Near-surface seismic properties characterizations with a roadside Distributed Acoustic Sensing (DAS) array

Cars and earthquakes induce Rayleigh waves that are recorded on a roadside section of the Stanford DAS-2 fiber array. They can be directly used for near-surface S-wave velocity inversion. Cars driving along the road and small earthquakes excite Rayleigh waves with complementary frequency bands. The surface waves induced by passing cars have a consistent fundamental mode and a noisier first mode. By stacking dispersion images of 33 passing cars recorded in the same section of the DAS array, we obtain a stable dispersion image with two clear modes. The frequency range of the fundamental mode can be extended by adding the lowfrequency earthquake-induced Rayleigh waves. In order to assure clear separation from Love waves and aligning apparent velocity with phase velocity, we choose an earthquake that is approximately in line with the array. Thanks to the extended frequency range, we can achieve better depth coverage and resolution for shear-wave inversion. The inverted models match those obtained by a conventional geophone survey performed using active sources. Processing car-induced surface waves is dramatically cheaper than interferometry and reliable estimates can be obtained more frequently because surface wave analysis do not require convergence of the interferometric analysis. Furthermore, in order to automate the Vs imaging process, we introduce a new objective function that avoids manual dispersion curve picking.