Abstract: When a proposed tunnel is over a coal mining area, the accurate locating the underground mining voids within the surrounding rock mass of the tunnels is one of the critical issues in the geotechnical investigation. The coal mining voids might affect the capital investment, construction duration and even the realignment of a proposed tunnel.  This paper briefly introduces all possible geophysical methods that could potentially be applied to the detection of void location and size. Then, a case history of interpreting the surrounding rock mass of a proposed tunnel by using the combination of Transient Electromagnetic Method (TEM) and High Density Resistivity Method. The results indicate that there are no mining voids within the tunnel influence zone. This conclusion was consistent with the findings after the tunnel excavation. This case history presents an application of geophysical methods in the geotechnical investigation of the large size and long tunnels in the urban area.

Key words: Transient Electromagnetic Method (TEM), Large Size and Long Tunnels, HighDensity Resistivity Imaging, Coal Mines