Geoff,

At 10:49 AM 5/8/2011, you wrote:
>It looks like TUC saw the same thing.
>
>It is odd but I am unable to get a flat velocity response cheaply.
>
>I guess my waveform is displacement below 1 Hz
>and Acceleration Above 1Hz,
>Only right at 1Hz might you call it velocity.
>
>What kind of seismometer is that ?

An Accelerometer with a low corner frequency of 1Hz.  Its Velocity 
response will peak at 1Hz and fall away on either side of that.

To make it flat to velocity, you have to grossly overdamp it which 
clips off  and flattens the peak of the velocity response curve (also 
lowering its sensitivity) and gives the response curve distinct upper 
and lower corner frequencies.  It is difficult to overdamp it 
sufficiently, and it generally requires some tricky circuits.

http://www.vaxman.de/publications/teach_gp.pdf

http://www.geophys.uni-stuttgart.de/oldwww/seismometry/man_html/node26.html

http://www.ub.uib.no/elpub/2002/h/406002/hovedoppgave.pdf  (I may 
well be wrong, but I think his response curves are Velocity response 
not Acceleration as stated.)

Angel Rodrigues at the OSOP in Panama sells a three-axis strong 
motion instrument based on 4.5Hz geophones which are overdamped to 
obtain a 0.5Hz low frequency 
corner.  http://www.osop.com.pa/new.html  (you might be able to use 
Google or some such to translate if you don't read Spanish)

I would expect that if you started with a 1Hz sensor, you could 
probably get its velocity response to be flat from 0.1Hz to 10Hz, a 
reasonably useful range.

Brett 


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