PSN-L Email List Message

Subject: Re: "Giant Seismometer"
From: Bob Smith bobsmith5@........
Date: Wed, 27 Dec 2000 07:28:13 -0500

John --

I admire the creative process that thought this one up!!!

I wonder, however, if there isn't a basic flaw in this
plan.  I would expect that any such fiber optic cables are
installed with generous numbers of expansion loops.  Thus,
there would be very little variation in strain in conjuction
with geological events.  However the following does not
mention the mechanism which produces said strain so I may
have missed the point here.

	I hope they succeed, best to all, Bob Smith

John Hernlund wrote:
> Hi Everyone,
>    I came across some really good stuff at AGU last weekend.  One
> item that will interest you all is the construction of what may be the
> largest seismometer ever.  A friend of mine named Steve Gao (at
> Kansas State) got together with a fellow engineering professor at
> KSU that can measure the length of fiber optic cable very accurately
> using a simple reflection technique (the technology is in the sending
> and receiving unit).  They got an NSF grant to go around hooking this
> thing up to existing fiber optic networks and monitoring the strain
> across the fiber optic cable in time.  They haven't yet conducted their
> first major experiment, but they expect to measure a lot of subtle
> effects.  The advantage of this device is that it is truly broadband,
> and the technique can conceivably sample at any frequency without
> any frequency dependence.  The sending/receiving unit costs around
> $10,000 to build, which is cheaper than many commercial broadband
> seismometers.  Since most of the fiber optic cables in the ground for
> telecommunications are not in use (due to super-high bandwidth) they
> can go around renting an optic cable here and there.  Their biggest
> ambition is to wire up a huge network of these things, especially in
> the area of the San Andreas where strain precursors may be looked
> into for a very low price.  Traditional strain measurement at the San
> Andreas has consisted of laser interferometry through the air, which
> turns out noisy/messy data and is extremely expensive.  Plus the
> desired accuracy can be achieved in the fiber by simply increasing the
> length of the cable you use, since the error over time
> will be independent of the fiber length.  If the instrument yields an
> error of X (for example), and you wish to measure a strain above a
> noise level E, then just hook it up to a cable of length L = X/E.  For
> them, X is typically less than 1 mm, so for a 10 km cable the lower
> limit of effective measurement is 10^(-7), which gives them a very
> useful range.  Over time these things will become more accurate and
> less expensive to use...
> Cheers!
> John Hernlund
> Department of Earth and Space Sciences
> University of California, Los Angeles
> hernlund@............
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Robert L. (Bob) Smith			Smith Machine Works, Inc.
internet   bobsmith5@.............. Lumlay Road
landline   804/745-1065	                Richmond, Virginia

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