I've heard rumours of supposedly highly classified aerial or satellite systems that can detect and accurately map out tunnel systems at substantial depths. 

Shouldn't be that hard from the ground. There are systems used by paleontologists, to find buried bone depostis and such. It is like an underground sonar system.

Most seismic systems require either small explosions (like from a cartridge), "thumpers", or striking a hammer to propogate signals through the earth. I would imagine there could be air-dropped systems that could do so and transmit signals back. There are other systems such as digital magnetotellurics (google it...) that could also be air-dropped. They'd be less detectable. While you're at it, the digital magnetotelluric system could probably give you a good indication if there is any oil to be found there...

You might also be able to air-drop a ground-resistivity system, but that might be more complicated.