Larry,
The primary reason for poor signal to noise ratio at long periods wit=
h a coil/magnet system involves the physics employed. Such a sensor functi=
ons on the basis of Faraday's law, which says that a time changing magnetic=
flux gives rise to a voltage. The key word in this process is 'changing';=
the output depends on the flux rate. If the boom of the seismometer is sw=
inging sinusoidally at a frequency f, then the amplitude of the output volt=
age of the sensor is proportional to f-which is the very nature of the time=
derivative of the sine function, involving the chain rule. Because the si=
gnal is proportional to f, the signal level decreases by the same amount as=
f decreases. Regardless of the nature of the noise, this means there is a=
significant reduction in the signal to noise ratio for accelerations havin=
g a frequency below the natural frequency of the instrument. When operatin=
g without the high-pass filter with your S-G instrument, your output is not=
rate sensitive; therefore it isn't afflicted with this low frequency loss.=
The 'wandering' you experience is actually representative of the various =
changes occurring. Although some of those changes are oftentimes the undes=
irables of instrument thermal coefficient or whatever, there are also signi=
ficant variations associated with the Earth itself. When you high pass the=
output, any earth motions having frequencies lower than the cutoff are su=
ppressed. I operate the VolksMeter with a high pass and also low pass filt=
er of recursive type in WinSDR. The primary reason is so that the helicord=
record is well behaved for purpose of seeing earthquakes. But for viewing=
tides, or magnetoelastic phenomena, or earth hum, or .... It is necessary =
to look at the unfiltered output, which is what I save.
The obsession with 'velocity' sensing is almost universal. Even forc=
e balance instruments of the highest dollar type use a network that causes =
the seismograph to behave just like a magnet/coil system with a low corner =
frequency, usually about 30 s. Thus the greatest advantage of the capaciti=
ve sensor is actually lost, for signals having frequencies lower than the d=
esign corner. For signal variations having a frequency below the corner, t=
he output from such an instrument is not 'velocity'; it is instead the deri=
vative of acceleration, called the 'jerk'. Only for drive frequencies abov=
e the corner can one use the term 'velocity' appropriately.
Randall
Larry,
The primary reason for =
poor
signal to noise ratio at long periods with a coil/magnet system involves th=
e
physics employed. Such a sensor functions on the basis of FaradayR=
17;s
law, which says that a time changing magnetic flux gives rise to a voltage.=
The key word in this process is ‘changing’; the output depends =
on
the flux rate. If the boom of the seismometer is swinging sinusoidall=
y at
a frequency f, then the amplitude of the output voltage of the sensor is
proportional to f—which is the very nature of the time derivative of =
the
sine function, involving the chain rule. Because the signal is propor=
tional
to f, the signal level decreases by the same amount as f decreases.
Regardless of the nature of the noise, this means there is a significant
reduction in the signal to noise ratio for accelerations having a frequency
below the natural frequency of the instrument. When operating without=
the
high-pass filter with your S-G instrument, your output is not rate sensitiv=
e;
therefore it isn’t afflicted with this low frequency loss. The =
‘wandering’
you experience is actually representative of the various changes
occurring. Although some of those changes are oftentimes the undesira=
bles
of instrument thermal coefficient or whatever, there are also significant v=
ariations
associated with the Earth itself. When you high pass the output, any
earth motions having frequencies lower than the cutoff are suppressed=
..
I operate the VolksMeter with a high pass and also low pass filter of recur=
sive
type in WinSDR. The primary reason is so that the helicord record is =
well
behaved for purpose of seeing earthquakes. But for viewing tides, or
magnetoelastic phenomena, or earth hum, or …. It is necessary to look=
at
the unfiltered output, which is what I save.
The obsession with R=
16;velocity’
sensing is almost universal. Even force balance instruments of the
highest dollar type use a network that causes the seismograph to behave jus=
t
like a magnet/coil system with a low corner frequency, usually about 30
s. Thus the greatest advantage of the capacitive sensor is actually l=
ost,
for signals having frequencies lower than the design corner. For sign=
al variations
having a frequency below the corner, the output from such an instrument is =
not ‘velocity’;
it is instead the derivative of acceleration, called the ‘jerk’=
..
Only for drive frequencies above the corner can one use the term ‘vel=
ocity’
appropriately.
Randall