Ground Penetrating Radar

or shortly GPR technology allows to detect different underground objects in a remotely way. It is also called "no dig technology".  It utilises the electromagnetics wave propagation phenomena inside the soil structure and its reflections from the target having different from medium electric properties.

The GPR is able to find underground urban structures, empty spaces, pipes, fluids leaks, minerals and even smal metal-free objects. Depending on radar technical parameters achievable depths varies from few inches to even over 0.5 of mile but the resolution toward the depth gets lower. The features makes the GPR technology very useful. It has many applications and is still developing.
 

see also links:

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

http://www.youtube.com/watch?v=oQaRfA7yJ0g

http://www.geo-radar.pl/en/methods/georadar/working/