Physical characterization of mine goafs using high-resolution seismic cross-hole tomography via an adjoint method

Mine goafs often cause severe ground settlement and collapse, posing significant endangers to the safety of buildings and engineering constructions. Quantitatively detecting the spatial distribution and physical characteristics of goafs has always been challenging. To evaluate the potential threat of a fluorite mine goaf on a proposed expressway, we inverted cross-hole dataset to image the P-wave velocities between boreholes using a newly developed Eikonal equation based adjoint-state traveltime tomography method (ATTOMO). Independent comprehensive geophysical prospecting, including core analysis, borehole optical televiewer imaging and ultrasonic wave logging, indicates that our tomographic results at the central section could discern anomalies on a scale of 1 m or even smaller and detects velocity variations at a scale of 100-200 m/s. Our results show that the mine goaf is distributed in a northeastward trend with a width of 20-30 m, consistent with the exploiting features of the fluorite mine. The goaf is located approximately 20 m away from the bridge site, where only a small-scale goaf tunnel with dimensions of 1.5x1.5 m was found at depths of 18-19.5 m. Our results suggest that the safety of the proposed expressway would not be significantly affected by the goaf, and thus no design changes are required.