Ground Penetrating Radar
Ground Penetrating Radar (GPR) is a high resolution technique that is designed primarily to image shallow soil and ground structures using electromagnetic (EM) waves, typically in the frequency band of 10-1000 MHz. GPR is an electromagnetic pulse reflection method based on physical principles to those of shallow seismic reflection surveys. The method has been increasingly applied for geological, engineering, environmental, and archaeological investigations since the 1980’s.
Reflections and diffractions of electromagnetic waves occur at boundaries between rock strata and objects that have differences in electrical properties. The electric permittivity (dielectric constant) and the electric conductivity are petrophysical parameters which determine the reflectivity of layer boundaries and penetration depth. In practice, the electromagnetic wave is radiated from a transmitting antenna and travels through the subsurface at a velocity which is determined primarily by the electrical permittivity of the material. The wave spreads out and travels downward until it hits an object that has different electrical properties from the surrounding medium, is scattered from the object, and is detected by a receiving antenna on the surface. The method is most effective when working with low attenuation media (characterized by low electrical conductivity) such as ice, sands and gravels, crude oil, bedrock, fresh water, etc.
Example GPR systems; Sensors and Software Noggin system, mounted on a GPR cart, acquiring geotechnical data in a warehouse structure (left) and Sensors and Software Pulse Ekko Pro system measuring ice thickness on a glacier (right).
A GPR system essentially measures the signal travel-time. We collect GPR data by moving a transmitter and receiver antenna over the area of interest, continuously broadcasting and receiving the radar waves. The transmitter and receiver antenna are usually separated by a fixed distance and moved together in a constant-offset profile. Because GPR surveys are non-invasive, the ground is not disturbed by impact or insertion of electrodes. This aspect makes GPR popular for structural engineering and archaeological applications.
Ground penetrating radar can be a good choice for high resolution subsurface imaging in the right conditions. However, ground penetrating radar does not work well in all environments. It does not penetrate into materials of high electrical conductivity, such as regions with wet clays near the surface or conductive fluids like seawater.
Example 3D GPR survey to locate rebar in a concrete slab.