A recent study published in Engineering presents a novel approach to urban geohazard investigations, specifically focusing on karst features. Karst hazards, which can cause surface subsidence, ground collapse, and groundwater pollution, pose significant threats to urban areas. Traditional geophysical methods often face challenges in complex urban environments, such as restricted land accessibility and noise interference.
The research team, led by Jianghai Xia from Zhejiang University, designed a synchronous-asynchronous ambient noise observation system. This system allows for dense-array observations with a limited number of stations, making it a cost-effective solution for urban geohazard investigations.
The study was conducted in an abandoned parking lot in Hangzhou, China, where karst caves beneath the surface threatened planned construction. The researchers used 197 nodal receivers with an average station spacing of 7 m and carried out two rounds of asynchronous observations, each lasting approximately 24 hours, along with synchronous backbone stations.
Analysis of the ambient noise characteristics showed that the noise in urban environments is highly complex, with significant diurnal variations and non-uniform source distributions. To address these challenges, the team proposed several innovative methods. They calculated the C1 functions by enhancing the noise sources in the stationary-phase zones. By using the similarity coefficients between multicomponent C1 functions related to Rayleigh waves as weights for stacking, they were able to suppress the influence of non-uniform noise source distributions.
For the calculation of C2 functions, which are used to obtain the empirical Green's functions between asynchronous stations, the researchers filtered the C1 functions to eliminate artifacts from higher-mode surface waves. This was achieved by leveraging the differences in particle motions of fundamental- and higher-mode Rayleigh waves.
After obtaining the C1 and C2 functions, the team measured the dispersion of Rayleigh waves and used surface wave tomography to invert for the subsurface 3D shear-wave (S-wave) velocity model. The inverted model revealed two significant low-velocity zones at depths ranging from 40 to 60 m, which were consistent with the karst caves found in the drilling data.
The addition of asynchronous observations significantly increased the ray path density of surface waves, improving the coverage of the study area. This new observation system reduces the number of stations required for dense-array observations, offering a practical alternative for future studies in urban geohazard investigations.
The short-term synchronous-asynchronous ambient noise tomography method shows great promise in providing high-resolution seismic imaging of subsurface structures in urban areas, which can contribute to better urban geological hazard prevention and control.
The paper "Short-Term Synchronous and Asynchronous Ambient Noise Tomography in Urban Areas: Application to Karst Investigation," authored by Ya Liu, Jianghai Xia, Bo Guan, Chaoqiang Xi, Ling Ning, Hao Zhang. Full text of the open access paper: https://doi.org/10.1016/j.eng.2025.02.001
