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DeWolf, S, Wyatt FK, Zumberge MA, Hatfield W.  2015.  Improved vertical optical fiber borehole strainmeter design for measuring Earth strain. Review of Scientific Instruments. 86   10.1063/1.4935923   AbstractWebsite

Fiber-based interferometers provide the means to sense very small displacements over long baselines, and have the advantage of being nearly completely passive in their operation, making them particularly well suited for geophysical applications. A new 250 m, interferometric vertical borehole strainmeter has been developed based completely on passive optical components. Details of the design and deployment at the Pinon Flat Observatory are presented. Power spectra show an intertidal noise level of -130 dB (re. 1 epsilon(2)/Hz), consistent within 1-3 dB between redundant components. Examination of its response to Earth tides and earthquakes relative to the areal strain recorded by an orthogonal pair of collocated, 730 m horizontal laser strainmeters yield a Poisson's ratio for local near surface material of 0.25 that is consistent with previous results. (C) 2015 AIP Publishing LLC.