Abstract
Low‐frequency seismic waves are widely used to detect oil/gas reservoirs, and study the properties of Earth’s interior. Here, we investigate the application of low‐frequency active seismic waves (tens of Hz to a couple of hundred Hz) for long‐range wide‐area detection of shallow underground tunnels. The main objective is to image wide areas of the ground at the longest possible ranges to identify potential underground tunnel hotspots. The tunnel is modeled as a circular air‐filled cylinder. The incident field at the tunnel is calculated in terms of the horizontal (u) and vertical (w) displacements by the elastic parabolic equation. The scattered field from the tunnel is modeled by application of Green’s theorem, approximating the radial dependence of the scattered field by the Hankel function, and satisfying the boundary conditions. Compressional and shear waves scattered from the tunnel will be calculated to determine which wave type is most suitable for detection purposes. Scattered field levels from the tunnel will be compared with seismic ambient noise and reverberation from surrounding medium heterogeneities. Tunnel detection ranges will be quantified as a function of tunnel diameter and length, seismic wave frequency, receiving array length and orientation, and medium properties.
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