PSN-L Message - Subject: RE: Anyone seen this MS thesis: Improving a Geophone

Chris,

Great post! =

I couldn’t use the url you = gave for the Lennartz document,

any suggestion about what to search = for to find the document you

were pointing = at?

Thanks,

Jack

From: psn-l-request@.............. [mailto:psn-l-request@............... = On Behalf Of ChrisAtUpw@.......
Sent: Saturday, December = 02, 2006 2:38 PM
To: psn-l@..............
Cc: angel@............... apsn@............ mariotti@.........
Subject: Re: Anyone seen = this MS thesis: Improving a Geophone

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/DefenseBarzilai= FinalCopyWeb/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=3Dcom_remo= sitory&Itemid=3D45&func=3Ddownload&filecatid=3D3&chk=3Dbc= 273996d12702c9fc96b4f638492797
       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

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