Thanks Chris--  Well Done, I was wondering if long base tiltmeters might =
be employed.  Thanks for the references--
  Jim Lehman
  ----- Original Message -----=20
  From: ChrisAtUpw@..........
  To: psn-l@.................
  Sent: Monday, October 25, 2004 10:23 PM
  Subject: Re: Earth Hum!


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


    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.=20
      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 =
are detected?


  Hi Jim,=20

        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.=20
        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.=20
        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.=20
        I hope that this is of some help.=20

        Regards,=20

        Chris Chapman=20







Thanks Chris--  Well Done, I was =
wondering if=20
long base tiltmeters might be employed.  Thanks for the=20
references--
  Jim Lehman

  
----- Original Message ----- 
  From:=20
  ChrisAtUpw@....... 
  
To: psn-l@.............. 
  
Sent: Monday, October 25, 2004 =
10:23=20
  PM
  
Subject: Re: Earth Hum!
  

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



  Recently I saw reference to an article on the parameters =
of=20
    "Earth Hum"---Real long period waves of 4 to 5 minutes.  This=20
    translated to a frequency 16 octives below (256 Hz) middle C. =
 These=20
    waveforms are detectable seemingly anywhere on the Earth's surface.=20
    
  The article can be found in the Sept. 30th '04 issue =
of=20
    Nature magazine, but didn't give much help in the recording =
mechanics=20
    of such long waveforms. Can anyone simplify an explanation of how =
these=20
    waves are detected?

Hi Jim,=20
  

      The original Streckeisen =
STS-1=20
  seismometer had a period of ~850 sec and used a LVDT feedback =
detector. The=20
  Kinemetrics FBA-23 accelerometer works from DC up. The Guralp CMG-3 =
can work=20
  at 360 sec and uses a capacitative detector. It is also possible to =
use a=20
  seismometer with a shorter period and extend the period using digital =
signal=20
  processing, by a factor of up to about 10. Other methods which may be =
used for=20
  sensing 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=20
  read 'The leaf spring seismometer: design and performance' by E =
Wielandt and G=20
  Streckeisen, Bull Seis Soc America, Vol 72, No. 6, pp2349-2367, 1982. =
LaCoste=20
  came up with an infinite period spring design back in 1934. Basically, =
you=20
  start with a longish period and then apply a feedback loop which =
defines the=20
  response - the S&G seismometers also did that with a 1 sec =
pendulum - see=20
  psn account. 
      There was an Iris =

  conference at Granlibakken last March discussing long period =
instrument=20
  development. See http://www.iris.edu/stations/seisWorkshop04/ The =
papers are=20
  mostly available on line. 
      I =
hope that=20
  this is of some help. =


      Regards,=20
  

      Chris Chapman=20