Broadband Electrical Impedance Tomography for Subsurface Characterization Using Improved Corrections of Electromagnetic Coupling and Spectral Regularization

Abstract

The low-frequency complex electrical conductivity in the mHz to kHz range has been shown to enable an improved textural, hydraulic, and biogeochemical characterization of the subsurface using electrical impedance spectroscopy (EIS) methods. Principally, these results can be transferred to the field using electrical impedance tomography (EIT). However, the required accuracy of 1 mrad in the phase measurements is difficult to achieve for a broad frequency bandwidth be-cause of electromagnetic (EM) coupling effects at high frequencies and the lack of inversion schemes that consider the spectral nature of the complex electrical con-ductivity. Here, we overcome these deficiencies by (i) extending the standard spa-tial-smoothness constraint in EIT to the frequency dimension, thus enforcing smooth spectral signatures, and (ii) implementing an advanced EM coupling re-moval procedure using a newly formulated forward electrical model and calibra-tion measurements. Both methodological advances are independently validated, and the improved imaging capability of the overall methodology with respect to spectral electrical properties is demonstrated using cross-borehole EIT measure-ments in a heterogeneous aquifer. The developed procedures represent a signifi-cant step forward towards broadband EIT, allowing transferring the considerable diagnostic potential of EIS in the mHz to kHz range to geophysical imaging appli-cations at the field scale for improved subsurface characterization.