In a message dated 24/10/04, lehmancj@........... writes:

> Recently I saw reference to an article on the parameters of "Earth 
> Hum"---Real long period waves of 4 to 5 minutes.  This translated to a frequency 16 
> octives below (256 Hz) middle C.  These waveforms are detectable seemingly 
> anywhere on the Earth's surface.
>    The article can be found in the Sept. 30th '04 issue of Nature magazine, 
> but didn't give much help in the recording mechanics of such long waveforms. 
> 

Hi Jim,

       The original Streckeisen STS-1 seismometer had a period of ~850 sec 
and used a LVDT feedback detector. The Kinemetrics FBA-23 accelerometer works 
from DC up. The Guralp CMG-3 can work at 360 sec and uses a capacitative 
detector. It is also possible to use a seismometer with a shorter period and extend 
the period using digital signal processing, by a factor of up to about 10. 
Other methods which may be used for sensing very long period signals include 
extensometers and tiltmeters, often placed deep underground in mine shafts to avoid 
surface noise. 
       For more information, I suggest you read 'The leaf spring seismometer: 
design and performance' by E Wielandt and G Streckeisen, Bull Seis Soc 
America, Vol 72, No. 6, pp2349-2367, 1982. LaCoste came up with an infinite period 
spring design back in 1934. Basically, you start with a longish period and then 
apply a feedback loop which defines the response - the S&G seismometers also 
did that with a 1 sec pendulum - see psn account. 
       There was an Iris conference at Granlibakken last March discussing 
long period instrument development. See 
http://www.iris.edu/stations/seisWorkshop04/ The papers are mostly available on line.
       I hope that this is of some help. 

       Regards,

       Chris Chapman
In a message=20=
dated 24/10/04, lehmancj@........... writes:



Recently I saw reference to=
 an article on the parameters of "Earth Hum"---Real long period waves of 4 t=
o 5 minutes.  This translated to a frequency 16 octives below (256 Hz)=20=
middle C.  These waveforms are detectable seemingly anywhere on the Ear=
th's surface.

   The article can be found in the Sept. 30th '04 issue of =
Nature magazine, but didn't give much help in the recording mechanics of=
 such long waveforms. Can anyone simplify an explanation of how these waves=20=
are detected?



Hi Jim,



       The original Streckeisen STS-1 seis=
mometer had a period of ~850 sec and used a LVDT feedback detector. The Kine=
metrics FBA-23 accelerometer works from DC up. The Guralp CMG-3 can work at=20=
360 sec and uses a capacitative detector. It is also possible to use a seism=
ometer with a shorter period and extend the period using digital signal proc=
essing, by a factor of up to about 10. Other methods which may be used for s=
ensing very long period signals include extensometers and tiltmeters, often=20=
placed deep underground in mine shafts to avoid surface noise.=20

       For more information, I suggest you=
 read 'The leaf spring seismometer: design and performance' by E Wielandt an=
d G Streckeisen, Bull Seis Soc America, Vol 72, No. 6, pp2349-2367, 1982. La=
Coste came up with an infinite period spring design back in 1934. Basically,=
 you start with a longish period and then apply a feedback loop which define=
s the response - the S&G seismometers also did that with a 1 sec pendulu=
m - see psn account.=20

       There was an Iris conference at Gra=
nlibakken last March discussing long period instrument development. See http=
://www.iris.edu/stations/seisWorkshop04/ The papers are mostly available on=20=
line.

       I hope that this is of some help.=20



       Regards,



       Chris Chapman