PSN-L Email List Message

Subject: Re: fiber optic strainmeters
From: John Hernlund hernlund@............
Date: Wed, 27 Dec 2000 12:17:48 -0800

sean@........... wrote:

> For the idea of using terrestial fibers, we can presume that long distance
> runs will be used. These are popularly installed using abandoned oil
> and gas lines as conduits, and the thinking for the strainmeter is that
> simple friction will convey the change of length of the earth to the
> conduit and then to the fiber cable lying loosely inside: the change
> is very small, and distributed over a relatively large distance. At
> slack points, there would be no coupling, but these would represent a
> very small portion of the length. And although such pipes/conduits are
> buried at least a meter, temperature is a major concern; it changes the
> index of the glass, so it can be determined; at CCM, the very shallow
> (.5 m) cable changes its loss by about 4 db (out of about 20 db total
> loss) from summer to winter.

Yes, this is something they will have to address.  I think their first
mission is to measure the tides over a large area.  This kind of data
can be quite valuable because the gravitational forces which induce
the tides are very well known, thus the measured data can be used to
get material properties if the measurement is sensitive enough.

> Of further interest in fiber optic strainmeters for geodetic strain
> measurements is a plan to use "extra" fibers in the undersea cables.
> Since much of the earth is oceanic and off limits to practical
> seismic instruments, measuring strain across the oceans would be
> a great contribution to geodynamics.

This would be fanstastic.  It would actually be perfect to measure strain
in segments across the Atlantic to see how it varies perpendicular to
the ridge.  I would be interested in seeing how strain accumulated at
the ridge propagates outward, and how it varies in time and space.  It
would certainly be nice to get some real numbers to compare with our
numerical models.

> The first fiber optic strainmeters were developed by UCSD at the
> Pinon Flat strain observatory (PFO) about a decade ago. They were
> installed in boreholes drilled at a 45 degree angle, and used a
> complex tensioning device. They had a resolution of about 10^-6,
> limited by the slanted borehole length.

The engineering fellow currently uses fiber optic strain meters on
structures such as bridges.  They have been very useful for him in
measuring a structure's response to wind and other forces.


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


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