Journal of the Korean Society of Mineral and Energy Resources Engineers ISSN:2288-0291(Print) 2288-2790(Online) 한국자원공학회지

This study describes the applicability of high resolution seismic reflection survey with an efficient traveltime inversion to the detection of abandoned underground tunnel through an example of field application. The field measuring condition was that (1) an inclined pit filled with water lay just over the abandoned railway tunnel that might be constructed in the past, (2) in front of the inclined pit, only a very limited space was given so that a regular survey(e.g. roll-along technique) could not be performed. Under these circumstances, the inclined pit was used to deploy hydrophone chain(36 channels) situated on the bottom of it, whereas at the entrance of the pit a seismic detonator(2 g) was used as a source. Using these optimized measuring devices with high resolution of 1/32 ms sampling interval, desirable reflection events from the tunnel could be observed. However, due to the abnormal source-receiver geometry, a special evaluation algorithm, i.e. reflection time inversion was needed. To this end, the underground model was simplified as three model parameters, i.e. depth of the abandoned tunnel from the shot point, dip angle of the inclined pit and P-wave velocity around the tunnel. The model parameters could be stably converged regardless of the initial values only in case of using reflection traveltimes from two adjacent shot gather data and they were also in good agreement with the in situ underground structure.