Ground penetrating radar ? will it clear the haze at your site?

Abstract

Ground penetrating radar (GPR), of all the commonly practiced geophysical techniques, has the greatest ability to provide clear high resolution images of shallow sub-surface structure. To date, however, the perceived unpredictability of its performance at different sites has limited its use. Factors which control the performance of GPR can be summarised by the radar range equation. The site dependence of GPR is a result of the wide variation between the wave attenuation rates of different geological materials and the variation in the reflectivities of the different targets. The attenuation rate of a material depends on its conductivity and dielectric constant while the reflectivity depends on the contrast of these properties between the target and host materials. Unfortunately, since conductivity varies with frequency, conductivities obtained by resistivity and other low frequency electromagnetic geophysical measurements are different to those at GPR frequencies. One way to obtain these properties is to make measurements of the radio-frequency electrical properties of rock samples from prospective GPR sites. For this purpose the open-ended coaxial line technique has been found to be the most practical method. Above 100 MHz measurements can be made on small samples with a single flat surface. Below 100 MHz the accuracy of this technique deteriorates and the capacitance bridge technique, which requires small disk-shaped samples, is more suitable. In homogeneous media, nomograms can be used to convert the attenuation rates determined from the sample measurements, to a maximum depth that can be imaged by a GPR system. The maximum depth in layered media can be determined by a simple graphical method involving summing the attenuation and spreading in each layer. Case histories from 4 different sites show that despite the uncertainties involved in making any measurement on samples which have been removed from their in-situ conditions, ranges calculated in this way provide a valuable guide as to how well GPR will perform at a particular site.