Wikipedia:Reference desk/Archives/Science/2023 November 2

= November 2 =

Tunnel construction
Not wanting to get political, but inspired by recent events and frustrated by the delays of rush hour traffic, I'm working on surreptitiously building a few hundred miles of secret tunnels underneath San Francisco, something like this. My question is about how to not get caught.


 * Could construction be detected with seismographs?
 * Would the tunnels themselves be visible on ground penetrating radar, if they are a few hundred feet deep? If not radar, what about the tomography-like schemes that oil companies use to locate oil deposits at far greater depths?
 * If I manage to get away with this without San Francisco Mayor London Breed noticing and having me arrested, would be appropriate to compare myself to Hogan's Heroes, with Mayor Breed as Colonel Klink? Keep in mind that I've carried out this scheme in the past and been discovered, so I will have to be vewwy vewwy quiet.

Obviously this is partly facetious, but thanks for any thoughts. I just can't figure out why Mayor Breed isn't onto me yet. 2601:644:8501:AAF0:0:0:0:D2BC (talk) 03:50, 2 November 2023 (UTC)


 * Elon, is that you? --Lambiam 10:06, 2 November 2023 (UTC)
 * Thanks for a good laugh. I needed that! JaydenBDarby (talk) 17:19, 5 November 2023 (UTC)


 * Seismographs are pretty sensitive. They can detect waves pounding on the coast in a storm 100 km away, people dancing at a concert 20 km away, even thermal cracks developing in a nearby road. If you'd be digging through clay or if no seismographs were present in the area, you might get away with it, but San Francisco is on solid rock and one of the best seismically monitored places on Earth.
 * If shallow enough, the tunnels would be visible on ground penetrating radar. I don't know how deep you'd have to go; it depends on the diameter of your tunnel. But I doubt the ground under San Francisco is routinely monitored with such equipment. The impedance mismatch of your air-filled tunnel would cause a seismic reflection that would be seen in the kind of studies that oil companies might do. They wouldn't be able to resolve the tunnel, but would see that something is there. Note that with metamaterial cloaking it may be possible to hide the tunnel, both from ground penetrating radar and from seismic studies (other than the vibrations caused by construction).
 * There's also the slight issue of getting the dirt out and reinforcements in without being noticed and the ventilation openings you might need. PiusImpavidus (talk) 10:49, 2 November 2023 (UTC)
 * Ventilation openings are not a problem. You need to have a couple of pipes: one to release fresh air at intervals and one to extract foul air in between those intervals.  Consider sub-sea tunnels – opening up a ventilation shaft to the sea bed is definitely not a good idea! (See Thames_Tunnel last para) Martin of Sheffield (talk) 10:59, 2 November 2023 (UTC)
 * See The Hum.  This is a real phenomenon - it makes things vibrate.   It went on for months and the local press speculated it was caused by tunnelling work near the river. 86.173.101.72 (talk) 11:08, 2 November 2023 (UTC)
 * Assuming they will all converge on your basement, your biggest problem is making sure your house doesn't collapse, which is rather common when attempting to extend one's basement. . Shantavira|feed me 13:27, 2 November 2023 (UTC)
 * You'd also have to dodge or incorporate the pre-existing thousand mile network of sewer tunnels under SF. Water intrusion might be a problem too. Alansplodge (talk) 22:12, 2 November 2023 (UTC)
 * Happy anniversary! —Tamfang (talk) 23:28, 3 November 2023 (UTC)