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Re: ELF



Kyle,
Geophysicists use naturally-occuring low frequency EM radiation to study
the electrical conductivity structure of the Earth. This technique is
known as "magnetotellurics" (MT). The Society of Exploration Geophysicists
(SEG) publishes "Magnetotelluric Methods" edited by Keeva Vozoff
(Geophysics reprint series #5) which gives some information about what we
MT-ers call "source fields."

In a nutshell, there are two generic sources for these signals. For
frequencies lower than about 1Hz, coherent motion of ions trapped in the
ionosphere interact with the Earth's principle magnetic field to emit EM
radiation. For frequencies about about .001 Hz these fields can be
idealized as plane waves propogating normally into the Earth. Above 1 Hz,
wide-band EM emissions from lightening strikes around the world are carried
around the world by a wave-guide comprising the (mildly conductive) surface
of the Earth and the bottom of the ionosphere. Certain frequencies are
preferentially transmitted, these are known as the "Schumann resonances."
Given sufficient equiment (modern-day MT data aquisition systems), one can
actually determine the azimuth to source for these fields, although this is
not routinely done.

The power spectrum for these signals is roughly bowl-shaped, with weakest
signal strength from about 0.5 to 10Hz (what we call the "dead band," and
where we spend the most time trying to acquire data). MT-ers usually
measure time series for three orthoganal components of the magnetic fields
and two, horizontal E-field components.

I won't ramble on much further, except to say that "electromagnetic
precursors to earthquakes" should be taken with a very large grain of salt.
The most plausible example of these, to my mind, is the precursor to the
1989 Loma Prieta earthquake near Santa Cruz, as reported by Bernardi et al.
(Physics of the Earth and Planetary Interiors, 68, 1991). I have my own
favorite explanation for how this "precursor" might have occured, but that
is another matter.

Hope this helps,

A few students of mine have built an extreamly low frequency EM antenna
(under 100Hz) following directions in the Amateur Scientist section of
Scientific American (May 1996). (This was described as a project to detect
possible earthquake precursor signals which purportedly are in the 4Hz
range.) We are seeing all kinds of signals (but not at 4Hz!). We know what
the 60Hz signal is, we think we know that an 8Hz signal (and multiples) is
lightning. We also see signals occasionally (lasting several hours) at
65Hz, 75Hz and 80Hz. Can anyone point me to any information about naturally
occuring (or artificially occuring for that matter) low frequency signals?
I am in contact with an amateur group in CA who publish a newsletter but I
can find nothing from the professional world about this end of the
spectrum.

Thanks,

kyle

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
! kyle forinash 812-941-2390 !
! forinas@indiana.edu !
! Natural Science Division !
! Indiana University Southeast !
! New Albany, IN 47150 !
! http://Physics.ius.indiana.edu/Physics.html !
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!


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