Importance

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Abstract

A ground-penetrating tunnel-detecting active sonar launches two different monotonic acoustic beams down into the ground from the surface. If the two separate monotonic acoustic waves arrive at a stress field, they will mix and produce a frequency difference heterodyne due to the inherent pressure nonlinearities in the solid medias. Any sonar returns are bandpass filtered so only an acoustic frequency difference heterodyne can pass through. The existence of a tunnel is revealed by the return of acoustic frequency difference heterodynes all coming from a more-or-less horizontal line of phase-delayed sources and directions. These phase differences can be derived from the vector values provided by the acoustic vector sensor. Three or more acoustic vector sensors on the surface can be used effectively to provide triangulations down to the tunnel to better estimate the tunnel track.

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First Claim