Ground Penetrating Radar

OUTLINE

image030This is an active method that uses a towed antenna to pulse microwave electromagnetic energy into the sub-surface. As the polarized pulse travels downwards it interacts with materials within the ground and part of the energy is reflected back to the antenna at surface.

The GPR unit measures the amplitude of the reflected signal and the time delay between the transmitted and received pulses in order to map sub-surface features. The frequency of the antenna can be changed depending on the required depth of investigation and the nature of the expected target. Whilst the method is highly versatile it is not suitable for use over highly conductive ground.

DETAIL

Ground Penetrating Radar (GPR) profiling involves transmitting pulses of electro-magnetic energy at microwave frequencies (typically 50-1000MHz) into the sub-surface and measuring the amplitude and travel-time of the returned signals.

The signal is introduced into the ground as polarized pulses via an antenna that produces energy of a specific central frequency. Each pulse propagates downwards through the ground where it may interact with sub-surface materials in a variety of ways. (These include attenuation, reflection, refraction, diffraction and scattering). However, the two most important physical conditions, which impact on the behavior of radar waves are the material’s dielectric properties and its conductivity.

The dielectric constant of the medium determines the velocity of the EM wave; the lower the dielectric the faster the propagation of the wave. A sudden reduction in the dielectric constant, such as might occur at a geological boundary, will result in an increase in the velocity of the wave and a consequent reflection of some of the energy back to the surface (analogous to the reflection of seismic energy in reflection profiling). Slowing of the EM wave also results in a concomitant energy loss. This explains why radar penetration depth is limited in a water-saturated material as groundwater has a dielectric constant of approximately 81.

The conductivity of the sub-stratum is the most important factor determining the rate of signal attenuation. Materials with high conductivities will cause rapid dissemination of the transmitted pulse through the transformation of the EM energy into heat, as ions within the medium become excited (similar to the effect of a microwave oven). Signal loss is consequently greatest in clayey soils. Signal loss can also result from scatter of the transmitted pulse during interaction with large inhomogeneities within the sub-surface, such as cobbles or bricks.

The antennae used in a GPR survey are selected on the basis of the depth of interest and the size of the target. Penetration depth varies inversely with frequency and, the higher the central frequency of the antenna, the smaller the size of object that can be resolved.

GPR is currently the subject of active research with respect to contamination, plume mapping and automatic target identification.

Ground Penetrating Radar data is generally presented as gray scale images (radargrams) illustrating the amplitude of the reflected radar energy against two-way time (on the vertical axis) and with distance along the survey line (horizontal axis). Strong reflections and multiple reflections appear as bright white areas on the Radargram. These are typical of buried metallic features or voiding. Vertical lines represent survey markers indicating distance along the survey line. Where an appropriate dielectric constant is available the travel time axis can be converted to indicate approximate depth.