Ground-Penetrating Radar (GPR)

Abstract

Principles of operation Ground Penetrating Radar (GPR) systems operate by transmitting polarised pulses of electromagnetic energy into the ground and then recording the energy that is reflected back to the surface. The method responds to variations in the electrical properties of subsurface materials (dielectric constant and conductivity) that are a function of material type, moisture content and pore fluid type. Where a contrast in dielectric properties exists between adjacent materials a proportion of the electromagnetic pulse will be reflected back. Subsurface structures are mapped by measuring the amplitude and travel time of this reflected energy. More information is given by Reynolds (2011). The electromagnetic energy is transmitted via an antenna at a specified central frequency and can be recorded with either the same antenna or a separate antenna (monostatic mode or bistatic mode, respectively). Digital radar systems can also operate with multiple receiving antennae. The frequency of the transmitted energy (antenna frequency) is selected with reference to the size and depth of potential targets, i.e. the lower the frequency of the transmitted energy, the greater the depth of penetration, and the higher the frequency, the smaller the size of target than can be resolved. GPR manufacturers produce antennae with a range of frequencies from 25 MHz up to 2.6 GHz. The low-frequency antennae are normally used for geological applications while the high-frequency antennae (> 500 MHz) are used for engineering applications, although most geo-environmental applications utilise the mid range of frequencies (200 MHz to 500 MHz). Figure 1: Example radargram from a GPR survey conducted along a road surface. successful in its aim, which was to locate possible subsurface voids.