Subsurface TE scattering by tunnels using a surface line current

The objective of this research is to develop a general method for remotely detecting the presence of unauthorized tunnelling activities by means of one or more boreholes and a surface source. A line current or dipole-dipole array, positioned on or near the surface of the earth, is used as the TE current source. Subsurface electric and magnetic field measurements are made in a borehole that is situated near a suspected tunnel location. The presence of a tunnel causes subsurface scattering of the field components created by the source. Both the electric and magnetic field strength and phase data are perturbed by the presence of a nearby tunnel. The scattered fields are observed on both sides of the tunnel relative to the source position. Two different models have been used to investigate this technique. The first model assumes that a line current is located in a homogeneous whole-space, while the second model includes the effects of the earth-air half-space. The authors compare the two models and show when the whole-space model can be used with minimal error.<>