Quasi-static waveform inversion from DAS observations

As a vehicle approaches the fiber-optic cable, the distributed acoustic sensing (DAS) records a broadband strain rate, which corresponds to propagating seismic waves at high frequencies (>1Hz) and to quasi-static strain fields at low frequencies (<1Hz). However, characterizing the subsurface media through quasi-static deformations remains challenging. Here, we propose a new method for imaging shallow urban subsurface structures using quasi-static strain waveforms, measured with fiber-optic cables. This technique utilizes the quasi-static waveform of a single DAS channel to generate a local 1D velocity model, thereby enabling high-resolution imaging of the underground using thousands of densely packed channels. We employed the Markov Chain Monte Carlo (MCMC) inversion strategy to investigate the depth range of inversion using car-induced quasi-static waveforms. The synthetic data demonstrates that the quasi-static strain field generated by a standard small car moving over the ground enables detailed imaging of structures at depths from 0 to 10 meters. Additionally, we conducted field experiments to measure the 2D shear-wave velocity model along a highway using quasi-static strain waveforms generated by a four-wheeled small car. The velocity structure we obtained is closely aligned with that derived from the classical surface-wave phase-velocity inversion. This consistency indicates that the inversion depth range is comparable to the simulation results, which confirms the applicability of this method to real data. In the future, we anticipate using the city's extensive fiber-optic communication network to record quasi-static deformations induced by various types of vehicles, thereby enabling imaging of the urban subsurface at a citywide scale. This will provide valuable insights for the design of urban underground infrastructure and for assessing urban hazards and risks.