In a message dated 2006/12/02, lcochrane@.............. writes:

> Subj:Re: MS thesis: Improving a Geophone 

Hi All,

       Before reading the article below, I suggest that you visit Lennartz 
GMBH at http://www.lennartz-electronic.de/MamboV4.5.2/index.php
    They have three very successful seismic sensor lines from 40 Hz down to 1 
sec, to 5 sec, and to 20 sec periods. The sensors themselves are of about 2 
Hz with various types of period extension.

    A response flat with velocity, from 40 Hz down to 20 sec period is broad 
band. The 5 sec response sensors are popular, since they cover teleseismic P 
and S waves. Very low noise amplifiers are required. 

    In general, a geophone type sensor may be extended to 1/10 of it's 
natural period, usually without problems. Getting any more out of it may be 'hard 
work'. It may be desireable in some cases to limit the high frequency response 
to 20, or even 10 Hz.

    It is not clear if Sean was aware of the techniques used by Lennartz. 
However, he used an additional in line LRDT where Barzilai used a capacitative 
sensor.

> Hi Everyone,
> 
> This is what Sean-Thomas wrote about using a 4.5Hz geophone as a broadband 
> sensor:
> 
> "I have been experimenting for several years with making almost any
> seismometer into a VBB sensor if the physical parameters are suitable,
> like the coil resistance. THe largest is a WWNSS LP (15 second T0 and
> 11 kg mass) vertical that I am running at 600 seconds.
> 
> As you may have gathered, the small geophones have been an attraction.
> I have been experimenting for about two years with making the 4.5 hz HS-1
> (by Geospace, but similar to the GSC-11d and the Mark Products L-15B)
> into a broadband instrument. I use the VRDT displacement sensor mounted
> externally above the case, with the sensing vane attached to the upper
> mass ring.  With VBB parameters set for 20 seconds and the proper coil
> resistance, the VBB output is 750 volts/meter/second and the calibrations
> fit the transfer functions.
> 
> But the data is too noisy for a sensitive broadband sensor, and I am usually
> barely able to see the normal 6-second microseism background of 1 to 2
> microns/second. Of course these were very clear (~10x) when hurricane
> Bonnie passed last August. I also made a nice record  (from St. Louis)
> of the Ohio quake in September, with peak velocities of Lg of about 10
> microns/second at about 8 seconds.

       NB. Sean didn't say what were the sources of this noise. An unsealed 
vertical geophone will be sensitive to atmospheric density changes, wind noise 
etc. Also, his remarks apply to broad band use, not to limited period 
extension.

> The trouble with the 4.5hz phone is that the mass is only 23 grams, and
> the intrinsic damping of 0.28 means that the Q is not very high.
> From the Riedesel paper this would be expected to have a Brownian noise
> power spectral density (PSD) level of about -165db (figure 12). (For
> reference, the USGS low noise model has the 6-second microseism peak
> at about -140db, the 12-second peak at -160db, and the quiet earth minimum
> between 40 and 200 seconds is about -185 db.) But the Brownian noise
> is only one of many noise sources; the circular suspension leaf springs
> and the fine-wire signal output leads are significant contributors.
> The Reidesel paper finds that when using the velocity signal coil and
> a properly selected amplifier, the noise level is -130db at 1 hz, and
> the 6-second microseisms cannot be seen. We can do much better with a
> VBB fedback configuration.

       Quite a lot better!

> The PSD of several noise samples was about -145db at 6 seconds, but
> levels off at about -155db at 10 seconds. It has trouble recording
> teleseisms compared with a larger VBB seis, like a Mb5.O west of Mexico
> or a 6.2 in China, where the 20-second surface waves were only about
> 2X the noise. It did make a reasonable record of a Ms 6.0 in the Queen
> Charlotte Islands (51N,130W).
> 
> It may be possible to reduce the suspension noise, and I have a new
> geophone to modify with great care to try to minimize it. Other
> problems are with the thermal sensitivity of the mass position and
> suspension resonances within the high-frequency portion of the VBB
> passband. The manufacturers are mum on these; the most notorious are
> the resonances of the 1-hz L4-C at 16 and 22 hz. The mass position change
> with temperature is a "don't care" for a velocity sensor, but it causes
> problems with a displacement output of 250 millivolts/micron, even with
> reasonable VBB loop gains."
> 
> Regards,
> Larry Cochrane
> Redwood City, PSN
> 
> >    In a message dated 2006/12/01, apsn@........... writes:
> > 
>  
> http://micromachine.stanford.edu/smssl/projects/Geophones/DefenseBarzilaiFinalCopyWeb/DefenseBarzilaiFinalCopy.pdf> 
> > 
> > Hi Bob,
> >        This has been around for ages.
> >        I would give him high marks for effort.
> >        I wish that I could be equally enthusiastic about the 
> > modifications themselves, or about his circuitry.
> >        The VLF noise level is high, but he does not seem to have 
> > addressed this, or cured it.
> >        Regards,
> >        Chris Chapman

       Let us turn the question around from what we CAN'T do with a 4.5 Hz 
geophone, 
       TO what we can FAIRLY EASILY DO to improve it's UTILITY + PERFORMANCE 
for Amateur Seismic Work?

