On Fri, 2012-09-21 at 18:56 -0400, Brett Nordgren wrote:
> Hi All,
> 
> Dave Nelson (CA) has been operating an instrument based on a 
> significantly modified version of the Lippman approach, using a 4.5Hz 
> geophone, extending its low corner down around 2-3 seconds.  Using a 
> single dual op-amp, it works great on local and regional quakes, 
> giving results for them that are virtually identical to the outputs 
> of his broadbands.  Though for teleseisms all you can see are the 
> p-waves.  It is flat to velocity above 0.33 or 0.5 Hz with a 
> sensitivity of around 3,000 V/m/s, driving the 16-bit A/D with no 
> additional amplification.  The instrument noise level seems 
> reasonably low.  Though all geophone period-extension techniques are 
> noisy, this one might be quieter than some other approaches such as 
> inverse filtering.
> 
> We have been able to make a SPICE model that we think is accurately 
> predicting its performance and now we're working on fine-tuning the 
> circuit.  This is definitely seems to be a good way to go.
> 
> The negative-resistance approach also greatly improves the dynamics 
> of the geophone, including significantly increasing its clipping 
> level and reducing the motion of its mass.  This has some 
> similarities with the force-balance designs, in that it uses a form 
> of electronic feedback, though in a significantly different way.
> 
> Here's a screen capture of its recording of part of the 26 August, 
> 2012 Brawley, CA earthquake swarm.  They were roughly 200km from the 
> instrument.
> 
> http://bnordgren.org/seismo/GeophoneZ260812.jpg

Very nice!  I have an inverse filter setup using a 10Hz phone and
results are not great.  I have been meaning to try the Lippmann approach
for some time - maybe now is the time!

Is Dave Nelson willing to share his circuit?

Thanks,

-- 
Bob Smither                                 bob@...........
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            Argentum et aurum comparenda sunt

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