David Saum DSaum@............
Subject: Subject: Damping Resistor


  Is the damping resistor useful if you are using a 4.5 Hz geophone to dete=
ct=20
signals from below 1 Hz and filtering out all the higher frequencies?
 I am using Sunfull vertical 4.5 Hz geophones that spec as 60% damped=20
without any damping resistor, and I have never been able to detect any=20
difference when I add an 2.8k external resistor for critical damping.
Hi Dave,=20
.
    A geophone needs to be damped to about 0.7 critical to keep the respons=
e=20
linear with frequency above it's resonant frequency, otherwise it can have =
a=20
large peak at that frequency. The response below resonance falls off as f^2=
..=20
    However, you can fairly easily use a frequency compensating amplifier t=
o=20
extend the linear response down to 1/10 the resonant frequency. A 4.5 Hz=20
geophone can thus be extended to make it a 0.45 Hz geophone, which is GREAT=
=20
since it then covers the whole range of the P and S waves.
    One circuit which does this is the Roberts' Circuit, which is listed in=
=20
the documentation on Larry's web site http://psn.quake.net/bibliography.htm=
l =20
and excerpts John Lahr's web site.=20
See http://www.jclahr.com/science/psn/roberts/index.html
    You need two circuit 'elements' in series to get the 1/f^2 relationship=
..=20
It works really well for me.=20
.
    Regards,
.
    Chris Chapman   =20

=




=
David Saum DSaum@............

Subject: Subject: Damping Resistor





  Is the damping resistor useful if you are using a 4.5 Hz geophone to <=
FONT size=3D2>detect 
signals from below=
 1 Hz and filtering out all the higher frequencie=
s?
  I am using Sunfull vertical 4.5 Hz geophones that spec as =
60% damped 
without any damping resistor, and I hav=
e never been able to detect any 
difference when I add an 2.8k external resistor for=
 critical damping.

Hi Dave,=
 
.
    A geophone=
 needs to be damped to about 0.7 critical to keep the response =
linear with frequenc=
y above it's resonant frequency, otherwise it can have a 
=
large peak at that frequen=
cy. The response below resonance falls off as f^2. 
    However, you can fairly easi=
ly use a frequency compensating amplifier to 
extend the linear response down to 1=
/10 the resonant frequency. A 4.5 Hz 
geophone can thus be extended to make it a 0.4=
5 Hz geophone, which is GREAT 
since it then covers t=
he whole range of the P and S waves.
    One circuit which does this is the Roberts'=
 Circuit, which is listed in 
http://ps=
n.quake.net/bibliography.html  
and excerpts John Lahr's web site. <=
/PRE>
See http://www.jcla=
hr.com/science/psn/roberts/index.html
    You need two circuit 'elements' =
in series to get the 1/f^2 relationship. 
It works re=
ally well for me. 
.
    Regards,
.
    Chris Chapman