Geological features such as fractures and voids pose potential hazards to the safe construction and operation of tunnels. The high resolution of seismic computerized tomography has been successfully applied to the detection of such features prior to the construction of a railway tunnel through Qiyue Mountain in central China. Data were acquired using three boreholes drilled to 280 m in Triassic carbonates and interpreted using seismic computerized tomography (CT) based on ray theory. We used a sparker source in one borehole and an array of hydrophones in a second borehole as receivers. First arrival times were picked automatically, and we applied the algorithm SIRT to reconstruct the variation of seismic velocity between the boreholes. Combining the results with information from engineering drilling, we were able to map precisely the distribution of fractures, rock fragmentation and small voids in the plane defined by the three wells. The results show that there are four linear low-velocity features, which are interpreted as fracture zones. Discrete low-velocity anomalies along these fracture zones are interpreted as small voids. These are consistent with the drilling results. The results provide an excellent example of the applicability of seismic elastic CT in mapping geological hazards such as voids and fractures.

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