Seismic Velocity Estimation Using Passive Downhole Distributed Acoustic Sensing Records: Examples From the San Andreas Fault Observatory at Depth. Issue 7 (22nd July 2019)

• Record Type: Journal Article
• Authors: Lellouch, A.; Yuan, S.; Spica, Z.; Biondi, B.; Ellsworth, W. L.
• Is Part Of: Journal of geophysical research. Volume 124:Issue 7(2019)
• Page Start: 6931
• Page End: 6948
• Publication Date: 2019-07-22
• Subjects: Seismic velocity -- Distributed Acoustic Sensing -- borehole seismology -- SAFOD -- Downhole; Geomagnetism -- Periodicals; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 551.1
• DOI: 10.1029/2019JB017533
• Languages: English
• ISSNs: 2169-9313

Abstract

Abstract: Structural imaging and event location require an accurate estimation of the seismic velocity. However, active seismic surveys used to estimate it are expensive and time‐consuming. During the last decade, fiber‐optic‐based distributed acoustic sensing has emerged as a reliable, enduring, and high‐resolution seismic sensing technology. We show how downhole distributed acoustic sensing passive records from the San Andreas Fault Observatory at Depth can be used for seismic velocity estimation. Using data recorded from earthquakes propagating near‐vertically, we compute seismic velocities using first‐break picking as well as slant stack decomposition. This methodology allows for the estimation of both P and S wave velocity models. We also use records of the ambient seismic field for interferometry and P wave velocity model extraction. Results are compared to a regional model obtained from surface seismic as well as a conventional downhole geophone survey. We find that using recorded earthquakes, we obtain the highest P wave model resolution. In addition, it is the only method that allows for S wave velocity estimation. Computed P and S models unravel three distinct areas at the depth range of 50‐750 m, which were not present in the regional model. In addition, we find high V P / V S values near the surface and a possible V P / V S anomaly about 500 m deep. We confirm its existence by observing a strong S ‐ P mode conversion at that depth. Key Points: A seismically active area is recorded using distributed acoustic sensing of a downhole fiber We estimate P‐ and S‐ wave velocities using fiber data and compare them with conventional surveys Velocity analysis unveils a geological structure at the San Andreas Fault Observatory at Depth location