Ground Penetrating Radar

In principle, Ground-penetrating Radar (GPR) is a very simple geophysical technique. An antenna transmits a pulse of electromagnetic energy into the ground and when that energy encounters a material with different electrochemical properties, part of it bounces back toward the surface--where a second antenna detects it. In reality, this geophysical technique is one of the most complicated, both in terms of instrument technology and data processing. However, the complications are well worth the effort because of the incredible utility of GPR.

In essence, GPR surveys generate cubes of data. The top of the cube is the ground surface and the bottom is the maximum depth at which reflections can return to the surface and be detected. The sides of the cube are the horizontal limits of the survey area. Perhaps the most unique and useful aspect of GPR surveys is that the cube of data can be cut into horizontal slices, each of which represents a horizontal plan map of the survey area at different depths (depth is estimated from radar velocity). The image below is a time slice map of a 20x30 meter area in a park in Worthington, Ohio. Numerous utility trenches, and the pipes within them, were detected below ground. Note how the actual pipes do not show up in the time slices until at least 50 cm below surface, though in some cases the trenches are visible at shallower depths.