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Seismic Velocity Estimation Using Passive Downhole Distributed Acoustic Sensing Records: Examples From the San Andreas Fault Observatory at Depth
Author(s) -
Lellouch A.,
Yuan S.,
Spica Z.,
Biondi B.,
Ellsworth W. L.
Publication year - 2019
Publication title -
journal of geophysical research: solid earth
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.983
H-Index - 232
eISSN - 2169-9356
pISSN - 2169-9313
DOI - 10.1029/2019jb017533
Subject(s) - geophone , seismology , geology , vertical seismic profile , seismic velocity , anomaly (physics) , observatory , seismic wave , distributed acoustic sensing , range (aeronautics) , geodesy , fiber optic sensor , computer science , engineering , telecommunications , physics , optical fiber , astrophysics , aerospace engineering , condensed matter physics
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.