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. Regards, Sean-Thomas __________________________________________________________ Public Seismic Network Mailing List (PSN-L)
Larry Cochrane <cochrane@..............>