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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]

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]

The frequent occurrence of groundwater-related damage in operational high-speed railway tunnels in China calls for swift and accurate diagnosis. Thus, this paper describes a way to detect water-bearing defects at 5–10 m using the train-borne transient electromagnetic method in operational tunnels. Due to the short detection distance, the early electromagnetic response of the receiving coil is crucial for determining the size and location of defects. First, the turn-off current in the transmitting coil is elaborated by the equivalent electronic circuit of the transmitting loop. Then the impact of changes in the relative position between the transmitting coil and the receiving coil on the early electromagnetic response are analyzed by a numerical simulation, which shows that a square transmitting coil is suitable for ultra-close detection due to the stability of the electromagnetic response at early times in the receiving coil. Finally a laboratory experiment of close-range TEM detection is carried out to clarify the necessity and feasibility of taking the early electromagnetic response of the receiving coil into consideration at this ultra-close range (5–10 m). This research provides an important theoretical basis for the development and application of train-borne transient electromagnetic method for automatic detection of tunnel defects. • The early EM characters in the transmitting coil plane are affected by loop shape. • The early EM characters in receiver coils are affected by loop relative position. • The square loop has the most stable EM response in the receiving coil at early times. • The test shows early EM response in the coil is essential for close-range detection. [ABSTRACT FROM AUTHOR]