Wide-band Rock and Ore Samples Complex Permittivity Measurement

Abstract

Ground penetrating radar (GPR) is based on high-frequency electromagnetic wave propagation and its detecting targets are below the ground surface (Daniels, 2004; Jol, 2009). Velocity and attenuation are two important factors describing the electromagnetic wave in the media composed of rocks or soils. Velocity is inverse proportional to the root of the permittivity while other parameters are fixed generally. In order to understand the performance of GPR, permittivity testing and analysis are critical. In addition, during the metal ore exploration by borehole radar which is an operating mode of GPR, the permittivity difference between ore body and surrounding rock is the foundation for exploration. As the sampling site, geological environment, and seasons are changing, the permittivity are different even for the same rock. Therefore, permittivity measurement is very important. Currently, measurement methods are basically indirect methods which are based on transmission line theory, characteristic impedance, and propagation constant. These variables have intrinsic relationship with permittivity which can be inverted from the measured data by certain calculation procedure. At the RF frequency band, common measurement methods include short-circuited wave-guide measurement, coaxial line transmission/reflection method, open-ended coaxial probe, resonant cavity method, freespace transmission technique, parallel-plate capacitance method, etc. Roberts and von Hippel (1946) developed the short-circuited wave-guide measurement, sample is inserted at the end of the wave-guide or coaxial line, the standing wave is formed as the incident wave and the reflected wave coexist in the wave-guide. The sanding wave ratios (SWR’s) were required to measure in the case with and without sample. Permittivity can be determined by the change in the widths of nodes, sample length, and the waveguide dimension. Resonant cavity method is a perturbation technique, which is frequently used for measuring permittivity because of its simplicity, accuracy, and high temperature capability (Venkatesh & Raghavan, 2005). This technique is based on the resonant frequency shift, and the change in absorption characteristics due to the insertion of sample material. The measurement is made by placing a sample completely in the center of a waveguide. The size of the cavity is