Wide-band Rock and Ore Samples Complex Permittivity Measurement

Abstract

IntroductionGround 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 www.intechopen.comBehaviour of Electromagnetic Waves in Different Media and Structures 102 designed for special frequency.Calibration is required before measurement, once the calibration is finished, the measurement is fast.The samples preparation is little difficult.Free-space transmission technique is carried out by placing the sample between the transmitting and the receiving antennas (Kraszewski, 1980).The electrical parameters can be calculated by measuring the attenuation and the phase shift in the media.Free-space measurement can measure permittivity in a wide frequency band.It is a no-destructive and contact-less method.The frequency can be as high as 30GHz.The usual assumption for this technique is that a uniform plane wave incident upon the flat surface of a homogeneous material, and the planar sample has infinite extent laterally (Venkatesh & Raghavan, 2005).Parallel disk capacitor technique (Shi & Shen, 1989;Shen, Marouni, Zhang, & shi, 1987) is based on the theory that the capacitance is dependant on the medium permittivity filling the capacitor.As a capacitor is filled with medium having permittivity of r ε , its capacitance C is r ε times of vacuum capacitance C 0 .If we measure the capacitance before and after the capacitor is filled with the medium, we can obtain the permittivity.This method is mainly fitted for low-frequency range between 20MHz-200MHz.Coaxial reflection/transmission method was introduced by Nicolson, Rose, and Wire, in 1970s, therefore, it is also called NRW method (Nicolson &Ross, 1970;Weir, 1974).The material under test (MUT) is inserted into coaxial line (waveguide).As the electromagnetic wave is traveling in the line, it meets the MUT, a part of the wave is transmitted, and the other is reflected.Attenuation and phase shift occur at the same time.Vector network analyzer (VNA) can measure the reflection and transmission coefficients at a wide frequency range.The permittivity can be inverted by electromagnetic theory from these measured data.This technique was first used for rock measurement (Shen, 1985), and the waterbearing, oil-bearing, or gas-bearing rocks can be distinguished from the measured data.Coutanceau-Monteil and Jacquin improved this technique in 1993.The rock samples are coated with a low melting point alloy and this improvement enables measurements to be made on larger samples and avoids air-gap correction at the solid-cell interface.However, the sample preparation is still difficult.Open-ended coaxial probe technique (Zhen &Smith, 1991; Blackham&Pollard, 1997; Hoshina, Kanai, Miyakawa, 2001) is a technique of which the open-end of a coaxial is touched on the MUT, and the other end is connected to a VNA.The measured reflection coefficient is used to invert the permittivity.A flange is connected to the open-end to improve the accuracy.The circular waveguide probe, the rectangular waveguide probe, and the coaxial probe are often used.The coaxial probe can be used at frequency-domain, timedomain, point frequency, and it is suitable for kinds of media with different electric characteristic (low to high permittivity, lossy or lossless, magnetic lossy materials).This method is no-destructive, no-intrusive, and also characterized with wide frequency band, simple and open structure.It is fitted for on-line, in-vivo, and in-situ testing, it is receiving the attention from many scientists and engineers.Open-ended probe electromagnetic radiation dates back to 1950s or earlier.Open-ended measurement technique was improved rapidly in recent 20 years (Stuchly M.A. & Stutch, S.S, 1980).It was first introduced and applied for biological tissue measurement at RF frequency, and was used for dielectric material testing at RF band shortly, and the lumped parameter model is used at the time for data processing.Later, as the testing frequency expanded to X and Ku band, quisi-static analysis model (