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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 Piñon Flat Observatory are presented. Power spectra show an intertidal noise level of -130 dB (re. 1 ε²/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. [ABSTRACT FROM AUTHOR]

We are developing a new vertical seismometer, motivated by a desire to have an instrument whose performance is similar to that of observatory sensors yet can operate within a borehole without electronics. This has led us to an all-optical seismometer consisting of a spring-suspended mass whose position is monitored inter-ferometrically. We use a Michelson interferometer illuminated with a 1 mW laser that can be linked to the seismometer with optical fibers only. A digital signal processor samples the interference fringe signal and produces a 400 samples/sec record of the seismometer mass displacement with a root mean square noise per octave band that varies from about 4 x 10-12 m at 0.001 Hz to 4 x 10-13 m at 1 Hz. The maximum displacement is limited by mechanical issues to a few millimeters at present, providing a dynamic range of at least 109, equivalent to 30 bits (180 dB). Experiments to test this idea have been performed on a modified STS1 vertical seismometer whose electronics have been replaced with an optical system, Comparisons with other seismometers show that, in terms of both noise and signal fidelity, the optical approach is quite viable. [ABSTRACT FROM AUTHOR]

Describes the design and testing of a low-finesse Fabry-Perot optical fiber vibration-isolated interferometer for use in absolute gravity meters. Design features; Fringe-processing technique for the interferometer; Practical details of construction.