Hi Chris,

Remind me again, why is it that we are worrying about dither.  What 
problem are we solving and how is it affecting what we are seeing?  I 
was under the impression that once we stress-relieve the spring, such 
problems (spring pops) almost completely disappear.  Normally they go 
away by themselves, but it takes a lot longer.

We could float the seismometer in a well, but I'm having trouble 
finding one that has ripples smaller than a few nanometers.  I like 
nice solid bedrock better.

Using a reference and a test seismometer can only give you the sum of 
their noise powers.  You can't tell which one is better than the 
other.  Comparing three allows you to derive their individual noise 
spectra.  It's a very clever procedure that Sleeman et.al. came up 
with.  As it stands, I suspect that we might possibly be a little 
better than our reference, the Trillium Compact, at a few 
frequencies.  We have been running four instruments, three of Dave's 
and the Trillium.  When sufficiently magnified, even on a quiet day, 
their traces all track very closely, and we should be getting good 
data to do a Sleeman correlation analysis.  We also need to do the 
same test on the A/D channels to understand what is their 
contribution to any measured noise.  I think it's reasonably small, a 
fraction of a count, but that's often the kind of instrument noise 
we're talking about.

Regarding the amount of feedback, it varies with frequency.  At a 
6-second period, the loop gain (feedback strength) is something over 100.

Regarding strain in the leaf-spring, it is quite high:  it's bent 
into a u-shaped, sort of eliptical arc.  However variation of that 
large static strain due to ground motion is very small.

Thanks for your thoughts,
Brett


At 06:56 PM 8/22/2011, you wrote:
>Brett Nordgren brett3nt@.............
>Sent: Tue, 16 Aug 2011 18:14
>Subject: Re: tin cries - dithering
>
>
>Hi Chris,
>
>****Thanks for your suggestion. I understand your
>approach, though implementing it will be a bit difficult.
>
>Hi Brett,
>
>     That rather depends on how you go about it !!
>     An alternative might be to fit two coils close to
>the centre of the spring and just excite them ?
>
>.
>****Normally, to get a decent idea of long-period noise we
>have to record for at least a day, and preferably several.
>And to separate instrument noise unambiguously from true
>ground noise we would need to record three instruments and
>use correlation techniques to distinguish their individual
>
>noise contributions. See:
>Sleeman, Wettum and Trampert
>"Three-Channel Correlation Analysis: A New Technique to
>Measure Instrumental Noise of Digitizers and Seismic Sensors"
>BSSA v96 n1 p258
>.
>****As it stands, at the lowest frequencies, the noise spectrum
>we normally see rises smoothly as 1/f and looks almost exactly
>like the best instruments except, of course, it's not as low.
>It is entirely possible that some of the noise we see may be
>true ground noise. We just don't have a quiet enough location
>to tell without playing the correlation game, which could be
>a pain.
>
>.
>
>     You don't happen to have a well locally in which you could 
> float the seismometer?
>
>     That is if you want absolute readings, but they will still
>be variable. If you just want to check if the noise is lower,
>you might have a reference seismometer and an experimental
>seismometer and look at the long period noise of both ?
>
>****If anyone could suggest what is the magnitude of the
>effect on the spring characteristics, I could at least model
>the instrument response to such stepwise force changes.
>Someone must have studied this and come up with some real
>numbers.
>
>     You have traces of a sensor with an Annealed spring
>and an As Supplied spring.
>
>****Wouldn't the six-second microseisms be an effective
>dither mechanism?  They're usually large compared with
>everything else.
>.
>     I don't think so. You are using a FORCE FEEDBACK
>SEISMOMETER ! The increase in the strain of the spring
>may be << 1% of the indicated signal. You are trying
>to keep the position of the mass constant with respect
>to the frame.
>
>     How much feedback are you using?


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