Using Dark Fiber and Distributed Acoustic Sensing to Characterize a
Geothermal System in the Imperial Valley, Southern California
Abstract
The Imperial Valley, CA, is a tectonically active transtensional basin
located south of the Salton Sea; the area hosts numerous geothermal
fields, including significant hidden hydrothermal resources without
surface manifestations. Development of inexpensive, rugged, and
highly-sensitive exploration techniques for undiscovered geothermal
systems is critical for accelerating geothermal power deployment as well
as unlocking a low-carbon energy future. We present a case study
utilizing distributed acoustic sensing (DAS) and ambient noise
interferometry for geothermal reservoir imaging utilizing an unlit
fiber-optic telecommunication infrastructure (dark fiber). The study
utilizes passive DAS data acquired from early November 2020 over a
~28-kilometer section of fiber from Calipatria, CA to
Imperial, CA. We apply ambient noise interferometry to retrieve coherent
signals from DAS records, and develop a spatial stacking technique to
attenuate effects from persistent localized noise sources and to enhance
retrieval of coherent surface waves. As a result, we are able to obtain
high-resolution two-dimensional (2D) S wave velocity (Vs) structure to 3
km depth based on joint inversion of both the fundamental and higher
overtones. We observe a previously unmapped high Vs and low Vp/Vs ratio
feature beneath the Brawley geothermal system that we interpret to be a
zone of hydrothermal mineralization and lower porosity. This
interpretation is consistent with a host of other measurements including
surface heat flow, gravity anomalies, and available borehole wireline
data. These results demonstrate the potential utility of DAS deployed on
dark fiber for geothermal system exploration and characterization in the
appropriate contexts.