For remote borders with little or no infrastructure, there are various techniques that can keep an eye, or rather an ear, out. QinetiQ, a British firm, has a system called OptaSense, which uses a fiber-optic cable laid in the ground. Sound from above creates vibrations in this cable.
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Fiber optic sensing technology converts cables installed alongside transport infrastructure, such as railroads or highways, into an array of thousands of sensors which can detect acoustics, temperature or strain signals by using optical sensing techniques.
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For intrusion detection, OptaSense uses a distributed acoustic sensing setup that analyzes the Rayleigh scattering....The strain signature of a fiber-optic cable around 50 km long can be deconvolved to detect movement at distances of around 10 m from the fiber with a 1 m accuracy.