An Ultra-Wideband Measurement Method for the Dielectric Property of Rocks

Dielectric properties of rocks are the important indicators of subsurface formations when electromagnetic sensing methods are applied. Interpreting those in situ measurements relies on characterizing the dielectric permittivity of rock samples in the laboratory. For solid phase samples, the parallel-plate capacitance method (PCM) serves as one of the most accurate and feasible methods for charactering electrical properties. Low-frequency (~kilohertz) PCM measurements are most common; extending the PCM to an ultra-wideband measurement will establish a general framework for measuring both low-frequency and high-frequency (~gigahertz) properties (e.g., permittivity and conductivity). Major challenges of ultra-wideband measurements using the PCM are: 1) at high frequency, the measurement apparatus may introduce errors due to reflection and dissipation of the electromagnetic waves and 2) resonance caused by impedance mismatching in the apparatus design can occur at higher frequency, which will significantly compromise the accuracy of this method. Thus, the optimization and modification of the method is needed. In this letter, we: 1) show the validity and accuracy of the PCM for permittivity measurement by comparing numerical simulation and experiments; 2) propose a practical geometric parameter to conduct system-level optimization under giving constrains; 3) perform the optimization for the sample holder to increment the measurement accuracy at higher frequency without downsizing the sample. The results show that a satisfactory accuracy and stability can be achieved in an ultra-wide range of frequency spectrum from 100 kHz to 1.5 GHz with the improved PCM apparatus design.