Year
1990
Abstract
Underground tunnels can be detected with crosshole and combinations of downhole -surface electromagnetic (EM) wave instrumentation. Detection is based upon the measurement of secondary scattered waves from the tunnel structure. Detection and data processing methods are designed to maximize the secondary wave signal to \"geologic\" noise ratio. \"Geologic\" noise is the change in measured signal caused by changing rock mass. Tunnels may be free of or include finite length electrical conductors. Low and medium frequency (LF and MF) band EM waves are scattered from electrical conductors such as AC power/telephone cable, rail, and hydro/ventilation pipe. These conductors serve the utility needs of an active tunnel. Tunnels without electrical conductors are detected with high and very high frequency (HF and VHF) band EM waves. In these bands, EM waves are scattered by the tunnel rock-air boundary producing a diffraction pattern in the surrounding rock mass. When the wavelength in the rock mass is near the radius of a circular tunnel, resonant scattering reaches a maximum. A receiver on either the lit or shadow side of the tunnel can detect secondary waves. This paper describes secondary EM wave detection procedures and methods for maximizing the secondary tunnel signal to \"geologic\" noise ratio. Actual field test results are discussed.