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Open AccessArticle

Distributed Acoustic Sensing of Strain at Earth Tide Frequencies

1
Department of Geological Sciences, California State University Long Beach, 1250 Bellflower Boulevard, Long Beach, CA 90840, USA
2
Silixa LLC, 16203 Park Row, Suite 185, Houston, TX 77084, USA
*
Author to whom correspondence should be addressed.
Sensors 2019, 19(9), 1975; https://doi.org/10.3390/s19091975
Received: 19 March 2019 / Revised: 19 April 2019 / Accepted: 24 April 2019 / Published: 27 April 2019
(This article belongs to the Special Issue Distributed Optical Fiber Sensing)
The solid Earth strains in response to the gravitational pull from the Moon, Sun, and other planetary bodies. Measuring the flexure of geologic material in response to these Earth tides provides information about the geomechanical properties of rock and sediment. Such measurements are particularly useful for understanding dilation of faults and fractures in competent rock. A new approach to measuring earth tides using fiber optic distributed acoustic sensing (DAS) is presented here. DAS was originally designed to record acoustic vibration through the measurement of dynamic strain on a fiber optic cable. Here, laboratory experiments demonstrate that oscillating strain can be measured with DAS in the microHertz frequency range, corresponding to half-day (M2) lunar tidal cycles. Although the magnitude of strain measured in the laboratory is larger than what would be expected due to earth tides, a clear signal at half-day period was extracted from the data. With the increased signal-to-noise expected from quiet field applications and improvements to DAS using engineered fiber, earth tides could potentially be measured in deep boreholes with DAS. Because of the distributed nature of the sensor (0.25 m measurement interval over kilometres), fractures could be simultaneously located and evaluated. Such measurements would provide valuable information regarding the placement and stiffness of open fractures in bedrock. Characterization of bedrock fractures is an important goal for multiple subsurface operations such as petroleum extraction, geothermal energy recovery, and geologic carbon sequestration. View Full-Text
Keywords: distributed acoustic sensing; fiber optic sensors; earth tides; low frequency strain; geomechanics distributed acoustic sensing; fiber optic sensors; earth tides; low frequency strain; geomechanics
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Becker, M.W.; Coleman, T.I. Distributed Acoustic Sensing of Strain at Earth Tide Frequencies. Sensors 2019, 19, 1975.

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