Three-dimensional density and air–rock interface reconstruction with muography: Application to the TianQin tunnel

Muography is a non-invasive imaging technique that uses cosmic-ray muons, commonly divided into transmission (absorption) and scattering muography. For transmission muography, the inversion algorithm critically determines reconstruction quality. However, widely used schemes may produce smearing artifacts when measurement locations are limited and data are sparse. We develop an optimized Metropolis–Hastings (M–H) algorithm that mitigates smearing and retrieves sharper, more accurate density distributions without auxiliary data. Additionally, we implement an inverse distance weighting (IDW) approach to reconstruct the air–rock interface from muon measurements. The optimized M–H algorithm is applied in Monte Carlo simulations and applied to field data from the TianQin Tunnel experiment using the MuGrid-v2 detector. The IDW-reconstructed air–rock interface is validated against Light Detection and Ranging measurements. In simulations, the optimized M–H algorithm improves detection precision ranging from −1% to 72% across evaluated thresholds for both high-density and low-density anomaly detection. The largest improvement, 72% (from precision 17% to precision 89%), occurs for high-density anomaly at 3.9 g/cm3 threshold. These results, together with the TianQin Tunnel reconstructions. demonstrate the effectiveness of the proposed approach.