Seismic Velocity Estimation and Earthquake Detection Using Downhole Optical Fiber Sensing (DAS) - Examples from SAFOD

Ariel Lellouch

Stanford University

Date & Time
Building 3, Rambo Auditorium
Grace Parker

Distributed acoustic sensing (DAS) has been delivering us records of very high spatial and
temporal resolution. During the summer of 2017, about 22 days of passive recorded data were
acquired using a vertical downhole DAS array in the top 800 m of the San Andreas Fault
Observatory at Depth (SAFOD) main hole. In this talk, I will show two seismological applications
that take advantage of the revolutionary DAS data quality in a downhole array. First, certain
earthquakes may be used for P and S wave velocity model estimation, yielding results
comparable to (or better than) conventional downhole geophone surveys. This analysis may
unveil previously unknown geological structures. Ambient field records can also be used to
retrieve the P-wave velocity structure with reasonable quality. Then, I’ll show how a seismic
velocity-based detection method can be applied for automatic earthquake detection. Despite
the high performance of the USGS network in the area, we discovered and analyzed a
previously uncataloged earthquake. This method is picking-free, easy to implement, and yields
some directional information during the detection process.

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