PSN-L Email List Message

Subject: fiber optic strainmeters
From: sean@...........
Date: Wed, 27 Dec 2000 13:29:41 -0600 (CST)

Darrell, John, and co., 

Yes, fiber cables are major backhoe attractors. I operate a 3 km
link for a super broadband station in a state park, with three 1 km
lengths between the cavern and the visitor center. The cable was 
installed in 1988, so they pretty well know where it is. It has had 
close calls before, but the double armor has saved it. Last spring they
called me to exactly locate the cable, which I have marked with magnets
every 10 m, but they went ahead and tore it up as I was driving there.
The break was 100 m from a splice pedestal, so by replacing 110 m of
the cable, I ended up with the loss of only one additional splice.

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.

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.

But a feature of undersea fiber cables is that the optical signal 
still has to be regenerated every 5 to 15 kilometers. So the plan is 
to use every cable segment as a separate strainmeter, with appropriate
instrumentation at each cable amplifier node, and with the data of 
each segment multiplexed to a continuous fiber, like the cable's
state-of-health/control fiber.

But this would be very expensive, even assuming a cable owner would share
the fiber bundle and allow the inclusion of the interferometers gratis.

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.


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Larry Cochrane <cochrane@..............>