Hi Ted,

At 06:59 AM 12/27/2009 -0700, you wrote:
>Hi Folks,  I have downloaded Bob McClure's great program 
>SpringCalc.exe.    I have put in some numbers and see it calculates very 
>long periods under certain conditions, periods of >20.
>
>My question is can long periods like these really be achieved?

Then again, maybe.  I took a look at Bob's results and he seems to have 
been quite successful at lowering the apparent low corner frequency of a 
vertical by a factor of 8 by using an inverse filter.  I would be 
interested in knowing what the noise of a 2.5second vertical would be in 
the neighborhood of 20 seconds since, in the process of extending the 
period, that noise also gets multiplied by 8.  If the 20-second noise is 
low relative to the 20-second ground motions you are expecting, then it 
should work fine--and Bob's results would tend to bear that out.

I'd like to look at the differences between extending the period by using 
feedback and doing it with an inverse filter.  May prove interesting, 
though I'm convinced that using a feedback design will have some 
significant advantages.

Brett

Watch our wiggles
http://bnordgren.org/seismo/gif_images.htm

or watch some very very good wiggles
http://aslwww.cr.usgs.gov/Seismic_Data/telemetry_data/ANMO_24hr.html


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