Similar to the Chirp Sub-bottom profiler, a Ground Penetrating Radar (GPR) is a tool used to image the subsurface. Unlike a Chirp that uses acoustic energy, a GPR uses electromagnetic energy in the radio portion of the spectrum. Sonar can be used in fresh water or salt water, but not on dry land. A GPR, alternatively, can operate quite effectively on land or in fresh water, but not salt water (due to the high conductivity of the salt water dispersing the radio energy).
Despite the Chirp using acoustic energy and the GPR’s use of electro-magnetic energy, many of the same principles apply. Higher frequency yields increased resolution, but less penetrative power. Depth is again calculated from Two-Way Travel Time, but instead of the speed of sound through water, it’s the speed of electromagentic propogation through assorted mediums (which can vary quite a bit depending on what material is being imaged).
The GPR detects changes in the dielectric constant of the sub surface features. As the EM waves pass from one material to another, the change in the dielectric constant causes some of that energy to be reflected back towards the radar antennae. Based on the TWTT we can identify the nature of the stratigraphic sequence without having to break ground.
We have a range of antennae spanning low frequency (100mHz) to high frequency (800mHz). The 250mHz antenna often provides the best balance of depth and resolution.