UC San Diego

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 unlit fiber‐optic telecommunication infrastructure (dark fiber). The study exploits two days of passive DAS data acquired in early November 2020 over a ∼28‐km section of fiber from Calipatria, CA to Imperial, CA. We apply ambient noise interferometry to retrieve coherent signals from DAS records and develop a bin stacking technique to attenuate the 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 ( V s ) structure to 3 km depth, based on joint inversion of both the fundamental and higher overtones. We observe a previously unmapped high V s and low V p / V s ratio feature beneath the Brawley geothermal system, which 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 geological settings. Plain Language Summary Geothermal resources are considered a valuable component of our global transition to a zero‐emissions sustainable energy future. In the Imperial Valley, CA, three of the four producing geothermal fields have no active surface features. The development of inexpensive, rugged, and highly sensitive exploration techniques for undiscovered geothermal systems is a critical step in accelerating geothermal power discovery and deployment. We utilize a novel technology that converts ∼28‐km section of existing unused telecommunication fiber into seismic sensors (called distributed acoustic sensing, DAS) to characterize the subsurface geothermal resources. Ambient seismic noise data are recorded and processed to retrieve high quality coherent seismic waves after localized noise sources are attenuated. Our results reveal significant high‐velocity anomalies beneath the Brawley Geothermal Field; these are coincident with observations from boreholes, heat flow, and gravity surveys which indicate hydrothermal alteration has a pronounced effect on the physical properties of the metamorphosed sediments. Key Points We utilize high‐resolution ambient noise imaging to characterize a geothermal system using distributed acoustic sensing and dark fiber We develop a bin stacking technique to attenuate the effects of persistent local noise sources and enhance the retrieved empirical Green's functions We image a zone of high shear wave velocity beneath the Brawley geothermal field, which we interpret to be a zone of hydrothermal alteration