Abstract
Borehole EIT measurements in a broad frequency range (mHz to kHz) are used to study subsurface geophysical properties. However, accurate measurements have long been difficult because the required long electric cables introduce undesired inductive and capacitive coupling effects. Recently, it has been shown that such effects can successfully be corrected in the case of single-borehole measurements. The aim of this paper is to extend the previously developed correction procedure for inductive coupling during EIT measurements in a single borehole to cross-borehole EIT measurements with multiple borehole electrode chains. In order to accelerate and simplify the previously developed correction procedure for inductive coupling, a pole–pole matrix of mutual inductances is defined. This consists of the inductances of each individual chain obtained from calibration measurements and the inductances between two chains calculated from the known cable positions using numerical modelling. The new correction procedure is successfully verified with measurements in a water-filled pool under controlled conditions where the errors introduced by capacitive coupling were well-defined and could be estimated by FEM forward modelling. In addition, EIT field measurements demonstrate that the correction methods increase the phase accuracy considerably. Overall, the phase accuracy of cross-hole EIT measurements after correction of inductive and capacitive coupling is improved to better than 1 mrad up to a frequency of 1 kHz, which substantially improves our ability to characterize the frequency-dependent complex electrical resistivity of weakly polarizable soils and sediments in situ.
Highlights
We presented an effective approach to correct inductive coupling in borehole Electrical Impedance Tomography (EIT) measurements for all possible electrode configurations using one or more borehole electrode chains
This approach considers both the mutual inductance associated with inductive coupling inside each borehole chain, which is determined with calibration measurements and the mutual inductance associated with inductive coupling between two borehole chains, which is calculated from the geometry of the cable layout
These mutual inductances were assembled in a convenient pole–pole matrix that allows a simple and straightforward estimation of inductive coupling for arbitrary electrode configurations
Summary
Electrical Impedance Spectroscopy (EIS) measurements, known as Spectral Induced Polarization (SIP), have. We solved this problem for measurements with a single borehole electrode chain using a method based on calibration measurements (Zhao et al 2013) This calibration must be done only once because it does not depend on the cable layout of the field measurement. EM coupling in cross-hole EIT measurements with multiple borehole electrode chains was not yet considered in great detail In this case, inductive coupling cannot be corrected by only using calibration data because the coupling between different electrode chains depends on the cable layout of the field measurement. To improve applicability of corrections of inductive coupling, there is clearly a need for an effective framework to jointly consider calibration measurements and numerical calculations based on cable geometry to correct EIT measurements with arbitrary electrode configurations using one or multiple borehole electrode chains. The correction procedures were applied to actual borehole EIT measurements
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
