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 the radar pulse bounces back towards the surface, where upon its return it is detected by the antenna. This process is very similar to what happens when a person yells and then a short time later hears an echo of their voice. In reality, GPR is the most complicated geophysical technique, both in terms of instrument technology and data processing. However, the complications are well worth the effort because of the incredible utility of GPR.
Sensors and Software Noggin Radar System, with a 500 MHz antenna.
Survey in DeRivera Park, Put-in-Bay, South Bass Island, Ohio, Lake Erie. Searching for War of 1812 graves.
Most people automatically associate GPR with cemetery survey. While the radar is an excellent tool for identifying some graves, it cannot detect all graves. Radar actually is much better at detecting buried building foundations. Whether the foundations are several feet down in the soil or covered over by asphalt parking lots, radar is the ideal tool for foundation delineation. It is also good at finding old buried walk ways (gravel, sand, cinder, etc.) wells, cisterns, tunnels, privies, and filled in cellars, among many other historic-era feature types.
Radar is less commonly used at prehistoric Native American sites, mostly because the magnetometer is so much better at finding most prehistoric features. Nevertheless, the radar can detect a range of prehistoric feature types, including some pit features, structure floors, layers of sand or gravel, some ditch-type features (as in earthworks), rock-chinked posts, and the original footprint of some deflated mounds (especially those built with clay).
Examples of GPR Surveys
- Historic-era Foundations and Cellars
- Wells and other Shaft-type Features
- Buried Paths, Roads, and Utilities
- Prehistoric Features
- Urban Lots
- Industrial Sites
How Ground-Penetrating Radar Works
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 series of radar amplitude slice maps of a 20×30 meter area in a park in Worthington, Ohio. Each represents the radar reflections from a 25-cm-thick slice of ground. The linear features are numerous utility trenches (the linear gaps in the reflections), and the pipes they contain. 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. This was a quick survey we did while testing out the machine when we first bought it. We have since found hundreds of archaeological features!