) to eliminate flexure of the base and noise from the =
changing=20
buoyancy of the proof mass. Both effects are the result of the normal =
short=20
term fluctuations in atmospheric pressure. This is the only =
complicating=20
factor but not a great technical challenge.
The results of one of my instruments is posted =
on the=20
PSN Current Seismicity page. Compare the response of this =
instruments=20
with others posted on PSN.
http://psn.quake.net/currentseismicity.html =20
The proof is in the traces. =20
More =
information and=20
photographs of two designs may be seen on http://bnordgren.org/seismo/gif_images.htm =
and http://bnordgren.org/seismo/Ygif_images.htm =20
By way of Chris Chapman, it has come to my =
attention that a=20
significant number of readers of this list-serve are
misinformed concerning the nature and performance =
of the=20
VolksMeter seismograph. Even though the gravitational pendulum=20
in the VolksMeter has a period of only about 1 =
second, that=20
doesn't mean the instrument is limited to sensing only local =
earthquakes, as with conventional short-period=20
seismometers. Several VolksMeters around the world--such as mine =
here at=20
Mercer University, or Michael Phillips' seismograph =
at the=20
Edward Pigot Seismic Observatory near Coonabarabran,
Australia--routinely pick up the surface waves from =
teleseismic events. The outstanding broadband performance of the=20
VolksMeter that allows these measurements, involves =
two novel=20
features: (i) the unique sensor of fully-differential
capacitive type that allows pendulum deflection =
measurements=20
(position rather than velocity) all the way from 10 Hz down =
to d.c., and (ii) the outstanding electronics that =
was=20
developed by Larry Cochrane to work with this sensor. Operation=20
is by means of WinSDR and WinQuake, and the heart =
of the=20
system is the Analog Devices AD7745 capacitance to digital (24 =
bit) converter. As such, the VolksMeter is =
the first=20
"fully-digital" seismograph--because the sensor, along with the =
support electronics for data storage and =
communication, is=20
completely digital (excepting the moving electrodes that are =
attached to the pendulum). Commercial =
seismographs=20
still use analog electronics to measure changes in a capacitive =
sensor of lower symmetry and reduced sensitivity =
compared to=20
the sensor of the VolksMeter. They use a separate analog =
to digital converter to provide data storage =
capability and=20
uplinks to the web. Operation of these commercial
instruments is further complicated by the =
challenges of force=20
feedback needed to maintain stable performance, free from =
the problems of mechanical creep. By =
comparison, a=20
gravitational pendulum can be made nearly immune to mechanical =
instabilities, without using the complicated =
electronics of=20
feedback. In general, sensitivity of an instrument can be =
increased by either of two means--(i) using=20
less-sophisticated electronics, but operating with a mechanical system =
made=20
highly sensitive by period lengthening, or (ii) by =
working=20
with a much more stable, shorter period instrument in the =
form of a gravitational pendulum, that is supported =
by=20
superior electronics. The former requires genius of the type=20
that was exhibited by Gunar Streckeisen in the =
building of=20
instruments like the STS-1 and 2. The latter requires a =
genius in circuit development, in the person of =
someone like=20
Larry Cochrane, to create a state of the art electronics
package for seismic use--the very package that is =
resident in=20
the VolksMeter. Because there are significantly different =
seismic instruments that are described by the term, =
I am=20
being here careful to associate the adjective 'gravitational' with the=20
word pendulum. Unlike these other =
instruments, where=20
the oscillating member is also called a 'pendulum', the equilibrium=20
direction of a gravitational pendulum (in the =
absence of=20
acceleration or tilt influence) is that of a plumb bob
(direction of the earth's gravitational field). =
A=20
detailed description of the physics of such instruments is provided in=20
my article titled "Tutorial on gravitational =
pendulum theory=20
applied to seismic sensing of translation and rotation",
Bulletin of the Seismological Society of America, =
Vol. 99,=20
No. 2B, (2009).