Monitoring Seismic Velocity Changes with Borehole DAS at SAFOD Using Ambient Noise and Active Source Surveys

Natalia Berrios-Rivera, Andrew J. Barbour, Stephen H. Hickman, William L. Ellsworth, Rufus D. Catchings, Coyn Criley, Mark R. Goldman, Martin Karrenbach, Robert R. Sickler, James W. Atterholt, Albert L. Aguilar, Alina Belyalova, & Ettore Biondi

Submitted August 30, 2026, SCEC Contribution #15474, 2026 SCEC Annual Meeting Poster #TBD

The San Andreas Fault Observatory at Depth (SAFOD) provides a unique opportunity to use downhole distributed acoustic sensing (DAS) for continuous monitoring and shallow velocity imaging along a seismically active and creeping section of the San Andreas Fault just north of Parkfield, CA. Previous DAS work at SAFOD recovered body wave velocities using ambient noise interferometry from about 22 days of DAS recordings in 2017, using a vertical fiber cemented behind casing to 864 m depth in 2004. Here, we analyze DAS recordings from the same cemented borehole fiber, over 14 months in 2024-2025, using a comparable ambient noise cross-correlation framework. The 2024-25 dataset spans over a year, allowing for passive body wave imaging with borehole DAS under varying source and seasonal environmental conditions, including variations in wind noise, hydrological conditions, and nearby seismicity.

To supplement this continuous passive monitoring, we conducted an active source experiment using four 5 lb shot hole charges, as well as a 500 lb accelerated weight drop source that was repeated every 30 minutes over 24 hours. This active source experiment provides independent constraints on the shallow velocity structure, DAS sensitivities and borehole coupling effects, and gives insight into which passive sources available at SAFOD can reasonably be used for time-lapse monitoring. Potential sources include ambient noise, individual local earthquakes, and repeating earthquake sequences. This work will help evaluate whether the SAFOD borehole fiber geometry and available seismic sources are conducive to monitoring shallow velocity changes associated with hydrologic loading, fault zone deformation, and pore pressure perturbations. Our results will also inform future deep active and passive source DAS experiments at SAFOD, including with the newly installed 3-km-deep DAS array. By combining passive monitoring and active source results, we aim to show how SAFOD could serve as a field laboratory for identifying source types and monitoring techniques that are effective for borehole fiber-optic imaging in active fault zones.

Key Words
DAS, SAFOD

Citation
Berrios-Rivera, N., Barbour, A. J., Hickman, S. H., Ellsworth, W. L., Catchings, R. D., Criley, C., Goldman, M. R., Karrenbach, M., Sickler, R. R., Atterholt, J. W., Aguilar, A. L., Belyalova, A., & Biondi, E. (2026, 08). Monitoring Seismic Velocity Changes with Borehole DAS at SAFOD Using Ambient Noise and Active Source Surveys. Poster Presentation at 2026 SCEC Annual Meeting.

Related Projects & Working Groups
Seismology