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Optical Fiber Strainmeters: Developing Higher Dynamic Range and Broader Bandwidth

Billy Hatfield, Mark A. Zumberge, Frank K. Wyatt, & Duncan C. Agnew

Published August 15, 2016, SCEC Contribution #7047, 2016 SCEC Annual Meeting Poster #261

We have been developing near-surface horizontal strainmeters and vertical borehole strainmeters that use passive optical fibers as their primary sensing element: optical-fiber strainmeters, or OFS. These instruments are useful in two areas. First, the sensing of deformation over long baselines (hundreds of meters), without calibration issues, over a band from 100 Hz to a few days: a potential alternative to borehole strainmeters, especially given lower cost and usefulness in a wide range of geologic settings. Second, use in Earthquake Early Warning (EEW), where the bandwidth provides a single record including both the dynamic signals from radiated waves and the near-field static offset, both of which are important in EEW; the long baselength allows more reliable measurement of the offset. The optical-fiber systems have a dynamic range that is
fundamentally limited only by the fiber becoming nonlinear; between this and the resolution of the system the range is 120 dB or better. The current systems are limited by the ability of the fringe-monitoring system to reliably track very high rates of strain; this occurred on the fiber strainmeters installed at Pinon Flat Observatory (PFO) at the time of the recent Anza earthquake in June (2016:162, Mw 5.2, hypocentral distance 23 km), though the systems behaved linearly otherwise. We are, with SCEC funding, developing a fringe-counting system with very high rate fringe tracking. For broadening the bandwidth at low frequencies, temperature-induced noise must be reduced; we have developed an OFS using a dual-fiber system, measuring changes in optical path length along two fibers with different temperature coefficients; the resulting time series can be combined to provide a record of the strain much less affected by temperature. We will report primarily on the performance of the OFS systems installed at PFO, where we can compare them with the existing vacuum-pipe strainmeters for microseisms, earthquakes, tides, and longer periods. But we also have useful results from the first deployment of a long-base strainmeter on the sea floor, at a depth of 1900 m and 93 km west of the Oregon coastline.

Hatfield, B., Zumberge, M. A., Wyatt, F. K., & Agnew, D. C. (2016, 08). Optical Fiber Strainmeters: Developing Higher Dynamic Range and Broader Bandwidth. Poster Presentation at 2016 SCEC Annual Meeting.

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