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\begin{document}

\title{Distributed Acoustic Sensing (DAS) as a Distributed Hydraulic Sensor in
Fractured Bedrock}



\author[1]{Matthew W Becker}%
\author[2]{Thomas I Coleman}%
\author[3]{Chris Ciervo}%
\affil[1]{California State University Long Beach}%
\affil[2]{Silixa LLC}%
\affil[3]{California State University, Long Beach}%


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  \date{May 29, 2024}


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\begin{abstract}
Distributed Acoustic Sensing (DAS) was originally intended to measure
oscillatory strain at frequencies of 1 Hertz or more on a fiber optic
cable. Recently, measurements at much lower frequencies have opened the
possibility of using DAS as a dynamic strain sensor in boreholes. A
fiber optic cable mechanically coupled to a geologic formation will
strain in response to hydraulic stresses in pores and fractures. A DAS
interrogator can measure dynamic strain in the borehole which can be
related to fluid pressure through the mechanical compliance properties
of the formation. Because DAS makes distributed measurements, it is
capable of both locating hydraulically active features and quantifying
the fluid pressure in the formation. We present field experiments in
which a fiber optic cable was mechanically coupled to two crystalline
rock boreholes. The formation was stressed hydraulically at another well
using alternating injection and pumping. The DAS instrument measured
oscillating strain at the location of a fracture zone known to be
hydraulically active. Rock displacements of less than one nanometer were
measured. Laboratory experiments confirm that displacement is measured
correctly. These results suggest that fiber optic cable embedded in
geologic formations may be used to map hydraulic connections in three
dimensional fracture networks. A great advantage of this approach is
that strain, an indirect measure of hydraulic stress, can be measured
without beforehand knowledge of flowing fractures that intersect
boreholes. The technology has obvious applications in water resources,
geothermal energy, CO sequestration, and remediation of groundwater in
fractured bedrock.%
\end{abstract}%



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