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Subject: Re: laser seismograph
From: John Hernlund hernlund@............
Date: Tue, 06 Nov 2001 18:48:47 -0800

A friend of mine has co-developed a laser seismograph.  Really, it is a
strain meter.  The way it works is to lay out a long line of fiber optic
cable and bounce light signals.  One end is reflecting, while the other
end has a transmitter/receiver.  The "time of flight" variations give
you the change in length.  Dividing the change in length by the over all
length gives the strain as a function of time.  His large prototype is
buried in his back yard in Kansas...

He caught on to this while gabbing with an engineer who uses this same
technique to measure strain in structures (eg. bridges) and apparently
it works quite well.  They are working on a proposal to rent fiber
optics communications lines and hook this stuff could be a lot
of fun.  The longer term arm-waving goal is to eventually measure
precursory strain transients near an active fault (eg. San Andreas).
They could also work on other fun problems with sufficient sensitivity,
such as the Chandler wobble...

Most people I have known prefer an interferometric method to time of
flight methods, but I am not sure how each technique stacks up against
the other with the current technology.  The interferometric method uses
a steady transmitted signal and measures the relative interference of
the waves to back out the changes in length.  For a laser with a
wavelength around 500 nm, the resolution is a small fraction of this.
Averaged out over a very long cable, this would allow very small strains
to be measured.  The nice thing about these types of instruments is that
they are truly only by the number and frequency of

The astronomers working on binocularizing the Keck scope have run into
difficulties in connecting the two sources via an underground
tunnel...this also uses interferometric methods.  I wonder how they are
doing now; whether they have fixed this problem or not.  It may be
interesting to find out how they fix it all, since the Keck could easily
become a strain meter as well as a bad ass telescope when they do figure
it out.


John Hernlund
Department of Earth and Space Sciences
University of California, Los Angeles


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