       The response of a 4.5 Hz geophone is down to 1/5 at 2 Hz and it falls 
off rapidly below this, but this may be of use for local or volcanic quakes.
       However, extending a 4.5 Hz geophone's response to 1/10 of it's 
natural frequency (0.45 Hz) just conveniently covers most of the frequencies of 
interest for teleseismic P and S waves. There are three ways of doing this. 

       You can increase your main amplifier gain by x100 and process the 
digital signals in software. This reduces your dynamic range by the same factor 
and the full period compensation will fail for small signals (may not be too 
important).

       You can add a x100 VLF boost amplifier to compensate for the f^2 roll 
off of the geophone characteristic below it's resonant frequency. This 
amplifier needs to be low noise and the compensation needs to be done in two stages 
to maintain the gain through the corner frequency. If you don't do this, you 
get zero output at 4.5 Hz, instead of the full signal. See Roberts at 
http://psn.quake.net/bibliography.html I modified Roberts' circuit to remove most of the 
excess VLF noise that he experienced. The 4.5 Hz modules are available 
commercially - see www.sara.pg.it

       You can fit the geophone with a negative input impedance pre 
amplifier, which converts the 'dog leg' f^2 + the flat geophone responces into a much 
smaller straight line response proportional to f. You then put this through a 
low frequency bandpass filter set at your minimum frequency of interest. See 
the ARRL radio handbook. This removes the slope :f and you then amplify / filter 
the signal further. However, this does need an extremely low noise and 
specialised first amplifier. See 
http://www.lennartz-electronic.de/MamboV4.5.2/index.php?option=com_remository&Itemid=45&func=download&filecatid=3&
chk=bc273996d12702c9fc96b4f638492797
       See also http://www.vaxman.de/publications/teach_gp.pdf

       You can download a general filter design program and application notes 
free from http://www.ti.com/litv/zip/slvc108f

       Regards,

       Chris Chapman
In a me=
ssage dated 2006/12/02, lcochrane@.............. writes:



Subj:Re: MS thesis: Improvin=
g a Geophone 




Hi All,



       Before reading the article below, I sug=
gest that you visit Lennartz GMBH at http://www.lennartz-electronic.de/Mambo=
V4.5.2/index.php

    They have three very successful seismic sensor lines fro=
m 40 Hz down to 1 sec, to 5 sec, and to 20 sec periods. The sensors themselv=
es are of about 2 Hz with various types of period extension.



    A response flat with velocity, from 40 Hz down to 20 sec=20=
period is broad band. The 5 sec response sensors are popular, since they cov=
er teleseismic P and S waves. Very low noise amplifiers are required. 



    In general, a geophone type sensor may be extended to 1/1=
0 of it's natural period, usually without problems. Getting any more out of=20=
it may be 'hard work'. It may be desireable in some cases to limit the high=20=
frequency response to 20, or even 10 Hz.



    It is not clear if Sean was aware of the techniques used=20=
by Lennartz. However, he used an additional in line LRDT where Barzilai used=
 a capacitative sensor.



Hi Everyone,



This is what Sean-Thomas wrote about using a 4.5Hz geophone as a broadband s=
ensor:



"I have been experimenting for several years with making almost any

seismometer into a VBB sensor if the physical parameters are suitable,

like the coil resistance. THe largest is a WWNSS LP (15 second T0 and

11 kg mass) vertical that I am running at 600 seconds.



As you may have gathered, the small geophones have been an attraction.

I have been experimenting for about two years with making the 4.5 hz HS-1
(by Geospace, but similar to the GSC-11d and the Mark Products L-15B)

into a broadband instrument. I use the VRDT displacement sensor mounted

externally above the case, with the sensing vane attached to the upper

mass ring.  With VBB parameters set for 20 seconds and the proper coil<=
BR>
resistance, the VBB output is 750 volts/meter/second and the calibrations
fit the transfer functions.



But the data is too noisy for a sensitive broadband sensor, and I am usually=


barely able to see the normal 6-second microseism background of 1 to 2

microns/second. Of course these were very clear (~10x) when hurricane

Bonnie passed last August. I also made a nice record  (from St. Louis)<=
BR>
of the Ohio quake in September, with peak velocities of Lg of about 10

microns/second at about 8 seconds.




