Determination of dielectric properties of rocks for Ground Penetrating Radar (GPR) images

Abstract

Ground Penetrating Radar is a method for analysing the sub-surface structure, and is currently being utilised to provide the accurate verification and location of possible buried hazards, such as mine shafts and voids, the identification of archaeological sites and the investigation of environmental conditions. Many of the cultural heritage sites in Malta were built using the Lower Globigerina Limestone. GPR can be used to provide data for analysis in the preservation and restoration of heritage sites in a non-invasive way. The GPR technique utilises electromagnetic radiation which results in various reflections. The GPR requires an initial guess of the dielectric properties of the target material/s for post-processing of the GPR images. Therefore, a study of the dielectric properties including the permittivity and the permeability values of the Lower Globigerina Limestone is required. This project, has sought to establish a foundation for further research on determining the dielectric properties of the Maltese rocks. Special attention was given to the Lower Globigerina limestone, samples of which were obtained from various boreholes around Malta. In order to achieve the dielectric values, a microwave measurement technique was chosen and measurements of the scattering parameters at a frequency range of 1.7 GHz to 2.6 GHz by the use of a Vector Network Analyzer (VNA) were taken. The sample was measured when it was fully saturated and fully dry. This frequency range was imposed by the GPR frequency operation at 2 GHz. After the rock sample was measured, the scattering parameters were processed through a conversion method to obtain the electromagnetic permittivity and permeability values. In order to achieve results with minimal uncertainty, a calibration procedure was conducted before each measurement session. This provided consistency and accuracy in the measurements. In this study, a set of values of the electromagnetic dielectric properties of the Lower Globigerina Limestone was established. Further testing was implemented to verify the permittivity value obtained for Lower Globigerina Limestone by the use of a GPR.