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Abandoned coal tunnels (ACTs) pose risks to coal mine safety by accumulating gas and water, causing surface subsidence, and endangering infrastructure. Accurate detection of ACTs is vital for advanced prediction and geotechnical hazard warnings. Seismic methods, particularly the surface-wave method, have demonstrated reliability in detecting shallow buried tunnels within complex geological environments, but improvements in accuracy and resolution are still needed. In this paper, we suggest utilizing the multiscale window analysis of surface waves (MWASW) method for ACT surveys. We conduct a synthetic test and a real-world case study to evaluate the feasibility and validity of this approach. The synthetic test on a 2D ACT model reveals that MWASW can reconstruct anomalous structures and identify the presence of an ACT, while also highlighting some limitations of the method. In a real-world scenario, we applied MWASW to a multi-survey line dataset from a coal mine in Shanxi Province, China. The resulting meter-scale resolution pseudo-3D S-wave velocity structure uncovers a primary ACT and two sub-ACTs buried at approximately 60 m, offering valuable insights into their morphology and orientation. To our knowledge, this is the first application of the MWASW method in such a case study, indicating its potential as an emerging alternative for characterizing complex near-surface media, such as abandoned coal tunnels or mined-out areas in coal fields. • Detect coal tunnels using spatial multiscale dispersion analysis method. • Directly reveal 2D S-wave velocity profiles with high resolution and accuracy. • Reconstruct a pseudo-3D structure model in field case. [ABSTRACT FROM AUTHOR]

The Passive Multichannel Analysis of Surface Waves (MASW) method is a non-intrusive and cost effective technology adopted in civil and earthquake engineering for site geological and geotechnical survey. However, the application of passive MASW method under urban environments meets some challenges. One of interesting problems is associated with underground railways, which can be treated theoretically as buried sources. Since Rayleigh wave energy is dominant among that of surface waves, it is critical to design a proper geophone configuration during field tests at a site close to the metro line to ensure significant Rayleigh wave energy and therefore reliable dispersion images. This paper studied the propagation of Rayleigh waves generated from a buried source and proposed an empirical formula to estimate the nearest offset, which was then verified by numerical simulations. A case study was implemented in Shanghai, China at a site close to a subway line. With a linear spread geophone configuration, the influence of different parameters, including nearest offset, spread length and geophone interval, was investigated regarding their influences on dispersion images. Field tests were conducted by following the geophone configuration designed based on the proposed estimation method. The one dimensional (1D) shear velocity structure, being inverted from field vibration measurements, matches relatively well with the geologic feature obtained from borehole testing. Study results are expected to help facilitate the application of the passive MASW method for cases in which Rayleigh waves are generated by the buried source like underground traffics in cities. • Near-field effects due to subway traffic were studied for surface wave method application. • A method was proposed for the nearest offset estimation in MASW test with buried sources. • A case study was performed at a site close to underground railway for validation. [ABSTRACT FROM AUTHOR]

Accurate characterization of the low-velocity anomalies such as utility tunnels in the near-surface by effective geophysical imaging methods has been a valuable but challenging task for urban hazard assessment, underground space development and infrastructure construction. As a current research hotspot, the full waveform inversion (FWI) has been proven to be a promising technique for high-precision imaging of substructures, and has been applied in related practical engineering fields. However, most previous studies have focused on single-parameter (S-wave velocity) or double-parameter (S-wave and P-wave velocities) inversion. So, a complete PSV/Rayleigh-wave multi-parameter (S-wave and P-wave velocities and density) FWI method is developed in this paper to achieve accurate detection of underground utility tunnels. In this approach, the multi-parameter of the model are updated simultaneously during the inversion process to alleviate the negative influence on the S-wave velocity inversion accuracy due to the far deviation of the other two parameters from their true values. We tested the performance of the approach with a synthetic test and a real experiment. In the synthetic test, this method well characterized the position, shape and size of the embedded tunnel in the model, proving its effectiveness. Subsequently, the approach was applied to a field case investigated in Changsha city, China, and the inversion images revealed a tunnel with a burial depth of 4.5 m, height and width of 2 m and 4 m, respectively, which was identified as the detection target. The available borehole data verified the conclusion. The above results demonstrate the validity and time-effectiveness of the approach. It is expected to provide a reference for practical engineering applications at the same scale. • Detect utility tunnels using 2D PSV/Rayleigh-wave full waveform inversion. • Illustrated the validity of the method via a synthetic test and a field case. • Provide a complete workflow for handling field data. • Verify the reliability of the results by the borehole data. [ABSTRACT FROM AUTHOR]