posted on Aug, 15 2012 @ 01:42 AM
The first link is the one of interest. It is a scientific paper written by the JASONs (DoD scientific advisory group) on how to detect underground
facilities. It covers the usual schemes .(geophones, VLF ground sensing, gravity surveys, etc.), but also one I never heard of but is totally obvious
once pointed out. Basically the ground will settle where it has been disturbed underground. The settling can be detected by synthetic aperture radar.
This makes it way more likely that to escape detection, underground facilities are dug into mountains rather than just some blank spot on the map.
[Sorry if that ruin the theory of nationwide train tunnels. ;-) ]
The paper also goes into a scheme to detect vents, but I really don't think it would work, at least not in the desert. The idea is to detect the
thermal disturbance of the vented air. Far better just to thermal map the area and look for a temperature difference.
Regarding geophones, present day UAVs can drop them from the air. Air dropped sensors go back to the Vietnam war.