       NB. Sean didn't say what were the sour=
ces of this noise. An unsealed vertical geophone will be sensitive to atmosp=
heric density changes, wind noise etc. Also, his remarks apply to broad band=
 use, not to limited period extension.



The trouble with the 4.5hz phon=
e is that the mass is only 23 grams, and

the intrinsic damping of 0.28 means that the Q is not very high.

From the Riedesel paper this would be expected to have a Brownian noise

power spectral density (PSD) level of about -165db (figure 12). (For

reference, the USGS low noise model has the 6-second microseism peak

at about -140db, the 12-second peak at -160db, and the quiet earth minimum
between 40 and 200 seconds is about -185 db.) But the Brownian noise

is only one of many noise sources; the circular suspension leaf springs

and the fine-wire signal output leads are significant contributors.

The Reidesel paper finds that when using the velocity signal coil and

a properly selected amplifier, the noise level is -130db at 1 hz, and

the 6-second microseisms cannot be seen. We can do much better with a

VBB fedback configuration.




       Quite a lot better!



The PSD of several noise sample=
s was about -145db at 6 seconds, but

levels off at about -155db at 10 seconds. It has trouble recording

teleseisms compared with a larger VBB seis, like a Mb5.O west of Mexico

or a 6.2 in China, where the 20-second surface waves were only about

2X the noise. It did make a reasonable record of a Ms 6.0 in the Queen

Charlotte Islands (51N,130W).



It may be possible to reduce the suspension noise, and I have a new

geophone to modify with great care to try to minimize it. Other

problems are with the thermal sensitivity of the mass position and

suspension resonances within the high-frequency portion of the VBB

passband. The manufacturers are mum on these; the most notorious are

the resonances of the 1-hz L4-C at 16 and 22 hz. The mass position change
with temperature is a "don't care" for a velocity sensor, but it causes

problems with a displacement output of 250 millivolts/micron, even with

reasonable VBB loop gains."



Regards,

Larry Cochrane

Redwood City, PSN



>    In a message dated 2006/12/01, apsn@........... write=
s:

> 

 http://micromachine.stanford.edu/smssl/projects/Geophones/DefenseBarzilaiFi=
nalCopyWeb/DefenseBarzilaiFinalCopy.pdf> 

> 

> Hi Bob,

>        This has been around for ages=
..

>        I would give him high marks f=
or effort.

>        I wish that I could be equall=
y enthusiastic about the 

> modifications themselves, or about his circuitry.

>        The VLF noise level is high,=20=
but he does not seem to have 

> addressed this, or cured it.

>        Regards,

>        Chris Chapman



       Let us turn the question around fro=
m what we CAN'T do with a 4.5 Hz geophone, 

       TO what we can FAIRLY EASILY DO to impr=
ove it's UTILITY + PERFORMANCE for Amateur Seismic Work?



       The response of a 4.5 Hz geophone is do=
wn to 1/5 at 2 Hz and it falls off rapidly below this, but this may be of us=
e for local or volcanic quakes.

       However, extending a 4.5 Hz geophone's=20=
response to 1/10 of it's natural frequency (0.45 Hz) just conveniently cover=
s most of the frequencies of interest for teleseismic P and S waves. There a=
re three ways of doing this. 



       You can increase your main amplifier ga=
in by x100 and process the digital signals in software. This reduces your dy=
namic range by the same factor and the full period compensation will fail fo=
r small signals (may not be too important).



       You can add a x100 VLF boost amplifier=20=
to compensate for the f^2 roll off of the geophone characteristic below it's=
 resonant frequency. This amplifier needs to be low noise and the compensati=
on needs to be done in two stages to maintain the gain through the corner fr=
equency. If you don't do this, you get zero output at 4.5 Hz, instead of the=
 full signal. See Roberts at http://psn.quake.net/bibliography.html I modifi=
ed Roberts' circuit to remove most of the excess VLF noise that he experienc=
ed. The 4.5 Hz modules are available commercially - see www.sara.pg.it



       You can fit the geophone with a negativ=
e input impedance pre amplifier, which converts the 'dog leg' f^2 + the flat=
 geophone responces into a much smaller straight line response proportional=20=
to f. You then put this through a low frequency bandpass filter set at your=20=
minimum frequency of interest. See the ARRL radio handbook. This removes the=
 slope :f and you then amplify / filter the signal further. However, this do=
es need an extremely low noise and specialised first amplifier. See http://w=
ww.lennartz-electronic.de/MamboV4.5.2/index.php?option=3Dcom_remository&=
Itemid=3D45&func=3Ddownload&filecatid=3D3&chk=3Dbc273996d12702c9=
fc96b4f638492797

       See also http://www.vaxman.de/publicati=
ons/teach_gp.pdf



       You can download a general filter desig=
n program and application notes free from http://www.ti.com/litv/zip/slvc108=
f



       Regards,



       Chris Chapman