Ambient noise shallow structure imaging with distributed acoustic sensing: A case study in Wenyu River area, Beijing

The shallow subsurface media in urban areas are closely related to human life. Near-surface media generally exhibit characteristics such as lower seismic wave velocity, lower density, stronger absorption of seismic wave energy, and significant heterogeneity both laterally and vertically. When seismic waves propagate upward from the deeper high-impedance bedrock to the low-impedance loose overburden near the surface, influenced by energy conservation and strong impedance contrasts, significant ground motion amplification occurs, characterized by increased amplitude and prolonged vibration duration. This site effect can trigger resonance phenomena, exacerbate the destructive power of strong earthquakes, and lead to severe disasters. Therefore, acquiring high-resolution shallow shear-wave velocity structures through advanced imaging techniques is crucial for site response evaluation and seismic hazard risk prevention and control. As an ultra-dense seismic observation method, Distributed Acoustic Sensing (DAS) technology offers a sensor spacing of 1–10 meters, enabling higher-resolution imaging of near-surface structures at a lower cost. The Wenyu River area in Beijing features complex geological structures, including potential geological hazards such as ground fissures and land subsidence, which significantly impact urban planning and underground space construction in Beijing. This study utilizes data collected from a DAS system deployed in the Wenyu River area to conduct near-surface imaging research, obtaining a high-resolution two-dimensional shear-wave velocity structure within a depth of 80 meters. The results reveal significant vertical stratification in shear-wave velocity from 0 to 80 meters depth: a low-velocity zone with Vs < 150 m/s at 0–10 meters depth, likely caused by backfill during fiber optic installation; a gradual increase in shear-wave velocity from 150 m/s to 300 m/s at 10–40 meters depth; and increased medium stiffness at 40–80 meters depth, with shear-wave velocities reaching approximately 450 m/s, reflecting a lithological transition from loose fill and silty clay to dense sand-gravel layers. Local low-velocity anomalies observed in channels CH036 and CH131 are likely attributed to the cavity effect of underground drainage channels and reduced soil shear modulus due to water infiltration from an artificial lake, as confirmed by field investigations.