Utility tunnel detection by 2D elastic PSV/Rayleigh-wave multi-parameter full waveform inversion

Abstract

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.