PSN-L Email List Message

Subject: Re: Anyone seen this MS thesis: Improving a Geophone
From: Larry Cochrane lcochrane@..............
Date: Fri, 01 Dec 2006 23:07:54 -0800

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 to 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.

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.

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."

Larry Cochrane
Redwood City, PSN

>    In a message dated 2006/12/01, apsn@........... writes:
>> Looks interesting.
> 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


Public Seismic Network Mailing List (PSN-L)

[ Top ] [ Back ] [ Home Page ]