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Improved vertical optical fiber borehole strainmeter design for measuring Earth strain
Scott DeWolf1, Frank K Wyatt2, Mark A Zumberge2
1Environmental Engineering and Earth Sciences, Clemson University, Clemson, South Carolina 29634-0919, USA.
The Review of Scientific Instruments
|December 3, 2015
Summary
A new passive optical borehole strainmeter effectively measures Earth tides and seismic events. This geophysical instrument achieves low noise levels, providing accurate strain data for geological studies.
Area of Science:
- Geophysics
- Optical instrumentation
- Seismology
Background:
- Fiber-based interferometers are passive, long-baseline displacement sensors ideal for geophysical applications.
- Existing strain measurement techniques require robust, sensitive instrumentation for detecting subtle ground deformations.
Purpose of the Study:
- To develop and present a novel 250 m interferometric vertical borehole strainmeter utilizing passive optical components.
- To detail the design and deployment of this new strainmeter at the Piñon Flat Observatory.
- To assess the instrument's noise performance and its ability to measure geophysical phenomena.
Main Methods:
- Construction of a 250 m vertical borehole strainmeter using entirely passive optical elements.
- Deployment at the Piñon Flat Observatory for field testing and data acquisition.
- Analysis of power spectra for noise level determination and comparison with collocated horizontal strainmeters.
Main Results:
- Achieved an intertidal noise level of -130 dB (re. 1 ϵ(2)/Hz), with high consistency between redundant components (1-3 dB).
- Successfully recorded responses to Earth tides and earthquakes.
- Calculated a local near-surface material Poisson's ratio of 0.25 by comparing vertical and horizontal strain data, consistent with prior research.
Conclusions:
- The developed passive optical borehole strainmeter is a viable tool for geophysical strain measurements.
- The instrument demonstrates excellent low-noise performance and accurately detects seismic and tidal signals.
- The results validate the use of this technology for characterizing local material properties like Poisson's ratio.
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