SIP laboratory data: frequency-domain versus time-domain

Spectral information from induced polarisation (IP) measurements can be used for several applications as the characterisation of the lithology, the estimation of hydraulic parameters, the localization of contaminated areas and many more.
So far, the spectral content has been mainly determined in frequency domain (FD). Due to significant instrumental developments and advanced data processing and inversion tools, it is nowadays also possible to extract spectral information from time-domain (TD) measurements. Even though the results from both domains should be theoretically identical, differences can be observed in the practical application. To compare both domains, we started with numerical models and measurements at test circuits. Thereafter, we conducted measurements on different types of well-known material (e.g., sandstones and wood samples) in a controlled laboratory environment. For the TDIP measurements, the AIE-2 instrument was used and the FDIP spectra were recorded with the SIP Fuchs III device. Besides the measurement results, shown as decay curves in TD and amplitude and phase spectra in FD, also the Differential Polarisation parameter (DP) is presented. This parameter is calculated from the TD decay curve and proves to be well correlated with the phase in FD. The comparison of DP and phase curves enables a first visual check and a discrimination between different samples. To compare both domains qualitatively, the relaxation time distribution (RTD) was calculated for all data. Theresults are in (partly very) good agreement between both domains, depending on the data quality. We found that the RTDs are in better agreement for the wood samples than for the sandstone samples. We attribute the differences to the lower data quality of the TD measurements of sandstones, which were performed with lower current in comparison to the wood samples. Therefore, the TD decay curves at later times are more affected by noise.