GPR stands for Ground Penetration RADAR (Radio Detection And Ranging). GPR works by sending high-frequency electromagnetic waves into the ground from a transmitting antenna. GPR is able to detect subsurface features whose electrical properties contrast with those of the surrounding soil or concrete.
A Ground Penetrating Radar system consists of a transmitter, a receiver, and a control unit for data acquisition.:
Principals of a GPR system
The control unit contains the electronics which triggers the pulse of radar energy that the antenna sends into the ground. The antenna receives the electrical pulse produced by the control unit, amplifies it and transmits it into the ground or other medium at a particular frequency. Antenna frequency is one major factor in depth penetration. The higher the frequency of the antenna, the shallower into the ground it will penetrate. A higher frequency antenna will also ‘see’ smaller targets. Antenna choice is one of the most important factors in survey design. The following table shows antenna frequency, approximate depth penetration and appropriate application.
Antenna Frequency Choice - GPR
High Frequency GPR - Structural/Concrete
Mid Frequency GPR - Utilities/Archaeology
Low Frequency GPR - Geological
GPR works by sending a tiny pulse of energy into a material and recording the strength and the time required for the return of any reflected signal. A series of pulses over a single area make up what is called a scan. Reflections are produced whenever the energy pulse enters into a material with different electrical conduction properties or dielectric permittivity from the material it left. The strength, or amplitude, of the reflection is determined by the contrast in the dielectric constants and conductivity's of the two materials. This means that a pulse which moves from dry sand (dielectric of 5) to wet sand (dielectric of 30) will produce a very strong reflection, while moving from dry sand (5) to limestone (7) will produce a relatively weak reflection.
Table of material dielectrics
While some of the GPR energy pulse is reflected back to the antenna, energy also keeps traveling through the material until it either dissipates (attenuates) or the GPR control unit has closed its time window. The rate of signal attenuation varies widely and is dependent on the properties of the material through which the pulse is passing. Materials with a high dielectric will slow the radar wave and it will not be able to penetrate as far. Materials with high conductivity will attenuate the signal rapidly. Water saturation dramatically raises the dielectric of a material.
Time-Slice image of GPR showing an existing structure