oa Characterizing Near-Surface Structures and Properties Using Distributed Acoustic Sensing

Distributed Acoustic Sensing (DAS) has emerged as a promising geophysical tool for seismic surveys, utilizing a continuous distributed fiber-optic cable to capture seismic signals. In this study, a DAS system employing the phase-generated carrier (PGC) algorithm was utilized in a near-surface seismic survey at a construction site in Hebei Province, China. The objective was to process and analyze the DAS seismic data to reveal near-surface layer structures and media properties, with a focus on its role in ensuring safety in high-rise buildings and mitigating geo-hazards in smart city development. The experiment layout featured an L-shaped fiber cable spanning approximately 430 meters along the X and Y lines, with seven seismic sources strategically positioned. The DAS recording had a 1-meter channel spacing, 0.4 ms time sampling, and 8-second recording duration. Seismic shots were generated using a vehicle with a heavy hammer, and DAS records revealed direct, refracted, and reflected seismic events, enabling the determination of velocities and thicknesses of shallow subsurface layers. Seismic profiles along the X and Y lines provided insights into fiber directivity and media anisotropy. Analysis of ray paths from seismic source P1 to fiber sensors indicated variations in arrival times and amplitudes, suggesting differential anisotropy within the shallower strata. Spectral analysis of seismic signals and coda noise revealed distinctive spectral patterns in the DAS records that are attributed to the source truck’s vibrations. These patterns were consistent across multiple seismic shots, except for seismic shot P1, highlighting the influence of source location. This study demonstrated the utility of DAS seismic surveys in characterizing near-surface structures and detecting variations in media properties. Shallow layers exhibited strong anisotropy and attenuation, while the deepest layer displayed weak anisotropy. Further waveform modeling could enhance the analysis. DAS near-surface seismic surveys offer a flexible and cost-effective approach critical for smart city development, providing essential insights into shallow subsurface structures for geo-hazard mitigation and high-rise building safety.