Brett,
I'm glad to see you thinking about these issues. I wrote a paper that =
you might want to look at, dealing with seismic instrument noise. It is re=
lated to the optics work that I used to do during the antisatellite program=
about 20 years ago. There is a standardized convention for evaluating thi=
s feature of performance involving optical systems, called the 'noise equiv=
alent flux density'. The seismic equivalent I have labeled "noise equivale=
nt power". My paper is on the Mercer Physics webpage at http://physics.mer=
cer.edu/hpage/inep/inep.html
and is titled "Seismometer design based on a simple theory of instrument-ge=
nerated noise equivalent power".
One has to work with the proper 'power spectral density' units (m^2/s^3/Hz)=
for this tool to be acceptable. The universal to seismologists accelerat=
ion spectral density (units of m^2/s^4/Hz =3D incorrectly called (specific)=
power) does not allow any meaningful understanding of the problem.
Concerning tilt. As Einstein well appreciated, a horizontal seismogra=
ph without low frequency response limitations cannot distinguish (from a si=
ngle measurement) the difference between horizonatal acceleration andd tilt=
.. The tilt associated with teleseismic surface waves from earthquakes is i=
nconsequential compared to their accelerations. On the other hand, when th=
e period is greater than a few hundred seconds, the dominant part undergoes=
a change. What was dominant acceleration at shorter periods becomes domin=
ant tilt at longer periods. The tilt should not be thought of as a noise. =
If the tiltmeter is situated at a line of nodes, then it will respond to c=
hanges in the direction of the earth's field when the earth oscillates in i=
ts normal modes. The changes are oftentimes of the order of nanoradians, e=
ven though the acceleration of earth's vertical motion is so small as to be=
observed only with a superconducting gravimeter.
I've also recently discovered something really, really interesting. I =
used to wonder why earth's free oscillations were (seemingly) not visible d=
uring the time of an earthquake. I discovered the reason--the mHz range ei=
genmodes amplitude modulate higher frequency components and so they would b=
e visible only in the form of sidebands, of which there are two many compon=
ents to sort through everything. But by demodulating the record before doi=
ng the FFT, they show up. I have written a paper on this subject, titled "=
free oscillations coincident with earthquakes", online at
http://arxiv.org/ftp/arxiv/papers/1010/1010.5690.pdf
Brett,
&=
nbsp; I'm glad to see you thinking about these issues. I wrote =
a paper that you might want to look at, dealing with seismic instrument noi=
se. It is related to the optics work that I used to do during the ant=
isatellite program about 20 years ago. There is a standardized conven=
tion for evaluating this feature of performance involving optical systems, =
called the 'noise equivalent flux density'. The seismic equivalent I =
have labeled "noise equivalent power". My paper is on the M=
ercer Physics webpage at http://physics.mercer.edu/hpage/inep/inep.html
and is titled "Seismometer design base=
d on a simple theory of instrument-generated noise equivalent power".<=
o:p>
One has to work with the proper 'power s=
pectral density' units (m^2/s^3/Hz) for this tool to be acceptable. T=
he universal to seismologists acceleration spectral density (units of=
m^2/s^4/Hz =3D incorrectly called (specific) power) does not allow any mea=
ningful understanding of the problem.
&=
nbsp; Concerning tilt. As Einstein well apprec=
iated, a horizontal seismograph without low frequency response limitations =
cannot distinguish (from a single measurement) the difference between horiz=
onatal acceleration andd tilt. The tilt associated with teleseismic s=
urface waves from earthquakes is inconsequential compared to their accelera=
tions. On the other hand, when the period is greater than a few hundr=
ed seconds, the dominant part undergoes a change. What was dominant a=
cceleration at shorter periods becomes dominant tilt at longer periods.&nbs=
p; The tilt should not be thought of as a noise. If the tiltmeter is =
situated at a line of nodes, then it will respond to changes in the directi=
on of the earth's field when the earth oscillates in its normal modes. =
; The changes are oftentimes of the order of nanoradians, even though the a=
cceleration of earth's vertical motion is so small as to be observed only w=
ith a superconducting gravimeter.
I've also recently discovered something really, re=
ally interesting. I used to wonder why earth's free oscillations were=
(seemingly) not visible during the time of an earthquake. I discover=
ed the reason--the mHz range eigenmodes amplitude modulate higher frequency=
components and so they would be visible only in the form of sidebands, of =
which there are two many components to sort through everything. But b=
y demodulating the record before doing the FFT, they show up. I have =
written a paper on this subject, titled "free oscillations coincident =
with earthquakes", online at
=
http://arxiv.org/ftp/arxiv/papers/1010/1010.5690.pdf
<=
/body>=