Field Evaluation of Broadband Electrical Impedance Tomography Measurements

Abstract:

Laboratory measurements of the complex electrical conductivity in a broad frequency range (i.e. mHz to kHz) using spectral induced polarization (SIP) measurements have shown great promise to characterize important hydrological properties (e.g. hydraulic conductivity) and biogeochemical processes. However, translating these findings to field applications remains challenging, and significant improvements in spectral electrical impedance tomography (EIT) are still required to obtain images of the complex electrical conductivity with sufficient accuracy in the field. The aim of this study is to present recent improvements in the inversion and processing of broadband field EIT measurements, and to evaluate the accuracy and spectral consistency of the obtained images of the real and imaginary part of the electrical conductivity. 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 of 100 mHz 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 that was reasonably consistent with laboratory EIT measurements. In a second case study, borehole EIT measurements were made in a well-characterized aquifer. These measurements were inverted to obtain broadband images of the complex conductivity after correction for inductive and capacitive coupling using recently developed procedures. The results showed good correspondence with reference laboratory SIP measurements in a broad frequency bandwidth up to 1 kHz only after application of the correction procedures.