Field evaluation of broadband spectral electrical imaging for soil and aquifer characterization

Spectral electrical impedance tomography (EIT) involves the imaging of the complex electrical distribution in the mHz to kHz range. Until now, field EIT measurements were limited to frequencies below 100 Hz because strong electromagnetic coupling effects associated with longer cables and high electrode contact impedances prohibit accurate field measurements at higher frequencies. In this paper, we aim to evaluate the capability of recent improvements in the pre-processing and inversion of wideband EIT measurements to improve the accuracy and spectral consistency of field EIT measurements of the complex electrical conductivity distributions in the mHz to kHz frequency range. In a first case study, time-lapse surface EIT measurements were performed during an infiltration experiment to investigate the spectral complex electrical conductivity as a function of water content. State-of-the-art data processing and inversion approaches were used to obtain images of the complex electrical conductivity in a frequency range from 100mHz to 1 kHz, and integral parameters were obtained using Debye decomposition. Results showed consistent spectral and spatial variation of the phase of the complex electrical conductivity in a broad frequency range, and a complex dependence on water saturation. In a second case study, borehole EIT measurements were made in a well-characterized gravel aquifer. These measurements were inverted to obtain broadband images of the complex conductivity after correction of inductive coupling effects using a recently developed correction procedure that relies on a combination of calibration measurements and model-based corrections. The inversion results were spatially and spectrally consistent in a broad frequency range up to 1 kHz only after removal of inductive coupling effects. Overall, it was concluded that recent improvements in spectral EIT measurement technology combined with advances in inversion and data processing now allow accurate broadband EIT measurements up to 1 kHz.