A cylindrical guarded capacitor for spectral permittivity measurements of hard rock samples in the MHz-range

A shielded and guarded capacitor for non-destructive measurement of the complex permittivity of cylindrical rock samples has been developed. In combination with a vector network analyzer, the measurement system allows for rapid determination of spectral permittivity. The geometrical construction of the capacitor allows for scanning the properties of cylindrical samples, such as drill cores, and determination of anisotropy. The complex permittivity is calculated from the scattering parameters, which are determined using a network analyzer. The calculation is based on a p-network model for the cell and takes stray capacitances and capacitances related to the guard electrode into account. Using this method, good measurement accuracy is achieved between 10 MHz and 200 MHz. The validity of this method was proven by finite element modeling of the field distributions in the cell based on 3D electromagnetic simulation. The influence of typical geometrical imperfections of drill cores, such as varying diameter, finite length and heterogeneities has been studied and quantified by extensive measurements of test samples. These tests confirmed that due to the short shielded electrode, spatial variations in permittivity can be resolved on a sub-centimeter scale.