Please show me some instruments that can detect underground formations.

[Cesrate]

Title is the question.

[OtherDalfite]

Well, there is a sonar type instrument that sends down a signal through certain densities such as dirt and bounces off once it reaches something dense like stone or wood. It's used by Archeologists when scanning certain buildings with a cellar/basement that isn't directly connected to the structure. Sorry, as I can't remember it's name.

[N3X15]

Seismometers.

http://www.lpi.usra.edu/lunar/missions/apollo/apollo_15/experiments/ps/

Also used on Earth to "see" the transitions between different types of rocks, find oil reservoirs, etc. Like OtherDalfite says, the process is much like sonar, except the pulses are from earthquakes.

[Cesrate]

Oh... I think what I need is OtherDalfite's archaeological instruments. But problem is we all don't remember the name.

[Scotius]

https://en.wikipedia.org/wiki/Ground-penetrating_radar

That should do

[softweir]

Also:

http://en.wikipedia.org/wiki/Magnetic_survey_(archaeology)

http://en.wikipedia.org/wiki/Electrical_resistance_survey_(archaeology)

[Cesrate]

Oh thanks.