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

Regards,
Brett

At 08:45 AM 9/21/2012, you wrote:

>On Sun, 2012-09-16 at 19:53 -0400, Edward Ianni wrote:
> > Hi all:
> >           I have two very old seismometers and I would like to extend
> > their "periods", electronically and EXTERNALLY if that is possible. I
> > tried this a long time ago but I am not heavy into electronics and could
> > not come up with the right values for some of the circuits that I looked
> > at. The two Seismometers are a Kinemetric SH1 "Intermediate"(about 5
> > second period), and a Sprengnether (square box type) with a 1.5 second
> > period. Is it possible to do this easily and is there a circuit
> > somewhere with relatively simple instructions for calculating component
> > values? Thank You.  Ed.
>
>Hi Ed,
>
>Besides the inverse filtering method that others have mentioned you
>might consider the Lippmann approach.  In brief this involves loading
>the seismometer with a negative resistance to greatly overdamp it.  The
>technique is used in the Lennartz seismometers.  A Google search for
>"lippmann seismometer or 'lippmann geophone' will turn up lots of
>information.
>
>Let me know if you need any more information.
>
>Regards,
>
>--
>Bob Smither                             bob@...........
>=======================================================
>History, n: An account mostly false, of events mostly
>   unimportant, which are brought about by rulers mostly
>   knaves, and soldiers mostly fools.
>     -AMBROSE BIERCE  The Devil's Dictionary (1906)
>=======================================================


__________________________________________________________

Public Seismic Network Mailing List (PSNLIST)

To leave this list email PSNLIST-REQUEST@.............. with 
the body of the message (first line only): unsubscribe
See http://www.seismicnet.com/maillist.html for more information.