Fiber optic Distributed Acoustic Sensing (DAS) is based on the principles of Coherent Rayleigh Optical Time Domain Reflectometry, where light pulses are fired through an optical fiber, and photon backscatter is measured with an optical sensor. Strain in the fiber causes changes in the amplitude and phase of backscattered light. Using light’s two-way travel time, the optical sensor measures strain at distributed points along the length of fiber. In this work, DAS was adapted to establish hydraulic connectivity in bedrock by measuring hydromechanical strain in an observation well, as periodic well tests were conducted at mHz frequencies at an interrogation well ~30 m away. A lognormal relationship with a strong degree of interdependence was found between measured displacements and pressure amplitudes. This behavior is consistent with the semi-logarithmic closure law of fractured rock. The nanometer scale displacements reported here, however, suggest closure occurring as in-contact asperities deform, rather than opposing fracture surfaces coming into contact.
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|Advisor:||Becker, Matthew W.|
|Commitee:||Hagedorn, Benjamin, Onderdonk, Nate|
|School:||California State University, Long Beach|
|School Location:||United States -- California|
|Source:||MAI 58/02M(E), Masters Abstracts International|
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