Tunnel Detection using Borehole Radar Survey

Abstract

Various borehole radar methods were tested at the three field sites in order to verify the applicability of borehole radar survey for the detection of underground tunnels or mining galleries of abandoned mines. The first field site is the granite area where there are many boreholes with 6 inch diameter around the underground tunnel. Borehole radar reflection survey using dipole antenna with main frequencies of 100 MHz and 250 MHz, borehole radar reflection survey with direction finding antenna, radar tomography and crosshole scanning are adopted to find how much the medium condition and the diameter of the borehole affect to the signal quality. The second one is the abandoned iron mine area where two borehole radar survey methods were applied for detecting mining gallery, one of which is borehole reflection surveys with various offsets and the other of which is radar tomography. The final one is the abandoned lead-zinc mine area where borehole reflection survey were applied for detecting mining gallery but failed because the radar wave was attenuated severely due to very high conductivity of the medium. From these three case studies, we found the characteristic of the radar wave in various mediums and the ringing phenomena would be increased due to impedance mismatching when the diameter of the borehole is large to be compared the length of antenna. It is found that the refection event from a tunnel could be detected effectively with changing the offset between transmitter and receiver. Finally we proposed the cost effective procedure for detection of tunnel using borehole radar survey that the first is the borehole radar reflection survey with dipole antenna, the second is one with direction finding antenna or crosshole scanning.