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Near-surface Characterization Using a Roadside Distributed Acoustic Sensing Array

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Document pages: 32 pages

Abstract: Thanks to the broadband nature of the Distributed Acoustic Sensing (DAS)measurement, a roadside section of the Stanford DAS-2 array can record seismicsignals from various sources. For example, it measures the earth s quasi-staticdistortion caused by the weight of cars (<0.8 Hz), and Rayleigh waves inducedby earthquakes (<3 Hz) and by dynamic car-road interactions (3-20 Hz). Wedirectly utilize the excited surface waves for shallow shear-wave velocityinversion. Rayleigh waves induced by passing cars have a consistent fundamentalmode and a noisier first mode. By stacking dispersion images of 33 passingcars, we obtain stable dispersion images. The frequency range of thefundamental mode can be extended by adding the low-frequency earthquake-inducedRayleigh waves. Thanks to the extended frequency range, we can achieve betterdepth coverage and resolution for shear-wave velocity inversion. In order toassure clear separation from Love waves and aligning apparent velocity withphase velocity, we choose an earthquake that is approximately in line with thearray. The inverted models match those obtained by a conventional geophonesurvey performed by a geotechnical service company contracted by StanfordUniversity using active sources from the surface until about 50 meters. Inorder to automate the Vs inversion process, we introduce a new objectivefunction that avoids manual dispersion curve picking. We construct a 2-D Vsprofile by performing independent 1-D inversions at multiple locations alongthe fiber. From the low-frequency quasi-static distortion recordings, we invertfor a single Poisson s ratio at each location along the fiber. We observespatial heterogeneity of both Vs and Poisson s ratio profiles. Our approach isdramatically cheaper than ambient field interferometry and reliable estimatescan be obtained more frequently as no lengthy cross-correlations are required.

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