Muon absorption imaging for the density structure of metro tunnel overburden

Detecting the geological structure of the overburden above subway tunnels is crucial for the safe operation of the subway. Cosmic-ray muon, due to their strong penetration capabilities, offer a non-invasive method to probe underground structures. This study leverages muon absorption imaging technology to map the geological density variations of the overburden between Shenzhen Metro's Apollo South and Yuanshan Stations. A muon detector measured muon flux at 22 points within the tunnel, which, combined with preliminary geophysical drilling data, allowed for the estimation of average densities using the differential evolution global optimization algorithm. The results show that muon flux variations corresponded well with changes in marble content within the soil, reflecting density increases from 2.08 g/cm³ to 2.27 g/cm³ between measurement points 1 to 13, and decreases to 2.03 g/cm³ at points 14 and beyond due to river erosion effects. This study confirms the efficacy of muon absorption imaging in identifying density anomalies in tunnel overburden. By integrating prior geological information, the differential evolution algorithm enhances the precision and efficiency of stratification analysis. This novel approach offers significant potential for mitigating geological hazards and ensuring subway safety, with broader applications in other underground infrastructure and continuous monitoring systems. The findings could influence future standards in subway construction and operation, highlighting the importance of innovative methods in geological assessment and infrastructure safety.