Hi Geoff,


Il 04/12/2010 23:50, Geoffrey ha scritto:
> I have been playing around with a dismantled SPZ geophone damping and find
> if you short the leads to the coil and then drop the mass
> the mass will descend slowly, I guess from meeting
> the generator current being dissipated by the
> internal resistance.
>

It is a electrodynamic back-force that induce an inverse magnetic field
which damp the motion. All geophones are shipped from the manufacturer
to the client with shorted input to keep the mass more steady.

>
> 1. if there were no internal resistance (superconductor)
> then when the leads are shorted the mass would fall
> a bit then stay forever hovering ?
>

In this case you would hava a greater generator constant which would
help in have the mass more steady, but when the mass become steady you
would have no inverse magnetic field so no sustain and the mass would
drop a bit increasing in velocity until the generate magnetic field
contrast the motion. The result would be in a just little slower motion
down.


>
> 2.the geophone is acting just like a rotating electrical generator
> armature developing torque under electrical loads ?

Exactly.

>
> 3. It seems to me that seismic noise rarely hits the resonant frequency and
> you might do better to not increase the damping over what already
> is there in the mechanical and physical sense ?

I don't know what is the resonance frequency of your geophone.
Common geophone are built from 1Hz (rarely 0.5Hz) up to 30 Hz and more.
Unfortunately this is the range of seismic noise. So if you overdamp the
geophone you only reduce the sensitivity (in your field of application).

>
> 4. Does the rate at which the geophone mass drop under heavy damping
> represent some new fundamental Eigen frequency ?

No.
Heavy damping reduce the real fundamental frequency.
To measure the real fundamental frequency you have to leave the mass
swinging without damping.


>
> 5. Can anyone provide me with high school math
> models which represent the mechanical and
> electrical behaviors of the geophone ?
> High school math being trig and algebra
> minus the calculus ?

If you google "geophone transfer function" you find a lot of info.

>
>
> note: it seems to me for greatest sensitivity you pick
> an Eigen freq not contained in EQ signals then do no damping at all. ??
> This [little damping] should work for weak EQ signals and not close
> strong ones ?
>

As a rough description i can say the EQ signals falls in the range of
0.1 to 30Hz. Standard 4.5Hz geophones with a good preamplifier can be
used to pick signal from 0.2Hz and up.
Damping in a geopohone to record nicely EQ should be always between 0.5
and 0.7, not above the critical damping.
Damping over 0.707 (also called over damped geophones) can be used for
different applications.

best regards

-- 
Mauro Mariotti
SARA electronic instruments s.r.l.
Via A.Mercuri 4 - 06129 - Perugia
Tel. +39 075 5051014 Fax +39 075 5006315

-- 
Mauro Mariotti

-- 
Mauro Mariotti
http://www.infoeq.it
http://www.sara.pg.it

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