Just as one would expect a construction site for a ten storey building to have significant seismic activity so would you expect a barren field with a single clapboard house to have far less seismic activity.
The construction of a 3,000-foot-long tunnel four feet wide and seven feet high requires the removal of hundreds of dump trucks of earth. The movement of those trucks can be detected and will be detected at ranges of over a mile even in urban terrain and with the masking noises of busy traffic.
Bi-static tunnel detection methods are also viable. If one imagines the thumps of vehicles not as something to be detected only for themselves but rather as a sound source to be used to detect something else, tunnel detection becomes an art.
A tunnel is a void. A tunnel is a hollow space in the earth. If you have ever blown over the top of a Coke bottle and made a whistle then you know that the frequency of the tone generated is in some way related to the amount of open space within the bottle.
As the cacophony of seismic noise arrives at your array of seismic sensors a pattern emerges. There will be slight reflections or
echoes from large rocks and big buried objects. As a tunnel is dug, it too affects the sound field or seismic field. Just as you banging on a bottle to make a noise or blowing over its top to make a whistle creates a signature event, so too do the seismic noises passing through the earth affect a tunnel. The tunnel absorbs certain amounts of seismic energy and it can also delay the transmission of such energy. The tunnel delays the transmission by creating a reverberation which is then detected as a
ghost trailing the seismic or a acoustic event.
If we think of the noise of a city as your measuring instrument, then the way that noise is attenuated and or delayed by a tunnel even gives you the size and length of the tunnel.
Those changes in the earth from a tunnel in the seismic field and even the sound field can be detected at distances of a few hundred feet. So long as the tunnel is eventually headed north and eventually crosses the border and there are any detectors nearby, then the tunnel will be detected.
Are we detecting tunnels? No.
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