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| Hauptseite > Workflowsammlungen > Öffentliche Einträge > Numerical correction of the phase error due to electromagnetic coupling effects in EIT borehole measurements |
| Hauptseite > Workflowsammlungen > Öffentliche Einträge > Numerical correction of the phase error due to electromagnetic coupling effects in EIT borehole measurements |
| Poster | FZJ-2013-05383 |
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2013
Please use a persistent id in citations: http://hdl.handle.net/2128/5561
Abstract: Spectral Electrical Impedance Tomography (spectral EIT) allows obtaining images of the complex electrical conductivity for a broad frequency range (mHz to kHz). It has recently received increased interest in the field of near-surface geophysics and hydrogeophysics because of the relationships between complex electrical properties and hydrogeological and biogeochemical properties and processes observed in the laboratory with Spectral Induced Polarization (SIP). However, these laboratory results have also indicated that a high phase accuracy is required because many soils and sediments are only weakly polarizable and show small phase angles between 1 and 20 mrad only. It is a challenge to reach this phase accuracy in a broad frequency range for EIT measurements in the field. In the case of borehole EIT measurements, electrode chains (>10 meters) are typically used, which leads to undesired inductive coupling between the electric loops for current injection and potential measurement and capacitive coupling between the electrically conductive cable shielding and the soil. Depending on the electrical properties of the measured transfer impedances, both coupling effects can cause large phase errors that have typically limited the frequency bandwidth of field EIT measurements to the mHz to Hz range. In this presentation we will i) derive correction procedures for capacitive and inductive coupling effects to extend the applicability of EIT to the kHz range and ii) validate these corrections using controlled laboratory measurements and field measurements. In order to do so, the inductive coupling effect was modeled using electric circuit models and the capacitive coupling effect was modeled by integrating discrete capacitances in the electrical forward model describing the EIT measurement process. The measured calibration data set of mutual inductances obtained for each individual electrode chain was combined with the numerically modeled mutual inductances to obtain the mutual inductance for each considered electrode configuration (e.g. cross-hole). The correction methods were successfully verified with measurements under controlled conditions in a water-filled rain barrel, where a high phase accuracy of 1 mrad in the frequency range up to 10 kHz was achieved and also verified in a first field demonstration using a 10 m borehole EIT chain with 8 electrodes. The results of a 1D inversion show that the correction methods increased the phase accuracy considerably.
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